US2421771A - Approach protective system - Google Patents

Description

June 10, 1947. G. H. BRowN|NG APPROACH PROTECTIVE SYSTEM Filed July 17, 1942 3 sheets-sheet 2 INVENTOR @n0 /Bram' BY g 9 Y my ATTORNEYWP? y June 10, 1947.

G. H.` BROWNING APPROACH PROTECTIVE SYSTEM Filed July 17, 1942 `heets-Sheet 5 9495 so 93 y *IML-#big BY M I ATTORNEY Patented June l0, 1947 Glenn H. -Browning, Winchester, Mass. n i u Application `July 17, 1942,`Serial No. `151,279

The present invention relates toa protective given `area and in which capacitance changes or other electrical changes may indicate andzdetect 1o claims. (ci. 177-352) v system to guard against; approach of persons or objects to a region or area to be protected, and, ,more particularly toa system in which signal `wirescr signal networksmay be strung around a the approach of persons or objects tothesignal .A

* wires or network.

The present Ainventionhas Ynumerous advantages overother systems ofthe prior art particularly in providing a system which isV little or not .t at all affected by climatic changes such as may occur during `dilferent times ofthe year or by accumulation of snow and iceorchange of ground` conditions oratmos'pheric moisture orother variable conditions bound to!l exist inthe use of the,

inventionY out of doors. These results are accomplished by the apparatus` itself, by reason ofathe design and layout of the signaling system or by thes'ffeatursfin combination with compensating means and methods which maintain a constant differential the system itself.

AK fundamental advantage of the presentV invention is a self-balancing feature which is relation between dual parts in brought about by employing two signal wire netl works connected to the end apparatus. yIf the `twosignal wire networks are substantially equal,

the end apparatus is preferably adjusted inthe initial installation so that equal capacitance changes on both signal Wire network allows the system to remain in balance.l If itis not feasible 'to havethe two signal wire networks of 'similar use of harmonics length, the end apparatus is prefenaibly adjusted on-'the initial installation, so that `if equal capacitance changes per unit length occur on bothsignal wire networks the system will `remain in balance. n

A further advantage is obtained in the present invention by the use oftwosmilarjsystems oscillating at frequencies whichvary but which under normal operating conditions when the alarm does cause a vsmallchange iny the fundamental frequency may be' made t'oproduce aconsiderably greater change inthe harmonics which areu'sed. j A further-*featurev of ferent `harmonics'of the fundamentals `preferably harmonics whiclivare sufilciently closeto'gether as to produce an audio frequency'note approximatelyv withinthe rangellof 1 000 cyc`1es."--'" The signal wire system or -signa1 :network A'system maybe used withotherloperati'ng `apparatus asffor instance a balancedV bridge c-ircuitin which the 'variation of .thecapacitance` in the signal f Wire system or network upsets the balance ofthe bridge but asa whole there/is amarkedrad-vantage in the use` of two' separate oscillators est- -pecially where conditionsvaryv from time totime. Further advantages will'be readily understood '1 from a-consideration ofthe specicationlset forth `below when taken -in connection with the drawings which are Vbriefly-described asfollowsrl t 'gure `1` is a blockdiagram of a simple form ofA electrical apparatus? embodying the present,` invention. :Figure 2` isablock diagram of adlfferent form `of electrical apparatus embodying thepresent invention.. Figure 3is al view illustrating one arrangement for protectingaregion t with the `signal` wiresaccording to the` practices Vof theipresent invention." Figure 4 4is 1a `circuit `diagram corresponding to the block diagram of Figure 1, Figure 5- istheclrcuit diagram of the arrangement corresponding to Vtheblock diagram of Figure 2. Figure 6 `shows the resonance char-` Vjacteristic' curve of the selectiveaudioamplier Awhich is used in the circuit diagram of-Figure 5.

