US2558670A - Electrical apparatus - Google Patents

Description

June 26, 1951 s. BREEN ET AL ELECTRICAL APPARATUS 2 Sheets-Sheet 1 Filed April 3, 1950 HLSW@ wai/KM QM@ d June 26, 1951 Q BREEN ET AL 2,558,670

ELECTRICAL APPARATUS Filed April 5, 1950 2 Sheets-Sheet 2 A In vena/".5

JaaW/ef/ reei? zxzae/ L. Ker/f @@Mslww Patented `lune 26, 154951 ELECTRICAL APPARATUSA Stanley Breen, Geneva, and Maxwell A. Kerr,

Wheaton, Ill., assignors to Operadio Manufacturing Co., a corporation of Illinois Application April 3, 1950, Serial No. 153,552

20 Claims. (Cl. 175-320) This invention relates to an electrical apparatus and particularly to a sensitive relay. The sensitive relay to which this invention pertains is adapted to work with sources or generators having an extremely low potential direct current output.

Generators of the type having a Search coil operating in the earths field, as one example, provide a direct or slowly varying current of minute value at low potential. Such currents in extreme cases may be measured in microamperes with potentials in microvolts. In connection with such devices, it is desirable to provide a relay which will respond to the output of such a generator and close a second relay circuit for supplying current to a final load.

Vacuum tube amplifiers for such purpose are not satisfactory. One reason is that a conventional vacuum tube amplifier is inherently unsuitable for operation on slowly varying or direct currents. Even elaborate means to balance the amplier and prevent drift still make the operation of such amplifiers erratic and unreliable. Secondly, is the fact that a vacuum tube amplifier requires a cathode circuit continuously energized. In certain types of apparatus, such as portable apparatus for neld use or apparatus having long stand-by periods, the matter of a power supply for energizing a cathode circuit is a serious problem and renders a, vacuum tube amplifier imr part of a relay circuit. Since moving coil galvanometers are sensitive and have very Weak forces normally generated therein, it follows that the movable element is unlikely to make good contact between the xed and movable contacts.

The most sensitive type of galvanometer is the moving coil type having current leads and torsion suspension combined into one structure. The movable contact will naturally move with the coil and may have a separate lead therefor or use the moving coil leads. two leads for the movable part of the galvanometer renders the structure impractical and impairs the sensitivity. If two leads are used in a conventional manner, a serious drawback becomes apparent. When the relay circuit is estab- Having more than f lished, the IR drop across the galvanometer coil system, including suspension, becomes large in comparison tc the E. M. F. normally developed by the original generator as a search coil. Hence the entire movable coil system becomes a source of E. M. F. which may exceed the safe value to be impressed across the original generator as a search coil with disastrous results to the latter.

This invention provides a construction utilizing a movable coil type of galvanometer whereby the movable element, when it reaches a predetermined position, is adapted to lock itself closed so that rm contact is established between the xed and movable relay contacts. The invention further provides means whereby relay currents are prevented from flowing into the generator, thus removing the possibility of damage to the generator due to excessively large currents. This invention further provides means whereby the coil will unlock itself when desired and open the relay circuit in preparation for a new relay operating cycle. A galvanometer embodying the invention utilizes the full sensitivity of the instrument and, apart from the movable contact arm, does not have elements upon the movable coil which detract either electrically or mechanically from the sensitivity of the galvanometer.

The invention, in general, is characterized by a combination of a novel galvanometer structure and a novel circuit for the galvanometer. The galvanometer is of the moving coil type operating in a constant magnetic field. The movable coil is preferably, though not necessarily, suspended in a torsion type of mounting. The mounting also functions to lead currents to the movable coil. The movable coil carries at least one Inovable contact in the form of a thin wire extending therefrom. This contact is connected to the movable coil at any place desired between the end and center of the coil. Two movable contacts may be thus carried and be connected symmetrically on opposite sides of the center point of the movable coil. While the movable contact may be connected to the terminal of the movable coil, it is preferred to make the connection to an intermediate eccentric point on the coil proper. The amount of eccentricity of the contact tap on the coil will be determined by other considerations which will be apparent in connection with a detailed description of the invention.

