Sept 24, 1946. . ' B E_ LENEHAN AL >- 2',4@8,H3 ' ELECTRICAL MEASURING INSTRUMENT Filed Nov. 10, 1945 _' - INVENTORS ?ernardELene/zan andé‘aaryaJM/ey. ATTORNEY Patented Sept. 24, 1946 2,408,218 . UNITED» srATEs PATENT OFFICE Bernard Lenehan, Bloom?eld, and George .1. . ~ Wey, East- Orange; N. .L, assignorsto West.-.. ' . inghouse Electric Corporation, East Pittsburgh, 3a., a corporationof Pennsylvania . ' . Application November 10, 1943, Serial No. ‘509,728 1. coil has aside» positioned for rotation through this air gapand is so proportioned that itmay be insertedand removedvwith respect to rthegmag netic~ structure through thechannel de?ned by Thisv invention relates to devices: responsive to the frequency of an alternating electricalquan tity and ithas particular‘ relation to frequency measuring instruments having long. -. circular ‘It has been-proposed 111413116‘. prior art'that- de the tip of thehook-shaped portion. v 7 It- is, therefore, an; object of the}v invention» to scales. ' _ , vices responsive to the. frequency ofxjan alternat provide an improved device responsiverto-the-free ing-current quantity bar-provided‘ with magnetic quency ' of an; alternating electrical quantity.» cores. For example, reference may be made to ' ‘It is a-further object of the'invention to pro the MacGahan Patent 1,549,664 andito the-book .10 vide a frequency measuring instrument compris ing means responsive to an alternating electrical entitled “ElectricalInstruments?’ ‘by H. C. Turner, quantity to be measured for establishing aimag published~ by Instrumentsv Publishing Company. netic ?eld wherein ;a;coil. is disposed} for move However,v in designing a v‘frequency responsive ‘de ment and; for ‘inducing-‘a- voltage in-ithe'coil; an ‘vice having a long circular scale,'such as one hav electrical - circuit : having’ inductance‘ and»; capaci ing anwangular length rinvlexces‘s of 200°,‘a numf- '7 tance'and- including the‘electrical coil; andimeans ber of‘ problems are presented. ' Among these 7 for introducingan‘ auxiliary; voltage in the-elec problems are the provision of adequateésensitivity and torque throughout the'range'of' indication'of trical circuit. ' . It is alstill“ further object: of‘ the invention to the device While avoidingtoogreat a spre'ad‘be tween maximum torque and~minimum ‘torque. 20 provide an improved magnetic core for: a; fre In addition,» it is desirable thatthe device. be-of quency:measuring'instrument; ily serviced. , _ - V .1 A ‘ ‘ - ' ' ' ' Other objectsof the invention-will be apparent simple construction, readily assembled and read- from the following descriptiontaken in-conjunc- » ,tion with the accompanying drawing, in which: ~‘ . In accordance with‘ the, invention, a frequency Figure 1 is'a view'in'perspective"with p'ar-ts bro responsive device, such as an indicating'freduency meter, is provided with ‘means-responsive to-_-an alternating current electrical‘ quantity to be meals; ,ured-for establishing a 'magnetic ?eld and‘for ~ ken away ‘ of‘ a frequency measuring" instrument embodying'tl'ie invention. ' Fig. '21s a view in top'plan of a magneticstruc inducing a volt'agein a coil having a'side dis ture suitable‘for the'instrument of: Fig. 1. ' ' posed’ for‘ movement through’ the ' magnetic ?eld. 30 r "Fig; 3 is‘ a schematiciviewlshowing circuit con nections suitable for the instrument of '_Fig. l; and _, The coil is included in an electrical circuit, having Figs." 4; 5- and 6' are simpli?ed vector repre inductance and capacitance, thevalue‘of'the-in'g ductance being dependent on thelposition- of the ‘ sentations of'electrical conditionsv'which' may oc coil with respect to the, magnetic ?eld‘; The-elec- .. ' cur-in the instrument oflFig. '1.‘ ’ trical circuit is so proportioned "that by proper 35 ' Referring 'to‘the drawing, Fig. 1 shows a fre quencymeasuring instrument comprising a stator positioning of the coil the electrical circuit- may assembly I and. a rotor assembly 3. The rotor be brought into resonance foreach'freque'ncyto which the instrument is'designed to‘ respond. ‘ assembly includes a shaft 5 having pivots at its endswhich are rotatably received insuitable, bear¢ With such a construction, the coil seeks a posi ing'screws ‘l and 9. A coil II is suitably secured ' tion such, that the electrical circuit is. in'ires to the’ shaft 5. Inv the'speci?c embodiment of Fig. l, the coil ll vis cemented to brackets 13 and ‘onance for the’speciflc frequency of the afo'rer said alternating electrical‘ quantity; In order to 15 which are securedto the shaft 5. decrease the ratio. of the maximum ' mum torque exerted‘ on the coil, provisr The shaft 5v also supports a; pointerrl'l which _ has an end positioned for movement adjacent a circular'scale l9. It will be noted that the point er l1 and‘ the coil II are on opposite sides of the for ‘introducinginto ther'eSonant electricalv cirl.