US2406870A - Apparatus for responding to magnetic fields - Google Patents

Description

Sept. 3, 1946. v. v; VACQUIER APPARATUS FOR RESPONDING TO MAGNETIC FIELDS Filed July 21, 1941 5 Sheets-Sheet 1 ll QUKQQQ/ INVENTOR. VICTOR V. VAC Q U 1BR, BY @l 727. j

ha ATTORNEY Sept. 3, 1946. v. v. VAQUlER 2,406,870

. APPARATUS FOR RESPONDING TO MAGNETIC FIELDS Filed July 21, 1941 5 Sheets-Sheet 2 $9. X 3 A k npw H INVENTOR. VICTOR v. VACQU 113R Sept. 3, 1946.

V. V. VACQUIER APPARATUS FOR RESPONDING TO MAGNETIC FIELDS Filed July 21, 1941 5 Sheets-Sheet 5 G Y Rm v v N A D ER R 1 7 o m+ i mmwm R ca R mm B m M N 0 IM 5 Q c 3 A V R O T C I V 2 5 J 6 e 5 .L||

Sept. 3, 1946. v. v. VACQUIER APPARATUS FOR RESPONDING TO MAGNETIC FIELDS Filed July 21,'1941 5 Sheets-Sheet 4 OUTPUT OUTPUT INVENTGR. VICTOR V VACQU 1BR BY -722 9 da- ATTORNEY Sept. 3, 1946. v. v. VACQUIER 2,406,370

APPARATUS FOR RESPONDING T0 MAGNETIC FIELDS Filed July 21, 1941 5 Sheets-Sheet 5 INVENTOR. V II VICTOR VIVACQUIER, 67 BY %-9:1- M

MAT T O RNEY Patented Sept. 3, 1946 APPARATUS FOR RESPONDING 1'0 GNETIC FIELDS Victor V. Vacquler, Oakmont, Pa., assignor to Gull Research & Development Company, Pittsburgh, Pa, a corporation of Delaware 9 Claims. i

This invention relates to improvements in apparatus for responding to magnetic fields.

In many arts it is desired to produce an electrical signal or impulse in accordance with relatively small changes in magnetic fields. For example, in detecting submarines from ships or airplanes, a device is desired which will produce a usable electric signal on the rather small change in local magnetic field due to the relatively distant iron mass of the submarine, and a similar need arises in military mines, intended to set off explosives on approach of a vehicle or ship within a predetermined distance. In automatic ship or aircraft piloting apparatus it is desired to provide a device responsive to the earths magnetic field and capable of producing an electric signal on a small deviation from some predetermined direction in such field. Similar problems arise in magnetometry, involving investigation of the magnetic fields of metallic or non-metallic specimens, or of the earths field as in prospecting work.

Magnetic field responsive apparatus of various types has been proposed. Most apparatus heretofore known has suffered from a lack of sumcient magnetic sensitivity, or has been unduly sensitive to mechanical shocks and vibrations, or is dependent, as regards its output, on acceleration.

One magnetically responsive system of the prior art, which is promising on its face but is rather disappointing in results, makes use of a pair of matched transformers having cores of high-permeability material, the primaries of which are arranged for periodic energization and the secondaries of which are connected in opposed relation to each other and are connected to a transducing device of a type which responds to any and all electrical signals applied to it. With such a circuit, in the absence of an ambient magnetic field, the voltages induced in the secondaries are equal and opposite and no signal appears. In the presence of a magnetic field, in each magnetization cycle of the transformers, the field and flux developed in one of the cores is increased by a certain amount and that in the other core reduced by the same amount, so that energy appears in the output circuit. This apparatus, however, is in practice seriously lacking in sensitivity.

Among the objects achieved in the invention are the provision of an apparatus adapted to respond to changes in magnetic fields, characterized by an extraordinarily high sensitivity, which, however, is accompanied by reliability and insen- Application July 21, 1941, Serial No. 403.455

sitivity to mechanical shocks and vibrations; the provision of such an apparatus the output of which is independent of acceleration; the provision of such a device adapted to actuate a relay or the like upon change in magnetic field beyond a predetermined level; the provision of such a device adapted for accurate quantitative measurement of magnetic fields; and the provision of such an apparatus involving use of magnetically susceptible transformer cores in which error due to residual magnetism in the cores is avoided.