The eccentricity of the contact tap on the movable coil is with reference to the electromagnetic properties of the coil. Thus the center of the movable coil will be that point which will divide the coil electromagnetically into equal and opposite coil parts. While such a coil center is near the geometrical wire center, it is not necessarily coincident, due to departure from theoretically perfect winding. Hence the term coil center will be used here with reference to the electromagnetic eiects and not the geometrical center or resistance center unless expressly stated.

An important feature oi the invention resides in the provision for constructing a winding so that an eccentric tap i'cr a movable contact may be substantially at the resistance center of the winding so that the direct current resistance of the' parts of the complete winding on opposite sides ci the tap are substantially equal. This may be true even though two movable contacts are pro-- vided. This effect is obtained by forming that part of the movable coil at the electrical or resistance center for equal distances on both sides of the center up to the movable contact tap or taps of wire having lower resistance than the remainder of the movable coil. In so disposing the low resistance section of the movable coil, it is assumed that the geometric center and ohmic resistance or electrical center of the coil are coincident. In practice, these two points are close enough so that satisfactory results are obtained.

By virtue of the above expedient, the galvanometer movable contact (or contacts, if two are used) is electromagnetically eccentric of the winding but electrically is substantially centered. Hence the galvanometer coil is substantially symmetrica] for ohmic resistance as seen from the movable contact or contacts and coil terminals.

rEhe relay circuit in which the galvanometer operates has a source of constant potential, such as a battery, and a three wire type of load. By virtue oi the arrangement of the relay circuit source of potential and three wire load in the relay circuit and the inclusion in the circuit of suitable balancing means, it is possible to provide a substantially perfect electrical balance across the terminals ci the galvanomcter when the load relay circuit is closed. As previously pointed out, the movable coil may be balanced for IR drop. The torsion suspension leads, however, may vary from instrument to instrument so that resistance balancing means external to the galvanometer are preferable. When balance at the galvanorneter terminals is obtained substantially no current from the load relay circuit will flow through the generator.

Where two fixed relay contacts are provided, it is possible to utilize a center Zero type of nstrument, thus rendering the instrument independent of polarity of the generator. Normally, the generator must be connected in such a manner that current of proper polarity actuates the galvanoineter coil in the direction so that the relay contacts approach each other.

In order that the invention may be understood, it will now be explained in connection with the drawings wherein Figure 1 is a diagrammatic view showing one form of the invention and Figures 2 to ll inclusive are diagrammatic views showing modified forms of the invention.

Referring first to Figure l, generator lll of any desired type is shown, this generator having terminals l l and l2 and furnishing, at desired times or under predetermined conditions, a current whose intensity is normally measured in microamperes at a potential as low as several microvolts. Such a generator, for example, may consist of coil ll of wire with or without a suitable ferromagnetic core and adapted to function as d an exploring coil. Such a generator will have an internal resistance represented by resistor I5. rlhis generator, in addition, may also have inductance and capacitance, these two properties of the generator' being unimportant for the purposes at hand.

Terminals ll and i2 of the generator are connected by wires ll and li? to terminals 20 and 2| oi a galvanometer generally indicated by numeral 22. Galvanometer is preferably of the suspension type although the conventional spiral hair spring Dnsonval 'type of galvanometer may b'e used. inasmuch as both types or galvanorneters are well known and highly developed, a detailed description of the galvanometer itself is not deemed to be necessary.

Galvanometer 22 has suitable permanent magnet system it having poles 2t and 25 respectively. In practice, the magnet would have suitable pole pieces ci soit iron shaped in a particular manner to distribute the flux from the magnet over a predetermined angular extent, all well known in the art. Cooperating with the magnet is movable coll winding which may consist of any desired number of turns on a suitable form. Coil 2.8 has terminals Erl and til attached to suspension wires tl and 32, these wires being aligned and consisting or ilegible, elastic good conducting material such as gold, tungsten, copper, bronze or any other material. Wires 3i and 32 may be tensioned by suitable means not shown, in a manner well known in the art with suitable adjusting means so that moving coil normally has a predetermined zero position.

Between one oi the galvanometer terminals and the suspension wire, in this case between terminal 2l and wire t2, there is provided electrical balancing means consisting or' variable resistor and variable inductcr shunted by Variable resistor 35 and variable capacitor 36 for a purpose which will be apparent later. In some instances, it may be possible to eliminate either or both resistors cr the inductor or combine the two in one unit. in the case of a DArsonval type of meter, the conventional spiral springs would replace the suspension wires.