-' ' cult ‘an auxiliary voltage having the-53mg he, Quenc'yas that of ‘ the alternating fe1gct17i¢a'_1' (mane shaft Consequently, the pointer I1 balances, , For establishing, the required‘ magnetic" paths, 2,50 at leastin part, the coil l I.‘ If the pointer slight titytol be measured; ' ' ' > - - '- . ly overbalances ' the coil, adjustable balance the ' instrument includes a‘rmagneticf' structure weights 2| may; be secured to the shaft 5',‘ as 7 formed of laminations. 7' Each-‘ofxthe'vlaminations ' includes a hook-shaped portion’ providing ' a sub stantially‘ annular-bore and“ an outer ‘magnetic V Rotation of the rotor 3 is dampedby means of 7 an ‘electroconductive damping disk ,‘23' secured‘ , to therewith element‘spaced‘ a substantially'annularfrom the annular core air gap. to: define The 55 the shaft E'and' formed of a material such as 2,408,218 3 4 aluminum. This disk 23 rotates between the poles of a permanent magnet 25 to damp rota in the art. It is desirable at this point to note that the self-inductance of the coil || depends on its po sition with reference to the magnetic structure 41. This may be explained with referenceto Connections to the coil H are made through ?exible conductor strips 21 and 29 which have tioned adjacent the end 59A of the annular air torque in response to a deviation of the frequency being measured from the indicated value. In a speci?c instrument designed in-accordance with trolled by the shape of theair gap. For example, if it is desired that the self-inductance of the tion of the shaft 5 in a manner well understood Fig. 2, if it is assumed ?rst that the coil is posi gap 59. When the coil II is so positioned, sub their inner ends secured respectively to insulat stantially all ?ux passing there through is dis ing collars 3| and 33 secured to the shaft 5. The outer ends of the spiral conductor strips 2.1.’. 10 tributed over substantially the entire air gap 59. Since the air gap has a large cross section the and 29 are soldered or otherwise secured to lugs reluctance offered to the magnetic ?ux is com— 35 and 31_which are ?xed with respect to the paratively small and the self-inductance of the stator I. Connections from external electrical coil II is large. On' the other hand, if the coil elements to the lugs may be effected through is positioned adjacent the'opposite end 59B of suitable conductors 39 and 4|. The coil H is connected through conductors 43 and 45. respec 15 the air gap,.magnetic ?ux passing through the coil passes through an extremely small portion tively. to the inner ends of the conductor strips 21 and 29. As shown in Fig. l, the spiral con- , of the annular air gap. Consequently, the re luctanceoffered to such magnetic flux is com ductor strips 21 and 29 are wound about the shaft paratively high and the self-inductance of the 5. Although the conductor strips are extremely ?exible, thev have a little resilience. Since the 20 coil is substantially smaller than that of the coil when'the coil is ‘positioned adjacent the end 59A shaft 5 is designed for a large angular movement, of the "air gap‘. ‘At intermediate positions of the such as one in eXcess of 200°, it is convenient to coil, the self-inductance of'the ‘coil has interme adjust the conductor strips so that they are un diate values. ' ‘ ‘ stressed when the coil H is adjacent a position The variation in self-inductance of the coil H wherein the instrument develops a minimum as it moves through the air gap 59 may be con coil decrease at a more rapid rate as the coil the invention, the conductor strips were posi tioned to be unstressed when the pointer indicat» .30 moves from the ‘end 59A to the end 59B of the air gap, the magnetic structure 41 may be con ed approximately two-thirds of the full scale structed to provide an air gap 59 having a length value. L which increases from-a predetermined value The stator includes a magnetic structure 41 adjacent the end 59A to a larger value adjacent for‘ establishing a magneticv ?eld within which the end 59B. Alternatively, the magnetic struc a portion of the coil H is disposed for rotation ture may be constructed to provide‘ an annular and for directing magnetic flux through the coil air gap having a- uniform length L but having a ||. The magnetic structure 41 comprises a hook width W which decreases from a predetermined shaped magnetic inner part 49 having a hook value adjacent the end'59A to a smaller value ad portion 5| and a shank portion 53. It will be observed that the hook portion 5| provides a 40 jacent the end 593 of the air gap. The desir ability of such variations in the air gap will be substantially annular magnetic core having a discussed below. 