Figures-f7` and @illustrate signal Wire installation outside-.anduinslde barbed wirefenceswrespectively, 'and FigureV 9-.illustrates signal wire innot sound have a constant differential frequency comprising a'beat note Vproduced by the beating C-f different harmonics of the oscillators.

lorthe production of the beat nete in-the present system has a furtherladvantage of permitting the Voscillators to operate .at considerably different frequencies and thereby preventing intercoupling or interconnection betwo'cscillators. n A still iurtheradvantage inthe use of the lharmonics of the oscillator'is tliattlie sensi- The .stallation inside. achain link fence. l f

an arrangement ofthe conductingwires guarding against the approachof anyone vto it; Itwill` be noted that these conducting wiresformtwo separate Wire networks lvlfand 2, which are preferably` counterparts of1 each` other having, as'

closely a'spracticaL the same lengths,` geometrical t arrangements and position with respectto the fil 4 tivity of vthe system vmay bergreatly increased be- 57 ground and other objects in theirvicinity. If `it is not possibleto make the two signal `,wire `networksor systems nearly the same `because ofv penmanent objects in the vicinitylor if it is inconvenient to do so, the apparatus canl beA so ad- -lus-ted that it maylopera'te 'in accordance with the present .invention either by` makingV they oscillators proportionally adjustable' inaccordancc n the present-,system isthat the beat notefused islthat -obtained from dif--` .In Figure 3- of the presentinvention isshown f end wires I5 and' I6 supported with the signal wire climatic changes or by use of a compensating system hereinafter described to compensate for unequal climatic and other slow changing conditions not brought about by the approach of foreign objects to within detection range of the signal wire networks.

As indicated in Figure the signal wires are .strung on insulator supports 3, 3, 3, ctc., which are mounted on supporting beams 4, 4, etc., and on supporting poles 5, 5, `5,`etc.,either on the inside or on the outside of the'barbed wire fence 6. The details of the construction and arrangement of the signal-wires with respect to the fence 4 2 I. This beat note may be chosen for instance as the beat note existing between the third har- -monic of the oscillator and the fourth harmonic oi the oscillator 2l, or other combinations of harmonics may be used as for instance the' stance be approximately 'l5 and 100 k; c. respecare indicated more in detail inFigures 7, 8, and9.`

It will of course be understood thatthe signal wires may be used alone without a fence, but in event that a metallic fence is used, the signal wire should be positioned atr a certain distance awayv from the wirefence for best results in the manner described below. Y

Figure. 3-also indicates the signal wire lead-in construction. ,-Each of thesignal wire networks I and' 2l have independent lead-ins. The lead-in wire 'I for thenetwork .I isstrung along` a series ff of `posts from one of the posts 5 to the housing 8 containihgftheapparatus'. `Frromthe post 5 the lead-in wire-:is extended downward to join the signalwirefnetwork at the junctionS.` Similarly the lead-in w'ire I0 extends from-the junction II ofthe network 2 to a post-5 and then'to the housing for the apparatus 8.

#Figure `3 also illustrates the dead end construction ofthe lsignalwire network. The network I, 1- for instance may be terminated at a certain cen- Vtral supporting section Ajat -its insulators 3, 3,

respectively on the outer side ofthe supports 4 vand `5 from which the wires extend to la support I2- located somewhere between the top signal wire I3 and "t -ie'l-ottom signal wire fI4 forming a part arnetworl: "l, These end wires I5 and joined 'together "or maybe strung as ls may thertopwire Il of the network! supported by an insulator I'on-V the inner side of the beam 4 an'd the signal wire I8'of the networkZ supported'on the 'inner side of the post 5 by the insulator 3".

These lwires are similarly joined together by the to the beam I2 by suitable insulators. v j