Movable coil carries movable relay contact El. This contact may consist of a ne wire eX- tending outwardly from the coil s-o 'that this wire moves over a predetermined path when coil 28 moves in response to a current passing through the same. Movable contact 55? is connected to point 28h on coil somewhat off electromagnetic center 28a of the lecil so that coil M3 is divided into two portions. In practice, coil 2t may consist of a nrmber ci turns cl wire, as 25 turns for example. Between 28a and 2gb there may be part oi a turn or a number or turns.

Coil E3 normally 'would of ine copper wire such as #au an example. However, between points 28h and ttc on the winding, these points being symmetrical to point it is preferred to use wire having substantially less resistance per unit length than is true of the winding as a whole. Thus if coil is or #4G wire, thenwire 25h-#Btc may cf #34; or #86 wire.

If #Si wire is used, the resistance will be 1/8' that of the wire, the wire ses n doubling for each two steps wire gauge. The low resistance part ESD--Zlc may of silver to further reduce the resistance, although the gain is ordinarily not worth the expense.

The departure of point from electrical center 28a. of coil il' depends in some measure upon the amount oi relay current Flowing in the relay circuit when closed, the amount of current which the galvanometer winding can safely carry, the amount of force necessary for locking and unlocking the relay contacts and the convenience of locating a suitable spot on the winding to which a connection be made.

Cooperating with movable relay contact El is fixed relay contact connected by wire 6l to terminal t2 of double pole double throw reversing switch 3.1. This switch has terminals 43 and 44 going to suitable movable contacts. The movable contacts cooperate with fixed contacts 42 and 45 at one switch position and contacts 42' and 45 at the other position or" the switch. Movable contact terminals 43 and Ml are connected to the terminals of a source of potential for the relay circuit, this being conveniently shown as battery terminal 44 being connected to the positive terminal and terminal 43 being connected to the negative terminal, as an example. The polarity ci the connections, as will be readily apparent, is unimportant and is given merely by way of illustration.

Terminal 45 of the switch is connected by wire 53 to intermediate terminal Si of a three wire load. This load, as shown here, consists of windings 52 and 53 having common intermediate terminal 5l and outer terminals 5c and 55 respectively. Terminal 5e is connected by wire 5E to terminal 2G while terr .na is connected by wire 5l to terminal Windings 52 and 53 are the windings of load relay these windings being so arranged that when current from terminal El goes through windings 52 and 53 as shown, the magnetic elds created by the two windings will aid each other. Load relay 58 is a conventional relay and has contacts and El? of usual construction going to a utilization circuit of any type whatsoever, contacts 59 and 60 being adapted to handle comparatively heavy currents.

The resistances of windings 52 and e3 of relay 58 are preferably substantially greater than the resistance of winding 2li. This has the effect of preventing excessive loading of generator IQ.

The operation of the relay system is as follows: Assume that double pole double throw switch Se is in the position shown and the galvanometer coil is at rest. Now assume that generator lil becomes active and generates current with terminal l! being positive and terminal l2 being negative. Current will flow from terminal l! along wire il to terminal 2,53 through the moving coil portion ci the galvanometer to terminal 2l and back along wire i8 to terminal l2 oi the generator. Movable coil 28 of the galvanometer begins to respond to this current, it being understood that the polarity of the current is such that movable contact 31 approaches Contact eil. Assume that movable contact 3l reaches ixed contact 4e. Even though contacts 3l and il do not close tightly nevertheless, some current from relay battery 43 will flow. A circuit for this current may be traced as follows: From the negative terminal of battery t to terminal 43, terminal 42, relay contact llt, relay contact El, tap 28h of coil 28, terminal wire 32. resistor 33 and inductor 34 to terminal 2i wire Si?, through winding 53 of the load relay, center terminal 5I, wire 5G to terminal [i5 and then to the positive terminal of the battery. An additional circuit through which relay current will flow er;-

tends from the negative terminal of battery 48 through to movable contact 3l of the galvanometer, then tap 23h of coil 28 and up to terminal 20 of the galvanometer and then through winding 52 of the load relay and then back to the positive terminal of the relay circuit battery.