9 channel 55 therein (see Fig. 2). The magnetic Suitable electrical connections for the instru structure 41 includes further ‘an outer magnetic ment of Fig. 1 are-illustrated in Fig. 3. In Fig. 3, element or portion 51 which surrounds and is spaced from the'hook portion or annular mag 45a pair of-conductors 1| and 13 represent an elec trical circuithaving an alternating voltage there netic core 5| to de?ne'therewith an annular air across. The-winding BI is connected across the gap 59. This air gap has a width W (Fig. 1) and circuit for energization in‘ accordance with the a length L (Fig. 2). By inspection of Fig. 1, it aforesaid alternating voltage. .Energization of will be observed that the coil “links the an nular magnetic’ core-5| and has a side disposed 50 the Winding 6| establishes a magnetic ?eld for the coil | | and also directs magnetic flux through for movement‘ through the annular air gap 59. the coil | | to induce any alternating voltage there It should be observed further that the coil II is in. ' The coil II is included in an electrical circuit proportioned to pass through the‘ channel 55 14 vhaving inductance represented by the coil H (Fig. 2) in response‘ to movement of the rotor in an axial direction. This permits the coil to be. 55 and an inductance coil 15 and having a capaci tance ‘represented by a capacitor 11. To decrease inserted in operative position or to be removed the size of the capacitor 11 required, the capacitor therefrom without disturbing the magnetic is connected to the electrical circuit through an structure 41. I auto-transformer 19, the high voltage side of the For energizing the instrument illustrated in Fig. 1, a winding 6| surrounds the shank por 60 auto-transformer being across thecapacitor 11. To facilitate calibration, one or more of the cone tion 53 of the magnetic structure 41. When an nections to the auto-transformer 19 are through electrical current is passed through the winding adjustable taps. 6|, magnetic ?ux is directed in series through In response to the voltage induced in the coil the shank portion 53, the annular magnetic core II, a current flows through the electrical circuit 5| and the air gap 59, as shown by dotted lines . 14. ’However, the voltage induced in the coil II in Fig. 2. Since a portion of this magnetic flux is dependent on the position of the coil with re passes through the coil II, a voltage is induced spect to the magnetic structure 41. Referring in the coil. Furthermore, a side of the coil H again to Fig. 2, when the coil is adjacent the end is disposed‘for movement through the magnetic ?eld produced by the magnetic flux crossing the 70 v59A of the air gap substantially all the magnetic ?ux passing. through -.the air gap also passes annular air gap 59. For a purpose hereinafter through the coil. . Consequently, a substantial pointed out, an auxiliary winding or coil 63 is positioned on the shank portion 53. If desired, voltage is induced in the coil | | . However, when the coil ,| | is adjacent. the end 59B of the air gap, the magnetic structure 41 may include a notch 65 for receiving the auxiliary winding 63. v75 very little of the magnetic ?ux passing through 9,408,218 5 6 the air gap. passes through; they coil. Therefore, the’ voltagainduced therein. is. comparatively small. This difference in voltage, if large, isv ob jectionable for the reason that to provide ade quate torque when the coil is adjacent the end 5930f the annular airgap, an extremely large torque is applied to the coil when. the coil is: ad' jacent the end 59A of the air gap. A. large torque‘ current 12- (Fig. 5) ?ows through the circuit 714. vIn ?owingrthrough‘the. coil II. the Current 12 produces a magnetic ?ux ¢2. The component of-Ithe magnetic ?ux ¢2, which is in phase with the. magnetic ?ux ¢, represents a torque T2 ‘which is. applied to the coil II. 