The present invention makes special application of the knowledge that a core of material of high permeability and low energy requirement for saturation (Hypernik, Mumetal or Permalloy, for example) exhibits a hysteresis loop of peculiar form, and upon the discovery that the unique flux-field relation can be taken advantage of, by suitable expedients, to afford a magnetically responsive circuit of extraordinary sensitivity; a sensitivity of the order of 10 to times that attainable with the most highly perfected apparatus of the prior art known to me. In detail, oscillographic studies show that on periodically magnetically energizing a small, thin,

core of high permeability alloy, to saturation, the hysteresis loop exhibits a sharp knee atthe saturation point. Flux changes very rapidly with applied field and then suddenly becomes constant, at saturation.

In my invention, in its best embodiment, two cores are provided, of minimal cross-sectional area (for reasons explained below), carrying windings energized by a periodic current source of suificient amplitude to energize the cores periodically (in opposite senses) past saturation, and other windings on each core for taking of! an induced voltage; all carefully balanced so that each unit is, so to speak, a mirror-image oi the other. By virtue of the described break in the hysteresis loop, the voltage induced in each output winding is a wave of extremely distorted shape, which rises more or less gradually and then drops suddenly to a low value or zero, at instants in time corresponding to the arrival, in each core, of the flux at said knee in the hysteresis loop. The output windings are connected in opposition. The cores and their windings being balanced, in the absence of any applied magnetic field the net output is zero. Now, if an ambient magnetic field is applied, the field and flux developed in one core during each energizetion cycle is correspondingly increased, and in the other diminished. The pulse in one secondary terminates, abruptly, slightly before the pulse 3 in the other secondary terminates. By virtue of the opposed connection of the secondaries this phase shift gives rise to a very sharp voltage pulse (1. e. a pulse of short duration but high intensity). The amount of energy in the pulse depends upon the degree of phase shift and 1n turn on the intensity of the applied field.

To take advantage of this pulse (which while non-oscillatory per se can be considered as a summation of vibratory components of high frequencies) and to distinguish it from other energy pulses or waves (of origin explained below), the amplifier or other transducing device is made to be selective to sharp pulses, by biasing it so that applied potentials below a predetermined value do not affect it, and by careful selection of the several circuit constants as described in detail below.

While the transformer primaries can be energized with almost any kind of periodically varying voltage, including pure sinusoidal alternating energy, there are advantages in employing a saw-tooth wave or pulse series for the periodically applied energy. These considerations are described in detail below.

In the accompanying drawings are shown diagrammatically several examples of specific embodiments of apparatus within the purview of the invention. In the drawings,

Fig. 1 is a circuit diagram of one form of the invention, embodied as a magnetometer,

Figs. 2 to 5 are charts illustrative of the electromagnetic phenomena taking place in the apparatus of the invention, with a saw-tooth energizing wave,

Figs. 6, 7 and 8 are similar charts, for the case of a sinusoidal energizing wave,

Fig. 9 is a circuit diagram of the invention embodied in a monitor compass Or automatic pilot,

Figs. 10 and 11 are circuit diagrams showing 'derivator-type exhibiting means useful in connection with the invention of Fig. 1,

Fig. 12 is a circuit diagram of means for automatically adjusting the compensating current in the apparatus of Fig. 1,

Figs. 13, 14 and 15 are diagrammatic showings of modified arrangements of the sensitive cores and their windings,

Fig. 16 is a circuit diagram of a simplified form of the invention, especially suited for use in military mines, and

Figs. 17 and 18 are circuit diagrams or simplified forms of the invention, similar to that of Fig. 16 but embodied as magnetometers.

Referring to the drawings and in particular Fig. 1, a pair of parallel transformer cores, 20 and 2|, is provided, these cores taking the form of very thin strips or ribbons of a magnetic material which has a high permeability and a low energy requirement for saturation. Among suitable materials for the cores are the'alloys known as Hypernik, Mumetal and Permalloy. Absolute and relative dimensions of the cores are important, as explained below. And for highest sensitivity, the cores and their windings should be very carefully matched.

The cores are provided with oppositely-wound primary windings 22 and 23, and oppositelywound secondary windings 24 and 25. The primaries are connected in parallel as shown and are periodically energized by an oscillator making use of a hot cathode gas trlode 26, in circuit with a battery 21, resistor 28 and condenser 29, to supply energy pulses through a connection 30 to the primaries. Screen voltage is secured by pearing at the output of circuit 4 a voltage divider 3| across the plate circuit, by means of which amplitude and frequency of the oscillations can be adjusted.

The secondaries are connected to each other in series opposition through a circuit including leads 32 and the primary of a step-up transformer 33, as shown, at the input of an amplifier, described in detail below, constructed and arranged to be selective to the high frequency pulses appearing at the transformer.