Whether this initial current is strong enough to operate load relay Eil is unimportant. The initial current from the relay battery passing along wire 56 is of the same polarity as current from generator le as far as its eiect upon that part of the coil between 28h and 29 is concerned. The current going along wire 5l to terminal 353 and tap 23h opposes the generator current in the galvanometer coil. Thus the eccentricity of connection of movable contact 3l causes the currents from the load relay battery to have unequal and opposing eiects upon the galvanometer coil. Thus as shown, current from wire 5e passes through more than one-half of coil 28 and since it aids the current from generator l, it will be noted that coil 2S will tend to close contacts 3l and A@ tightly against each other. As soon as suicient current passes through the two contacts in the galvanometer, load relay 5S will be operated to close its contacts 59 and El).

It will be evident that the galvanometer contacts will remain in locked position now independently of the condition of generator lil. Now assume that it is desired to open the galvanometer contacts and restore the system to the position shown. To do this, it is necessary to operate double pole double throw switch 39 so that terminals 43 and le are connected to terminals 45 and 42' respectively. It will be observed that this reverses the polarity of battery d8 with respect to the coil of the galvanometer and load relay 58. As far as load relay 58 is concerned, the direction of current is unimportant but it is obvious that the direction of current flowing through coil 28 is important. Thus, if the net effect of currents flowing through galvanometer coil 28 is reversed, coil 28 will tend to move contact 3l away from xed contact 4e. Hence, sufcient force can be generated to open these contacts even though some sticking may occur. Thereafter, the reversing switch may be returned to its original condition and the galvanometer is ready to operate anew.

It will be observed that any difference in potential between terminals Z5 and 2l will result in current ilow through generator l. It is desirable to limit such potential diirerences as may exist to the order of that developed across the generator by virtue of generator operation. Durn ing steady state conditions, existing while contacts 3'! and 4G are open or closed, a high degree of balance across terminals 2o and 2l may be readily attained. This is true even if part 28h-28o of the coil is not of lower resistance than the remainder of the coil. Thus any difference in resistance between tap 22h and terminal 2li on the one hand and tap 28h and. terminal 2l on the other hand, solely through the winding and suspension, can be compensated for by resistor 33. Hence, after steady state conditions have been reached with the galvanometer contacts closed, the potential difference across 2&3 and 2l may be made zero or small enough so that generator lll will not be damaged.

' It is clear that transients will be generated in the galvanometer system when the galvanometer contacts open or close. An important cause of the transients is believed to reside in the mutual inductance between the parts of the movable coil considered with reference to a tap whose contact is just closing or opening. Thus if contact 31 is just closing against fixed contact fill, the entire movable coil may be considered as consisting of one part from terminal 29 to tap 28h and the other part from tap 2th to terminal Sii. Obviously, the two parts of the entire coil are electromagnetically coupled and have mutual inductance. Upon initial closure of the contacts, the mutual inductance has some effect upon the coil tending to move the contacts together more closely. At the same time, the transient or transients generated in the coil result in a potential difference at the galvanometer terminals apart from the resistance drop in the coil. This transient may cause a surge in generator coil M. By choosing proper values for resistor 35, capacitor 35 as weil as inductor 36, a low pass lter may be provided to suppress transients. A condenser across the generator winding may also help.

When the reversing switch is opened, the mutual inductance effects have some tendency to generate forces to move the coil in a direction to open the contacts. While this may be relied upon in some instances, it is preferred to move the switch to the reversing position. The cornbination of transient effects and positive actua tion of the movable coil suffice to move the coil quickly into contact opening position.

The transient effects resulting from opening the reversing switch are generally more powerful than from contact closure. Thus after contact closure, opening of the reversing switch will cause the current to decay at a sharper rate than is true of the current increase at contact closure. However, the rates oi current rise and decay may be assumed to be of the same order so that a low pass lter will generally be effective against all transients.

It is clear from the foregoing, that the polarity of generator lo is important when connecting the generator to the galvanorneter system. For certain purposes, it may be desirable to have a system wherein the polarity or" the generator is not important. This is shown in Figure wherein similar' parts carry similar numbers. n this modification, movable contact 3l is connected as before to tap 2573 of coil 28. A low resistance conductor is between points 23o and 28e. In the system shown in Figure 2, the suspension system for movable coil 28 is adjusted so that the normal aero position of the winding is the same as in a aero center type of instrument. In such an instrument, current in one direction will cause the coil to move one way while current in reverse direction will cause the coil to move the other way.