'In' response to this torque the coil ll moves until the pointer l1 indicates a frequency of 50 cycles per second on the scale l9. Atthis point, the inductance 10 of the coil_ I l’has changed to a value which makes coil and associated portions» of the instrument. the electrical circuit" resonant at a frequency To decrease‘ the variation ofi'curren't'in the of~50 cycles per second and the vector relation electrical circuit 14 resulting from. .a: variation ships again are similar to those illustrated in in» the positionyof the-coil? Hi withrespect to the may- resultin excessive stress and vibration'iin the magnetic structure 41., an auxiliary voltage is Fig. 4". > - Let it be assumed‘ that the instrument of Fig. introduced into the electrical circuit. a This volt agemay be obtained directlyrfrom the conductors "H and 13 through a suitable phase control device 1 is indicating a. frequency of 60 cycles per sec-' ' 0nd and that the frequency applied to the wind ing 6| increases suddenly to a value" of 70 cycles per second. Since the electricalicircuit 74 initi for’ maintaining the auxiliary voltage substan tially in phase with th'e'voltag'e’ induced in the ' coil ll. Preferably, the auxiliary voltagehisi in 20 ally is resonant for a frequency of 60 cycles per second, the circuit for a frequency of 70 cycles troduced by means of the auxiliary‘ winding 63 per second offers a substantial inductive imped which is inductively coupled to the winding 6|. Since the same magnetic flux-passeslthrough the windings 63 and H, the voltages induced therein 1 are in phase. The auxiliary voltage induced in the winding» 63 is substantially independent of ance to the flow of current. Consequently, in response to the voltage V a current I3 ?ows 25 through the circuit. The current as shown in Fig. 6 lags the voltage V, and produces a mag netic ?ux ¢3 when ?owing through the coil H. thoposition of the coil H with respect to the magnetic structure 41. For this reason, theratio The component of the magnetic flux¢3 which is projected on the line of the magnetic ?ux c rep of-the- maximum to the minimum torque applied to the coil Il may beheld ‘to/Ia reasonably small value, v ~ ' resents a torque T3 acting on the coil H. This torque is directed oppositely to the torque T2 of. Fig.‘ 5 and urges the pointer I ‘I up-scale to a posi tion indicating a frequency of '70 cycles per sec I The operation of the measuring instrument il lustrated in Fig. 1 and connected as shown in Fig. 3, may be considered withreference to the par‘ ond. The resulting movement of the coil ll re duces the inductance of the coil H to a value ' tial vectorrepresentationsof Figs. 4', 5» andr6; When the winding 61 is ‘connected to the circuit whichmakes'the electrica1 circuit 14 resonant represented by the conductors 'H- and 13',‘ m'ag- ‘ at a» frequency of 70 cycles per second. netic ?ux ?ows in the magnetic structure 41. This‘magneticflux isrepresentedv in Figs. 4,5 and voltage across the winding 6| is an alternating voltage having a frequency of;6.0 cycles per-secs fore, with the pointer I1 indicating a frequency of 70 cycles per second, the vector relations in the circuit‘!!! are similar to those represented in Fig. 4 and the pointer ‘ll remains stationary. From this brief review of the operation of the 0nd; instrument, it will be appreciated’ that the coil 6 ‘by the vector ¢. Let‘ it. be assumed that the; .. Since the coils’ Hand 6.3 are-inductively . There coupled to the winding 5!, the alternating ?ux ¢ l_l always seeks a position such that the electrical in the magnetic structure 4-‘!v induces ‘voltages in v i circuit 14- is brought into resonance for the fre- ' quency of the‘alternating voltage applied to the the coils II and 63“Whi0h‘ are in'ph'asewith each other. The sum of these two voltages is. repre-. sented/ in Figs. ‘1,5 and 6 by the vector V which lags the flux ¢ by an angle of 90°. The voltage winding 61. ’ ' . , " The» scale distribution of the instrument is de termined by the rate at which the self-inductance V produces a flow of current in the electrical cir- ; i of the coil H changes in response to movement cuit 14. The parts are so proportioned that if of the coil with ‘respect to the magnetic struc- ‘ ture 41. 7 To provide a linearscale, the annular ‘magnetic core '5! is tapered‘as shown in Fig. 1 1This tapering of the annular magnetic core is the pointer’ H indicates a .frequency'of.6‘0'cycles per second on the scale-19,.as shown» ‘in? Fig. 1,' the electrical circuit 14 is- resonant-at a. fre-, quency of 60cycles .per second. Under these con such that the cross section of the annular-‘mag ditions, the electrical circuit. 