Considering the operation of the apparatus as thus far described:

Fig. 2 shows in its upper part the hysteresis loop, that is to say, the cyclic function of flux F versus field, for a core of Mumetal, one of the alloys suitable for the cores of the present apparatus. Each cycle of the periodically varying current applied to the primary causes the loop to be traversed one time, in the direction indicated by arrows. This loop is of pecular character. The flux increases (or decreases) very rapidly until at a certain point, the saturation point, it abruptly becomes constant, further increases in field producing substantially no change in fiux. The sharp knee at which this saturation takes place is indicated at A in Fig. 2. As stated, to utilize this effect it is important that the core be very slender. In a core of large cross-section eddy currents are induced which tend to oppose rapid changes in flux and thereby round off the corner A. In practice, strips of cross-section l g inch by .014 inch, and of length 2.5 to 10 or 12 inches, have been found especially satisfactory. The longer the core the higher is the magnetic sensitivity.

Below the hysteresis loop and plotted to the same horizontal scale are shown approximately saw-tooth wave forms S, as generated by tube 26 in Fig. 1 and applied separately to each core. As shown, each wave has sufiicient amplitude to saturate its core in both directions. Disregarding the effect of the compensating current from battery 21 (described below), the waves will be displaced with respect to each other by amounts +h. and h; It being the value of the earths magnetic field or other applied field. The applied field is added to the field of one core and is subtracted from the field of the other, as the cores are magnetized, in opposite senses.

The corresponding fiux changes with time are shown in Fig. 3.

The corresponding fiux changes give rise to voltage changes, in the secondary windings, shown in Fig.4. The signals are qualitatively similar in shape, but one leads the other slightly in time; that is, there is a phase shift or displacement therebetween. As these voltage signals are opposed, the net signal, obtained by subtracting one wave form from the other, is as shown in Fig. 5. This signal has for each energizing cycle a very high sharp peak P and a number of smaller peaks as shown.

These sharp peaks change in amplitude very rapidly as the ambient field is changed. Due to the very steep wave front of the magnetizing pulses S, the core material is magnetized in a very small part of one cycle so that the angular phase shifts produced by the ambient field are very small, and by reason of the abrupt change at saturation (point A) the differential E. M. F. ap-

is a very sharp pulse of short duration. Accordingly, th secondary windings are designed to have a sufficiently low distributed capacity to permit the very high frequency components to appear at the terminals.

Likewise, associated amplifier equipment is designed to handle the essentially unidirectional, high frequency pulse appearing at the output terminals.

Since the voltage induced in the secondaries is dependent upon the rate of change of magnetic flux, the more rapidly the magnetization cycle is traversed the higher will be the output voltage for a given ambient field. However, due to the effects of distributed capacity and time-constant limitations in associated amplifier equipment the optimum rate at which the magnetization cycle should be traversed is a compromise between these opposing effects. Rates of 60 to 1000 cycles are the most useful. W

Considering now the amplifier (Fig. l), which as stated is devised to accentuate the sharp pulses produced as described: the amplifier, the output of which is delivered to an exhibiting device 35, includes two amplifier stages 36 and 31 and a vacuum tube rectifier 38, arranged in a circuit, the constants of which are selected to emphasize high frequencies and rapid fluctuations. Thus, input transformer 33 and interstage transformers 39 and 40 are of a low inductance type capable of passing frequencies of 20,000 cycles or more. Certain small audio frequenc ,transformers are available for this purpose. A bias battery 4 I, selfbiasing resistors 42 of abnormally high values and bypass condensers 43 of low values are provided, in circuit as shown, so that the tubes are biased nearly to their cut-ofi points and only positive pulses operate the tubes. Such circuit degenerates low frequencies, and the net result is a high degree of discrimination against pulses of low amplitude, pulses of low frequency and pulses of undesired polarity. The transformers are all phased to make the highly sensitive sharp pulses received from the core windings 2d, 25 positive at the grids of the three tubes. Transformer 33 advantageously has a high step-up ratio. By having its primary inductance low, it favors the more rapid changes of the signal and tends to short circuit the slow changes.

Tube 38 is a rectifier of the grid leak type. Each pulse peak drives its grid momentarily positive, causing a grid current to flow. This current charges the condenser 44 to a voltage nearly equal to the peak value. Resistor 45 is large enough so that condenser 46 is but partially discharged between peaks. Hence the grid remains negative over most of the cycle by an amount varying with the strength of the signal. The plate current as exhibited at meter 35 thus decreases from its normal maximum value in proportion to the signal strength. A buck-out battery 50 and adjustable resistor 5| may be used with a sensitive meter if high sensitivity is desired.