Cooperating with movable contact Eil are fixed contacts fil! and ldd on opposite sides, these two contacts beingarranged so that the movable contact can close against one or the other, de* pending upon the direction of coil movement. Fixed Contact do is connected to contact i2 of double pole double throw reversing switch 39 exactly as in Figure l. Fixed contact Elia is connected by wire flic to the corresponding Contact c2c of double pole double throw reversing switch Bca. The two reversing switches are the same and are connected in the same manner, the corresponding part carrying the letter a with the exception that battery d8a has its positive terminal connected to contact 53o whereas in the lower reversing switch, Contact (i3 is connected to the negative terminal of this reversing switch.

8 As clearly indicated, the two reversing switches are tied to be simultaneously operated.

The operation of the modified system is as follows: If the polarity of terminal H of the generator is positive as in Figure l, then the action of the system is exactly the same as in Figure l, coil 28 moving clockwise as seen in Figure 2 to close a circuit with contact dii. lf, however, the polarity of generator it is reversed, coil 28 will move anti-clockwise and Contact 3'! will approach contact 5%. When contacts 3l and Mia touch, current from battery lac will act in the same manner as from battery iii in Figure 1, the polarity of 4ta being such that coil 23 is driven to close contact 3l against contact ma more tightly.

The operation oi the system as far as the load or secondary relay is concerned, is the same as in Figure 1 and needs no explanation. In order to unlock the contacts with the system shown in Figure 2, it will be necessary to throw the two reversing switches from the position during relay operation, such as shown in Figure 2, for example, to the reverse position.

Referring now to Figure 3, a further modification is shown. The galvanometer is generally the same as in Figure 2, having one movable contact operating between fixed contacts. However, terminals 2li and 2l of the galvanometer are connected directly to the coil suspension and do not have any resistors, capacitors or inductors as in lead 32 of Figure 1. instead, generator coil Ill has condenser ida across it and is in series with inductor lftb across galvanometer terminals 2c and 2 l Terminals 2o and 2l are connected to resistors 82 and 83 having common junction 8d. Wire 5o is connected through conventional relay 35 to junction M. The galvanonieter load here is still of the three wire type, consisting of resistors 32 and 33 and relay E5. Resistors 82 and 83 are not necessarily equal and may be proportioned relatively the saine as the resistances from tap 23h to terminals Zii and 2i.

This system may be considered as a bridge with galvanometer parts Zlib-dii as one arni, 23h-2 l as a second arm, resistor 32 as a third arrn and resistor 83 as a fourth arm. The diagonal across 2t and 2i is formed by the generator and inductor in series. A. difference in potential is impressed across the second diagonal from movable contact 3;? and junction il@ when the galvanometer contacts are closed. For perfect operation, the bridge arms should be so related that the bridge is substantially balanced across 2tand 2l'. As has been previously pointed out, this balance may be substantially achieved for steady state conditions by choosing proper values for resistors 82 and 33. By proper design of the rst diagonal containing the generator and inductor, a low pass filter effect is provided.

It will be noted that the system illustrated in Figure 3 resembles the system of Figure 2 in re-l duiring two sets of batteries in the galvanoineter relay circuit. This be undesirable. Reerring now to Figure fi, a modification is shown wherein only one battery necessary.

This system has generator Mc connected in series with resistor lea (which resistor may be the resistance of Hic) across galvanorneter ter minals 20a rand 2id. These terminals are connected directly to suspension elements 'dic and 32a going to terminals 29a and sila, of coil 2S. Coil 28 is the same as in Figure 1 except that it carries two movable contacts Elib and 31o at points 2817 and 28e respectively. The part of the coil between these contact taps preferably has a lower resistance than the remainder of the coil. Contacts 31h and S'lc cooperate with stationary contact 42a. Contact Mia is connectedto contact 42a of a double pole double throw switch having contacts 42a, c2c. 45a, 45a. movable contacts 43a and Mic. This switch is connected as in Figure l so that battery 53a may have its polarity reversed. From contact 55a, wire 50a goes to relay 85a, the other terminal of this relay going to junction 84a, or" resistors 32a and i3c. Resistors 82a and 83a are connected to galvanometer terminals 20a and 21a.

It is clear that the galvanometer is the center zero type. Coil 28 will move irrespective of the polarity of the generator. Normally the one ixed contact doc will be between the two movable contacts. Whichever way the coil will move, contact closure will result in locking action. By simply opening the double pole double throw switch, contact opening may result due to the action of the mutual inductance of the coil parts, as previously described. However, throwing the switch to the other position to reverse the battery will be found generally preferable and more reliable.