14', offers an .i'm-. pedance to the ?ow of current whichis repre sented only by the resistance of the circuit and the current which ?ows in the circuit is in phase with the voltage. V. This current is represented an in Fig. .4 by the vector-I. In ?owing ‘through the netlc core and the width of the air gap de crease from predetermined values adjacent the vend 59A of the air gap to smaller values ad ' jacent the end 59B of‘ the air gap.’ The magnetic structure 41 may be constructed in any suitable manner from a soft magnetic coil H, the current I produces a magnetic flux material, such as a-good grade of soft iron. which is represented in Fig. ,4‘ by the vector ¢'|. desired, the material employed for the magnetic If It will'be noted that the vectors it and‘ ¢l are in ‘ . structure may be similar to that shown in Patent quadrature. Consequently no torque is applied to the coil lhl and'the pointer I'ljcont'inuously in dicates a frequency of 60 cycles per second.’ Let it be assumed next ,that [the frequency drops suddenly froma value of 60 cycles-persec ond to a value of 50 cycles perisecond. Since the electrical circuit >14 initially‘ is resonant for a frequency of 60 cycles per ‘second, thecircuit offers at a frequency’ of‘ 50 cycles per second a "substantial capacitive impedance‘ to the ?ow ‘of (15 7N0. 1.807.021, of T. D.‘ Yensen, issued May 26, 1931, which is available on the market under the trade name “l-Iipernik.” The material‘dis~ closed in this patent is a low hysteresis loss heat treated magnetic alloy comprising iron and nickel in approximately equal proportions. Conven iently, the magnetic structure 41 may be formed from a. plurality of magnetic laminations M. Ml, M2, M3, M4, M5 which are attached'to each other in any suitable ‘manner. as. by means'of rivets current. , In response-to the voltage 7V, a leading -75 8|. The laminations all may be of. substantially 2,408,218 7 the same con?guration but certain of the lamina tions Ml have their tips cut oii along“ the line '83 (Fig. 2). Other laminations M2, M3, M4, M5 have their tips cut off along the lines, respec tively, 85, 81, 89 and M. This provides an an nular magnetic core 5i which tapers to provide the desired rate of change of the inductance of said coil for movement relative to said magnetic structure to direct said portion through'said air gap, means associated with said magnetic struc ture and responsive to, the alternating quantity tobe measured for directing a magnetic flux through said'air gap and through said coil, said magnetic structure being con?gured to effect a the coil H in its rotation through the air gap. change in inductance of said coil as said; coil It will be observed that the lines 83; 85,81, 89 moves through said air gap, a circuit containing and 9| are not radial with‘ respect to‘ the shaft 10 inductance and capacitance, said circuit includ ing said coil and being proportioned to be res 5. As clearly shown in Fig. 2, these lines-are onant for the range of frequencies to which said inclined to provide a gradual change in'cross sectionof the core as the coil ii moves there across. The laminations may be assembled in any desired sequence. Preferably, one or more of the laminations Ml are applied to each face of a stack of the laminations M. The laminations M2, M3, Mil and M5 are then applied succes sively to each face of the resulting stack to pro vide the stepped formations illustrated in Fig. 1 on each side of the laminations M.v This de» sign assures a magnetic structure or substantial rigidity. ' device‘is designed to respond, whereby magnetic ?uxpassing through said coil causes current to flow in saidv circuit having a phase relationship relative to the magnetic ?ux in said air gap which is dependent on the deviation of the frequency or the alternating quantity to be measured from the value corresponding to the position of ‘said coil relative to said magnetic structure, a sub stantial portion of the inductance in said circuit having a non-magnetic core, and auxiliary means for energizing said‘circuit in accordance with the alternating quantity to be measured. The current ?owing through the electrical cir 2. Ina device responsive to thefrequency c cuit ‘l4 depends on the voltage applied-to the ‘an alternating electrical quantity, a magnetic winding 6i and on the characteristics of- the structure comprising a substantially annular electrical circuit. It is desirable that a substan magnetic core and a magnetic element spaced tial portion of the inductance in the electrical from said annular magnetic core to de?ne