For compensating the effect of the earth's field or other field, a circuit is provided for supplying an adjustable direct current to the secondaries 2t, 25, in a direction to oppose the ambient field. The circuit includes a'battery 41, an adjustable resistor $2 and a by-pass condenser 33 to keep signals out of the battery circuit. It is convenient to incorporate a reversing switch 54 in the circuit. Current from the circuit magnetizes both cores in the same direction; ordinarily that direction which opposes the applied field. By suitable adjustment of the resistor the earths field or the field being measured is thus approximately com.- pensated and the instrument indicates small changes in the total field. In some cases, especially where current drain is to be kept low, it is more convenient to substitute permanent magnets, ar-

ranged as described in connection with Fig. 16 for the compensating system described.

Figs. 6, I and 8 are charts, similar to Figs. 2 to 5, for the case of an alternating current of 5 sinusoidal character applied to the transformer primaries. Fig. 6 shows a hysteresis loop for the ,alloy core as in Fig. 2, with sinusoidal GO-cycle waves T plotted below to the same horizontal scale. The amplitude of each wave is such as to saturate its core in both directions. The waves are displaced with respect to the center of the hysteresis loops of the two cores, by amounts +7: and -h, as in Fig. 2. The resulting cyclic flux changes are as indicated in Fig. '7. All the rising 15. and falling portions of both cycles arehere almost identical in shape, but one curve rises slightly sooner and falls slightly later than the other. The fiux changes induce voltages in the two secondaries, practically identical in shape but displaced in phase. The significant feature of the induced voltage is the abrupt discontinuity corresponding to point A in Fig. 6, at which point in time the induced voltage drops abruptly from its maximum to zero. Were the cores made of an 5 ideal material the wave form would be vertical at this point. One voltage wave being subtracted from the other, the net voltage output takes the form of a series of sharp pulses P (Fig. 8), with low amplitude maxima which are eliminated in subsequent parts of the circuit, as described.

The sensitivity of the apparatus as a magnetometer is dependent alone upon the single sharp pulse derived from the sudden break in the magnetization curve. All other portions of the cycle produce undesired E. M. F.s which in general do not cancel out perfectly at the differential output terminals and result in some interference or hash at other portions of the cycle which are not ordinarily troublesome except when attempting to detect an ambient field of extremely small intensity. (In such work, especial care is taken to adjust the amplifier circuit constants for selectivity to the sharp pulses.) These circumstances. considered alone, would indicate that for a magnetometer intended to operate at the highest possible sensitivity, an almost square wave of exciting current would be most desirable. However, perfectly square excitation waves would not permit an appreciable phase shift of the critical operating point, and such extremely small phase shifts would result in such a high frequency voltage pulse as to render dimcult the efiicient handling thereof in associated amplifier equipment. Hence, at present, the best excitation for the pri- 5maries is considered to be a symmetrical sawtooth wave with equally abrupt rises and drops in each peak.

In Fig. 2 the cores are represented as initially biased or magnetized to saturation in one direction and energized with asymmetrical saw-tooth pulses of such amplitude as to reverse the field of saturation. The initial magnetization or bias to saturation is not necessary. Saturation in only one direction need be used. For example,

the axis of the waves in Fig. 2 can be shifted to the right to coincide with the zero field axis of the hysteresis loop by removing the initial bias. For example, permanent magnets 95 and 96 of Fig. 16 may be removed. In this case the amplitude of the primary current pulse can be reduced one half.

Fig. 9 illustrates the invention embodied in a direction-sensitive instrument suitable for keeping a ship or aircraft on a predetermined course;

i. e., as a monitor compass.

Cores and 2| are disposed in parallel relation, on a horizontal support 55 orientable about a vertical axis. The opposed outputs of secondaries 24 and 25 are supplied to an amplifier, as in Fig. 1; a single stage 36 here suffices to afford accurate control, The output of this tube is supplied through transformer 39 to a gas tetrode tube I38, conveniently of the 2050 type, arranged to be sensitive to pulses of one polarity only. The plate of this tube is energized periodically (by means described below) during receipt of the signal pulse at its grid. If the signal pulses are positive the tube breaks down; if they are negative it does not. Each breakdown of the tube sends a signal to a tube 56, of type 6J5, whereby to change the average plate current in this tube. A suitable relay and rudder control or the like, of type known per se, indicated at 51, also a meter 35, if desired, are included in the plate circuit of tube 56 for operation by the plate current.