In general, where extreme sensitivity is desired, the galvanometer contacts must be ne and threaded to remain clean. A rugged but sensitive galvanom-eter construction is possible7 utilizing various constructional details well known in the Under certain conditions, the entire galvanometer be immersed in a liquid, as kerosene so that the contacts will remain clean for periods of time.

The load must be of the three wire type and the various circuit branches of the load preferably should have the same symmetry when coupled to the galvanometer system and generator would be required by an electrical bridge. Where a galvanom-eter coil carries two movable contacts, as in Figure 4, the two contacts may be considered as one for bridge analysis. Thus one arm of the bridge may consist of one coil suspension and the coil part going to the nearest movable contact Slo. The other arm may con- .sist of the other coil part going to the other movable contact, lill). The part of the coil between contacts 31h and 31o may be considered as a connection. having negligible effects on the bridge. actually, precise analysis would demand that whichever movable contact is closed against the iixed contact, is the bridge point, the temporarily idle movable contact being disregarded.

What is claimed is:

l.. A sensitive relay comprising a movable coil type of galvanometer, the movable coil consisting' of a `winding of wire having end terminals, a Contact movable with said coil and connected to a point on said coil electromagneticallv offcenter, a iiXed contact cooperating with said movable contact and touched by said movable contact when said coil has responded a predetermined amount to energization, a three wire load having an intermediate and end terminals, a connection from one terminal of the movable coil to one end terminal of the load, a connection from the other terminal or" the movable coil to the other end terminal of the load, a connection including at least one source of potential between the ixed contact and intermediate terminal of the load, a source of minute substantially direct current connected to said coil terminals, said sources of potential and current being properly poled whereby when said movable coil responds to current and closes the movable contact against the iixed contact, a relay circuit is closed With currents flowing between the movable contact and terminals of the movable coil in opposite directions generating a net electromagnetic coil effect due to the eccentricity of the connection of the movable contact to the movable coil tending to move the coil into tighter contact closing position.

2. The systeml according to claim l wherein only one source of potential is provided, this source being disposed in the circuit branch having the intermediate terminal of the load.

3. The system according to claim 1 wherein only one source of potential is provided in the circuit branch having the intermediate terminal of the load and wherein switching means are provided for reversing the polarity of this source of direct potential, said reversal serving to generate a force in the coil for opening the contacts.

4. The system according to claim l wherein one of said coil connections includes impedance balancing means whereby upon closure of the load circuit, no difference of potential due to the source of potential will be present at the terminals of the minute current source when connected thereto.

5. The system according to claim 1 in combination with a source of minute potential.

6. rlhe system according to claim 1 wherein said galvanometer coil has a section symmetrically disposed with respect to the electrical coil center and extending to the movable contact tap, said section having substantially less resistivity than the remainder of the coil.

7. A sensitive relay system.l comprising a galvanometer of the movable coil type, said galvanometer having a movable coil with terminals, a contact movable with said coil and connected to a point on said coil electromagnetically offcenter, said galvanometer being of the Zero center type, a pair of fixed contacts, said contacts being on opposite sides of the movable contact and spaced so that said movable contact must move from a zero position in either direction for a predetermined amount of travel to encounter one or the other of said Xed contacts, a three wire load having outer terminals and an intermediate terminal, a source of potential connected between the load intermediate terminal and one fixed contact, a second source of potential connected between the load intermediate terminal and the other fixed contact, the polarity of the fixed contacts being opposite to each other, and connections between the outer terminals of the load and the terminals oi the movable coil, said movable coil terminals functioning as terminals for connecting an energizing source of minute current to said system to move said coil from its center Zero position in one direction or the other depending upon the polarity of the minute current source, said movable contact when touching a Xed contact completing a load circuit which tends to lock the contacts, the polarity of the potential sources being such that when the movable contact touches a iixed contact, the load current through the movable coil generates a resultant force to lock the contacts together.

8. The system according to claim 7 wherein reversing means are provided for both sources of potential to reverse the polarities whereby unlocking of the contacts is provided.

9. The system according to claim 7 wherein impedance balancing means are connected in the movable coil circuit so that no relay potenc cil tial appears. across the minute current source when connected.

1 0. The system according to claim 'l wherein the branch of the system containing the minute current source includes low pass filter means for suppressing transient effects.