there circuit ‘id be independent of the current flowing in the circuit. To this end‘, the inductance 75 30 with a substantially annular air gap, a coil linked preferably is substantially an air core inductance. with said annular magnetic core, said coil having Such an inductance substantially eliminates a‘ portion positioned in. said air gap, >means errors resulting from changes in the voltage ap mounting said coil for rotation relative to ‘said magnetic structure to carry said portion through plied to the winding 5 I. vAs previously explained, the calibration of the the air gap, said magnetic structure being con instrument may be controlled by adjusting the ratio of the primary turns of the autoetrans ?gured to change the-inductance of said coil as the coil rotates with respect to said magnetic structure, means associated with said magnetic dition, a ?ne adjustment may be provided‘by structure for directing through said air gap and positioning a magnetic screw 93 for movement 40 through said coil alternating magnetic ?ux de into and out of the magnetic ?eld of theinduc pendent'on‘an alternating quantity to be meas tance coil 15.. By an adjustment of the screw ured, whereby an alternating voltage is induced 93, the inductance of the coil ‘I5 may be varied in said coil, a circuit containing capacitance and former T9 to the second turns thereof. ~ In ad 11 which has no temperature error; As a- gen inductance, said circuit including said coil, whereby said voltage produces a flow of current through said circuit having‘ a phase relationship eral rule, a commercial capacitor has avalue of relative to said magnetic flux whichis dependent to calibrate the electrical circuit. - - In practice, it is difficult to provide a capacitor capacitance which increases with temperature. on the deviation of said circuit from resonance, This increase may be'compensated' by a position and'a'uxiliary means for introducing a voltage 50 ing in the magnetic ?eld of the inductance coil in said circuit dependent on the alternating quan ‘l5, a magnetic element 95 having a substantial negative temperature coe?icient of permeability. tity to, be'imeasured, _ , _ 3. 'In a device responsive to the frequency of an alternating electrical quantity, a magnetic As well understood in the art, a magnetic ele ment having such a coefficient may be formed of an iron alloy containing 30% nickel; Such 55 structure ‘comprising a substantially annular magnetic core, a magnetic element spaced from alloys are known as temperature compensator alloys. ' - As previously explained, the inductance coil 15 is essentially an air core inductance. The ' ‘said annularmagnetic core to’de?ne therewith a substantially .annularair gap, and a_ magnetic member connecting saidnmagnetic core to said amount of iron added by the screw 93 and the 60 magnetic element, said magnetic core having a channel extending from the interior to the ex element 95 is small and the inductance coil 15 .terior thereof adjacent said magnetic member, a has, to a substantial extent, an inductive re coil linked with ‘said annular magnetic core, said actance which is independent of the value of coil having a portion positionedin said air gap, the current ?owing therethrough. Although the invention has been described 65 means mounting said coil for rotation relative to said magnetic. structure to carry said portion .through .the air gap, said magnetic structure being configured to change the inductance of said Therefore, the invention is to be restricted only coil as the coil rotates with respect to said mag by the appended claims as interpreted in view 70 netic structure, and a winding associated with of the prior art. with reference to certain speci?c embodiments thereof, numerous modi?cations are possible. We claim as our invention: 1. In a device responsive to the frequency of an alternating electrical quantity, a magnetic structure having an air gap, a coil having a por ‘said magnetic structure and effective'when suit ably energized for directing through said air gap and through said coil alternating magnetic ?ux dependent on an alternating [quantity to be meas tion positioned in said air gap, means mounting 75 ured,‘ whereby an alternating voltage is, induced 2,408,218 in said coil, a circuit containing capacitance and inductance, said circuit including, said coil; whereby said voltage produces a ?ow of current through said circuit having a phase relationship relative to said magnetic flux which is dependent in the deviation of said circuit from resonance, said coil being proportioned for removal from said magnetic structure through said channel. 