Primaries 22 and 23 are energized periodically from an oscillator tube 26, conveniently of 884 type, in a circuit generally similar to that described in connection with Fig. l. The oscillator is, however, duplicated as shown to provide B-supply energy to tube I38 in pulses in such manner as to prevent loss of control after each operation of tube I38.- The two oscillators are supplied plate voltage through a common plate resistor 58. This causes them to discharge alternately. However, this energizes the plate of tube I38 during receipt of the signal pulse at its grid by virtue of phase shift in the circuit coupling tube I26 to tube I38. Resistors 59, I59, 68 and I60 are arranged as voltage dividers to bias the grids of the 884 tubes (26 and I26). Resistors 6| and I6I act solely to limit grid current when the tubes discharge the condensers 62 and I62. Resistors 63 and I63, and 58, in combination provide charging paths for condensers 62 and I62 and also furnish the proper degree of coupling to insure alternate operation of the two oscillators.

In operation, when the rods are at right angles to the earths magnetic field, or approximately so, the sharp peaks of output signal disappear. A displacement in one direction then causes positive peaks to occur and opposite displacement causes negative peaks. The circuits of the 6J5 and 2050 tubes (tubes 36 and I38) are made sensitive only to positive signals so that relay and indicator current flow only for one direction of deviation. By having the relay operate a servomotor or the like, the rods may be kept oriented automatically with respect to the earths field. For example, the servo-motor may control the rudder of an aircraft to bring it onto a course which is predetermined by the angular relationship between the rods and the longitudinalaxis of the aircraft.

In one form of the invention in which.it is desired to indicate the total magnetic field of the earth continuously in an aircraft, it is desirable to use one indicating instrument like that of Fig. l and keep it oriented in the direction of maximum field by means of a second instrument like that of Fig. 9. The rods of Fig. 1 can be mounted on a gyro horizon or the like to keep them properly inclined with respect to horizontal, so that they will be pointed correctly when the automatic device of Fig. 9 reaches the balance point. In this arrangement it is preferable to have the Fig. 9 device control the orientation of the Fig. 1 device with respect to the axis of the aircraft rather than attempting to keep the aircraft on a predetermined course, as this allowed ficulty is experienced in that the indicator tends 10 to go off scale due to gradual changes in the earths field from place to place, and drift in the instrument itself. Fig. 10 shows a modification of the invention wherein such difliculty i avoided. by causing the indicator to show only the time the value of the field itself. In Fig. 10, tube 38 of the circuit of Fig. 1 is connected in a derivator circuit or rate-of-change indicator, of a type known per so, having it condenser 65 and resistor 66 selected to have a time constant of about one to three seconds, approximatingthe interval required to fly over a submarine. Slower changes are not transmitted effectively through the condenser. Slow drifts cause only a slight deflection of null-type meter 61 from its normal center position, but passage over a submarine or other magnetic mass of limited size causes strong upward and downward deflections of short duration.

Resistor 52 (see Fig. 1) need be adjusted only when the field strength reaches the limits of the sensitivity range of the instrument, which may be many times the range of indicator 35.

Fig. 11 shows a translating device. which achieves the same end as in Fig. 10 (that is to say, indication of time rate of change in magnetic fields) with a wider range of sensitivity, accompanied by a slight los in sensitivity. To this end, the input signal is rectified immediately in the grid circuit of a tube 68. The plate circuit of this tube contains a condenser 69 large enough to filter out the signal pulses and leave only the .rectified component which corresponds in amplitude to the peak value of the grid signal pulses.

Condenser I0 and resistor II form a derivator circuit which feeds the extremely-low-frequency amplifier stages, 12 and I3, and indicator 14.

In some cases it is desired to regulate the compensating current in the earths field automatically rather than manually by adjustment of resistor 52 in Fig. 1. Fig. 12 shows a modification of Fig. 1 in which the compensating current is made self-regulating. Box I5 contains an amplifier like Fig. 1 except that a resistor I6 is inserted in the cathode circuit of tube 38 to prow'de 55 bias control for a tube 11. Condenser I8 and resistor 19 provide a long time-constantusually many seconds-so that tube 'I'I follows the signal variations quite sluggishly. This is necessary to prevent the compensating circuit from ironing out the desired variations and to make the action stable. The D. C. output current of tube 11 flows directly through the pickup coil 24, 25. The circuit must, of course. be phased to give control in the proper sense. Any changes of field that ooour with reasonable rapidity cause fluctuation of the regular indicators, but slow changes m rely bring about a gradual adjustment of the iiilllpei' sation. Condenser 53 plays the same ,mgz-t is i Fig. 1.