11. The system according to claim 'l wherein said galvanometer coil has a section symmetrically disposed with respect to the electrical coil center and extending to` the movable contact tap, said section having substantially less resistivity than the remainder of thev coil.

12. A sensitive relay system comprising a galvanometer of the movable coil type, said galvanometer having a movable coil with terminals, a pair of contacts` movable with said coil and connected to points on said coil electromagnetically oi center butY generally symmetrically with respect to the electrical center of the coil, said galvanometer being of the Zero center type, a xed contact, said movable contacts being on opposite sides of the fixed contact and spaced so that' said movable contacts. must move from a zero position in either direction for a predetermined amount of travel to encounter the xed contact, a three wire load having outer terminals and an intermediate terminal, a source of potential connected between the load intermediate terminal and said fixed contact, connections between the outer terminals of the load and the terminals of the movable coil, said movable coil terminals functioning as terminals ior connecting an energizing source of minute current to said system to. move said coil from its center zero position in one direction or the other depending upon the polarity of the minute current source, one of said movable contacts, when touching the Xed Contact, completing the load circuit, the polarity of the potential source being such that when the load circuit is completed, current through the movable coil generates a resultant force to lock the contacts together.

13. The system according to claim 12 wherein switch means are provided for reversing the polarity of the potential source so that after contact has been established, reversal of the potential source will result in a force being generated in the coil tending to unlock the contacts` and move the coil away from contact making position.

14. The system according to claim 12 wherein said load consists of a pair of resistors connected in series across the movable coil and wherein the junction of the resistors is connected to the fixed contact through a load relay.

15. A sensitive relay galvanometer for use with a three wire type of load, said galvanometer being of the movable coil type, said coil having end terminals, at least one contact carried by said coil and movable therewith, a connection between said contact and a point on said coil electromagnetically off-center from said coil, at least one fixed Contact cooperating with said movable contact, said galvanometer having its coil terminals for connection to a source of current for moving said coil toward a contactmaking position, said galvanometer also having its coil terminals and xed contact for connection to the end and intermediate terminals re: spectively of a three wire type load with a source of potential included in the intermediate branch of the load, with the potential source of polarity i2 being so` selected that upon contact closure due to: galvanometer coil movement, a force is generated in the galvanometer coil tending to increase the force closing the contacts.

16. The galvanometer according to claim 15 wherein the galvanometer coil has a section symmetrically disposed with respect to the electrical coil center and extending to the movable contact tap, said section having substantially less resistivity than the remainder of the coil.

17. The galvanometer according to claim 15 wherein said galvanometer coil carries two movable contacts, said two contacts being connected to two points on said coil symmetricallyv disposed with respect to the coil center, said galvanometer being of the zero center type with one xed contact.

18. The galvanometer according to claim 17, wherein said galvanometer coil has a section symmetrically disposed with respect to the electrical coil center and extending to the points of connection of the movable contacts, said section having substantially less resistivity thanV the remainder of the coil.

19. A galvanometer system comprising a movable coil type of galvanometer having a movable coil with end terminals, a pair of movable contacts carried by said coil, connections between saidcontacts and points. on said coil symmetrically disposed with respect to the electrical center of the coil, a fixed contact cooperating with said two movable contacts, said galvanometer being of the zero center type, said galvanometer including conducting suspensions for said movable coil, a pair of resistors connected in series with the outer terminals connected to the suspension means so that the two resistors are connected across the coil and suspension means, a branch circuit between the iixed contact and common resistor terminal, said branch circuit including a source of potential and a load relay, said resistors having substantially the same ratio of Vresistance as one suspension means and the corresponding part of the coil up to the electrical center has to the remaining suspension means and coil part, a source of current connected to the coil terminals through the suspension means for actuating the movable coil toward a contact closing position, the polarity of the potential source being such that when contact is established, current from said potential source passes through the galvanometer winding to generate additional contact closing force.

20. The galvanometer system according to claim 19, wherein said galvanometer coil has a section symmetrically disposed with respect to the electrical coil center and extending to the movable contact taps, said section having substantially less resistivity than the remainder of the coil'.

STANLEY BREEN. MAXWELL A. KERR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,346,589 Lamb Apr. 11, 1944 2,357,524 Klepp Sept. 5, 1944 2,376,808 Ratz May 22, 1945-

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