4. In a device responsive to the frequency of an 10 through said channel, means mounting said coil for rotation substantially about the axis of said annular magnetic core, and a circuit having ca pacitance and inductance, said circuit including said coil and being proportioned to be resonant for the range of frequencies to which said device is designed to respond, and an auxiliary wind ing linked with magnetic ?ux produced by said ?rst-named winding, said auxiliary winding being included in said circuit. electrical quantity, a magnetic 6. In a device responsive to the frequency of structure comprising a substantially annular an alternating electrical quantity, a magnetic magnetic core having a channel extending from structure comprising a plurality of aligned mag the interior to the exterior thereof, a magnetic netic laminations, each of said laminations com element substantially surrounding said annular magnetic core but spaced therefrom to de?ne an 15 prising a hook-shaped magnetic inner part hav ing ashank portion and having a, hook portion annular‘ air gap therebetween, and a magnetic terminating in a tip, a magnetic outer part sur > alternating member adjacent said» channel for connecting _ said annular magnetic core to said magnetic ele ment, said magnetic structure being con?gured to provide the annular air gap with different re luctances at various angular positions around the axis of the air gap, a winding associated with said magnetic structure, said winding when en-> ergized in accordance with said electrical quan tity being effective for directing alternating mag netic ?ux through said annular magnetic core and said air gap, a coil linked with said annular magnetic core, means mounting said coil for ro tation substantially about ,the axis of said an nular magnetic core, and a circuit having ca pacitance and inductance,'said circuit including said coil and being proportioned to be resonant for the range of frequencies ,to.which said device is designed to respond.’ , V 5. In a device responsive to the frequency of an alternating electrical quantity, a magnetic struc-, ture comprising a substantially annular magnetic core'having a channel extending from the in terior to the ‘exterior thereof, a magnetic ele'_ ment substantially surrounding said annular magnetic core but spaced therefrom‘ to de?ne an annular air gap therebetween, and a magnetic member adjacent said channel for connecting said annular magnetic core to said magnetic ele',-, ment, said annularmagnetic core having a radial cross-section which diifers at successive points ‘ around said core, a winding associated with said magnetic structure fordirecting ‘magnetic’ flux through said annular magnetic core and said air gap, a coil linked with said annular magnetic core, said coil being proportioned to be inserted in'and removed from said magnetic structure rounding, and spaced from a substantial portion of the hook portion to de?ne therewith an arcuate air gap, each lamination forming a path for mag netic ?ux wherein said inner part, said air gap and said outer part are in series, certain of said laminations having tips terminating short of the tips of other of said laminations to provide a re sultant hook portion varying in cross-section, a coil linked with the resultant hook portion, and , means mounting said coil for rotation relative to said magnetic structure. 7. In a device responsive to the frequency of an alternating electrical quantity, a magnetic struc ture comprising a plurality of aligned magnetic laminations, each of said laminations comprising a hook-shaped magnetic inner part having a shank portion and having a hook portion termi nating in a tip, a magnetic outer part surround ing and spaced from a substantial portion of the hook portion to de?ne therewith an arcuate air gap, each limination forming a path for magnetic ?ux wherein said inner part, said air gap and said outer part are in series, certain of said one of said laminations having tips terminating short of the tip of said last-named lamination to provide a resultant hook portion varying in cross-section, a .coil linked with the resultant hook portion, means mounting said coil for rotation relative to said magnetic structure, and means positioned adja cent the shank portions of said laminations for directing magnetic ?ux through said path formed by each of said laminations. BERNARD E. LENEHAN. GEORGE J. WEY.