The two sensitive rods maybe used at an angle to each other as shown in Fig. 13, if desired. The direction of sensitivity is then along the bisector of the angle formed, as indicated by the double arrow. However, the sensitivity decreases as the 76 angle between the rods is increased, until it be rate of change of the magnetic field, rather than comes zero after one of the elements has been displaced 180 degrees. Yet, if one of the rods is rotated 180 degrees with respect to the other (which is equivalent to reversing its connections) and also displaced from it by a distance X as illustrated in Fig. 14. the device is again made sensitive, not to total field but to the space gradient of the field. which is to say. the change of field in the distance X. This type of, sensitivity is useful for spottin magnetic anomalies in somewhat the fashion of the derivator of Fig. 10 with the added advanta e that regional chan e of ambient field cause no indication. It is also possible to combine the time and space gradient responses to Figs. 10 and 14, respectively. in one instrument. In the gradient meter modification of the invention, application of a compensating current to the secondaries as in Fig. 1 i not very easily done, as one of the cores would be magnetized in the wrong sense for the preferred type of operation. However, the compensating current can be applied to the primaries, or to auxiliary coils such as coil 83 in Fig. 17.

In employing the invention as a relative intensity meter. if desired one of the cores can be rendered insensitive by disposing it within a magnetic shield, thus becoming a dummy against which the sensitive core is balanced.

Fig. 15 shows a simplified arrangement of the detector coil-s on the cores. While only two coils, I22 and H23. are required, the rods are nevertheless magnetized in opposite directions by the scillator and the resulting induced E. M. F.s are combined in opposition. Usually, separate primary and secondary windings as in Fig. 1 are preferable to this Simplified arrangement.

Fig. 16 shows a simplified embodiment of the invention, especially well suited as a magnetic relay for military mines which it is desired to explode automatically on approach of a vehicle or ship as the case may be.

Primaries 22 and 23 of the transformers are adapted to be energized at spaced intervals by a circuit comprisin a battery 21, a resistor 28 and a condenser 29. connected to a gas-filled cold cathode tube 226 in circuit with primaries 22 and 23 as shown. The battery charges the condenser and when the charge reaches a certain potential the tube breaks down and discharges a pulse of current through the primaries. Pulses occur at regular intervals, determined by the battery voltage, the values selected for elemerits 28 and 29 and the breakdown voltage of tube 226. The sensitivity of the apparatus is not aifected by the rate of impulse generation. In a naval magnetic mine where a low energy consumption is desirable, a rate of 30 discharges a minute (0.5 cycle), of tube 226, is enough. The drain on the battery in such case is negligible: a few microamperes. For uses where energy is not at a premium it is advantageous to raise the frequency of the oscillator. say to 25 to 200 cycles or more. substitute hot cathode tubes for the cold cathode tubes shown in Fig. 1.

secondaries 2d and 25 of the transformers are connected in series opposition and also in series with the primary of a step-up transformer '33 (cf. Fig. 1), the secondary d6 of which has a center tap grounded at 91, and ends connected to the grids of two gas-filled cold cathode tubes 89 and BI, the platesof which are in contact with junction 82 between the battery and resistor, and the cathodes of which are connected, as

In such cases it is also desirable to 10 shown, to a device 83, such as a blasting cap, which the apparatus is intended to actuate.

The sensitivity of the apparatus and the cf fective distance range can be adjusted conveniently by locating a pair of opposed permanent magnets, 95 and 96, adjacent the pair of transformers. The effect of these magnets is to impose steady fields on the cores, so that a smaller (or greater) extraneous field is capable of producing the same differential flux. For example, with the apparatus set up at a given orientation in the earth's field these magnets can be adjusted so that the apparatus gives zero or minimum response. These compensating magnets can be used in all embodiments, in lieu of the energy supply 41 of Fig. 1, but ordinarily the arrangement shown in Fig. 1 is preferable, especially where the core-coil arrangement is reasonably symmetrical.

The apparatus of Fig. 16, despite its simplicity, has a very high sensitivity. With a stage of amplification of gain twenty-five, between secondary 46 and each of tubes Bil and 8! in Fig. 16, a change in intensity of 20 X oersted's is readily detected, a magnitude corresponding to the anomaly produced by a passenger automobile thirty feet away, or a medium sized ship 300 feet away. Higher sensitivities are readily obtained when desired.

The simplified form of the invention shown in Fig. 16 can be embodied as a magnetometer. Fig. 17 shows one such good embodiment of the invention. The pair of transformers, the oscillator and the output transformer are provided as in Fig. 16. The pair of transformers is enclosed by a solenoid or coil 83 (the central portion being broken away for clarity) energizable from a battery 8d through a reversing switch 35; a variable resistor 86 and a meter 8! being provided in circuit as shown. Transformer secondary 46 is connected to a vacuum-tube voltmeter 38, which may include one or more stages of amplification.

In use, the pair of transformers and the mag- 45 netic body (which may be the earth) are brought in proximity. This unbalances the transformer circuit and voltmeter 88 registers a positive or negative voltage. Switch 85 is then closed, to whichever position causes the potential of bat- 50 tery 84 to tend to restore the reading of meter 88 to zero or null position. Resistor B6 is adjusted until meter 88 reads zero, and the corresponding current at meter 81 is read as a measure of the field strength of the unknown object.

This embodiment of the invention has special utility in prospecting for mineral deposits and in locating buried pipes or the like. It is also usefill in measuring the magnetic properties of metallic and non-metallic specimens.

The apparatus is readily adapted for'measuring gradients of magnetic intensity, as in Fig. 14, by reversing the connections of windings 22 and 24; or reversing the connections of windings 23 and 25.

Another simplified form of magnetometer is shown in Fig. 18. The transformer circuit is like that of Fig. 16. Hot cathode tubes are shown at 325, I80 and IN, in lieu of the cold cathode tubes of Fig. 16; such substitution being advantageous in the magnetometer form of the invention where low battery drain is not necessary. Resistors 89 and 90 are interposed in the circuits joining tubes I80 and 18! to point 82 and these circuits are grounded through condensers 75 9! and 92 as shown. The solenoid circuit is like ing positive signals.

that of Fig. 17. By reason or condensers 9i and 92 and resistors 89 and 90, the plate circuit currents of tubes l80and l8l, respectively, are not maintained unless the rid is continually receiv- In operation, when the transformers are brought into a region where the magnetic intensity is greater or smaller than a predetermined'value, tube I80 or l8l, as the case may be, glows. The current in solenoid 83 is adjusted until both tubes are dark. The current at 81 is taken as a measure of the field strength. A sensitivity of the order of two gamma is readily attained with this circuit.

In all embodiments of the invention, the apparatus sensitivity is very high, yet is accompanied by a high degree of inertness to mechanical shocks and disturbances. Except where deliberately provided for otherwise, the predetermined intensity level at which it gives a signal is independent of acceleration, that is to say the rate of change of speed at which the apparatus approaches, or is approached by, the foreign magnetic body. This makes possible fine measurements of magnetic intensity and gradient from a moving conveyance such as a destroyer or airplane.

What I claim is:

1. An apparatus responsive to magnetic fields comprising a pair of cores of material of high permeability and characterized in that the hysteresis loop thereoi exhibits a sharp knee where saturation is reached, means for generating periodic saw-tooth wave forms and for cyclically energizing said cores therewith to saturation whereby each core abruptly reaches saturation at a phase of the energizing cycle which is shifted by changes in the ambient field at said core, windings adjacent said cores connected in opposition to-deliver the difference between the voltages induced by said cores, whereby said phase shifts cause production or sharp voltage pulses which vary in magnitude with a change in field at either one of the cores, and transducing means selective-to said pulses, in energy-receiving relation to said windings.

2. An apparatus responsive to magnetic fields comprising a pair of cores of material of high" permeability and characterized in that the hysteresis loop thereof exhibits a sharp knee where saturation is reached, means for cyclically energizingsaid cores to saturation whereby each core abruptly reaches saturation at a phase of the energizing cycle which is shifted by changes in the ambient field at said core, and windings adjacent said cores connected in opposition to deliver the differences between the voltages induced by said cores, whereby said phase shifts cause production of sharp voltage pulses which vary in magnitude with a change in field at either one of the cores, and amplifier means in energyreceiving relation to said windings, biased to a level such as to respond to said sharp pulses while not responding to relatively low voltage fluctuations.

3. An apparatus responsive to magnetic fields comprising-a pair of parallel cores of material of high permeability and characterized in that the hysteresis loop thereof exhibits a sharp knee where saturaton is reached, oppositely phased primaries on the cores, means connected to said primaries for periodically energizing said cores to saturation whereby each core abruptly attains saturation at a phase of the energizing cycle which is shifted by changes in ambient field at said core, secondaries on the cores connected in I Hi opposition to deliver the diflerence between the voltages induced by said cores, whereby said phase shifts cause production of sharp voltage pulses which vary in magnitude with'a change 6 in field at either core, and amplifying means arranged to receive said sharp voltage pulses and to amplify them selectively while discriminating against low amplitude voltage pulsations.

4. An apparatus responsive to magnetic fields comprising a. pair of cores which are approximately 0.13 by 0.014 inchin cross section and 2.5 to 12 inches in length, of material of high permeability and characterized in that the hysteresis loop thereof exhibits a sharp knee where saturation is reached, means for cyclically energizing said cores to saturation whereby each core abruptly reaches'saturation at a phase of the energizing cycle which is shifted by changes in the ambient field at said core, windings adjacent 20 said cores connected in opposition to deliver the differences between the voltages induced by said cores, whereby said phase shifts cause production of sharp voltage pulses which vary in magnitude with a change in field at either one of the cores, and transducing means selective to said pulses, in energy-receiving relation to said winding.

5. An apparatus responsive to magnetic fields comprising a pair of slender cores of high permeability material not exceeding approximately 0.02 inch in thickness, characterized in that the hysteresis loop thereof exhibits a sharp knee where saturation is reached, means for cyclically energizing said cores to saturation whereby each core abruptly reaches saturation at a phase of the energizing cycle which is shifted by changes in the ambient field at said core, windings adjacent said cores connected in opposition to deliver the differences between the voltages induced by 40 said cores, whereby said phase shifts cause production of sharp voltage. pulses which vary in magnitude with a change in field at either one of the cores, and transducing means selective to said pulses, in energy-receiving relation to said windings.

6. An apparatus responsive to magnetic fields comprising a pair of slender cores of minimal cross-sectional area, of material or high permeability and characterized in that the hysteresis loop thereof exhibits a sharp knee where saturation is reached, means for cyclically energizing said cores to saturation whereby each core abruptly reaches saturation at a phase of the energizing cycle which is shifted by changes in the ambient field at said core, windings adjacent said cores connected in opposition to deliver the differences between the voltages induced by said cores, whereby said phase shifts cause production of sharp voltage pulses which vary in magnitude with a change in field at either one of the cores, transducing means selective to said pulses, in energyreceiving relation to said windings, and means responsive to slow changes in ambient field adapted to supply to said core windings a direct ourrent of magnitude dependent on the value of the ambient field.

'1. An apparatus responsive to magnetic fields comprising a pair of highly permeable cores of minimal cross-section having oppositely wound primary windings connected in parallel, a current'source to supply periodic energy impulses to the primary windings, oppositely wound secondary windings for said respective cores connected in series opposition for taking of! an induced voltage which by reason of sharp break in the hysteresis loop of the respective cores produces a resulting wave of distorted shape, the said windings and cores being so balanced in opposite senses that in the absence of applied magnetic field their net output is zero, but upon application of an ambient magnetic field the resulting phase shift in output gives rise to sharp voltage pulse, and an amplifier in energy-receiving relation to said secondary windings biased to respond to such sharp pulses and to amplify them selectively while not responding to relatively low voltage fluctuations.

8. An apparatus responsive to magnetic fields comprising a slender core of high permeability material not exceeding approximately 0.02 inch in thickness, characterized in that the hysteresis loop thereof exhibits a sharp knee where saturation is reached, means for cyclically energize ing said core to saturation in a manner whereby magnetically distinct parts of the core abruptly reach saturation at phases of the energizing cycle which are shifted by changes in the ambient field at said core, a secondary circuit having means therein linked with said core to deliver the difference between the voltages induced in said means by parts of said core. whereby said phase 14 shifts cause production of sharp volt: which vary in magnitude with a chan at the core, and transducing means s said pulses in energy-receiving relatii secondary circuit.

9. An apparatus responsive to magi comprising a core of material of higl bility and characterized in that the loop thereof exhibits a sharp knee i uration is reached, means for cyclical. ing said core to saturation in a mam by magnetically distinct parts of the cc ly reach saturation at phases of the cycle which are shifted by changes in t1 field at said core, a secondary circ means therein linked with said core the difference between the voltages said means by parts of said core, wt 7 phase shifts cause production of she pulses which vary in magnitude with in field at the core. and amplifier me ergy-receiving relation to said means a level such as to respond to'said si while not responding to relatively l fluctuations.

VICTOR V. VA4

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