The simplified block diagram of Figure l shows at the left the terminals of the'lead-in wires 'I and I0i of the signal wire networks I and 2. vThese Aare connected to the inputs of their respective oscillators 20 and 2 I', andthen to ground as shown I in Figures land 2. The oscillators will be more fully described' later,y it being suiiicient to note for vthe present that'each of theseoscillators is preferably tuned to considerably different frequencies differing for instance by 'approximately 25,000 cycles when each is tuned in 4theneighbor- .hood of`100 k. c. Higher' frequencies may be used for the fundamentals of these oscillators and the difference in' frequency between the two oscillatOrsma'y likewise be, greater. By keeping the fundamental frequencies of the oscillators 20 and `2"I sunlciently far apart, `itjis possible to avoid electrical coupling between the two oscillators as may occur with a common ground return and thereby, prevent variationsin the oscillator frequencies which occur between two coupled circuits. The mixer circuit 22 functions tofdetect a. desired beat note corresponding tothe diiTerence between certain desired harmonics of the fundamental frequencies of the oscillators 20 and of about 60 db. for-5 0 cycles either side of the normal operating frequency line.'v y A If therefore the sig-nalwire networks and' their associated oscillators'are `so adjusted under normal operating conditions when no vobject is approaching the wiresy to produce a frequency impressed upon the selective audio ampliiier correspending `to the normal operating frequency N,

then the amplitude Aof the signal delivered by the selective audio amplifier to the rectier^24 will be a constant maximum and remain so provided the ydifference frequency between the 'har-- monics of the two oscillators remain the same. In certain places and" under certain'conditions where the signal wire Vsystems are the,` same in 'most or all respects, this condition` may'remain true and changes occurringin weather and other v I n mentindicate'd in Figure 2 which will be deonecontinuous wire;'=f`Th`e signal net-work 2 is f 1" similarlyterminated-'in the ends'ectionA with scribed later, is preferable. l

In the system shown in Figure 1,'the Vrectifier 24 is connected t0 Va relay circuitk and supplies suiicient power to the relay circuit to maintain it in an energized state while the .normal beat frequency signal is being received'l and transmitted through the circuit. When the `beat frequency varies because the difference frequency between-the two oscillators 20 and 2l is changed, an amplitude less than the constant normal maximum amplitude is delivered to the relay circuit 25 which thereby releases and hcauses the operation of the alarm indicator 25. An advantage in this arrangement is that any failure of the power in the system will cause the alarm indicator to operate thereby assuring `the watchman or guard4 that the system is always working while the beat signal note is present.

The circuit diagram, Figurei, corresponding respectively, condensers 56 and 5l respectively,

and grid ccndensers 28 and 29 respectively with grid resistors 30 :and 3I respectively. The frequency of the oscillator '5D isV determined by the inductance 54, theV condenser 56 and the signal Wire network i which provides chieiiya capaci? tance across the inductance 54 which is in parallel.

with the condenser 56. The inductance54 may be variable and so also, if desired, the condenser tion exists f `tance ofthe two sections must 56. `Thefrequency 'of the oscillator 5| is similarly determined by the signal network Awhich isv in `parallel with the inductance 55 and the condenser 51 which may be tuned to `provide initial frequency adjustment between the two oscillators. i As' has been stated above it is preferable toimake the signal wire systems or networks similar as far as possible and equal in geometric. dimensions `so that the same changesin climatic" condif tions will affect both systems nearl`equally.` Since however thetwo oscill Vtors are-L tuned to Y diiferent fundamental"frequencies' it is necessary, in order that equal variations `in vthe capacitances to ground of the signal wire systems produce "equal `variation inV the harmonics of theV frei quency of the oscillators 5|l$and 5|, to provide the i oscillator operating at the'rlower `frequency with a greater inductance in its frequency determining matical relations worked ordinary oscillating circuit where `F is the*fundamentalfrequency of the circuit; L, the inductance and C, the capacitance. It the inductance .in thetwo oscillator circuits were equal and if `it isY further assumed that the third harmonic of the oscillator 50 is used to beat against the` 4th harmonic ofA the oscillator 5|, then it may Vbe'shownthat the length or capacibe in the relation of 1 to 1.78A for a seli" balancingsystem.` This 201 out below.` In the thejfollowing rela-` `39 that when' a signal voltage would'make one section of thezsignal Wire `system A very much longer than the other-,section and as protect the maximum stability. By making `the inductance offtheltwo oscillatorsin the ratio of ll78to V1, the two sections can be made equal in length (substantially equal in capacitance). This situation is arrived atmathematicalyby the following procedure.

f` 4f2-3f1=,K. l l 1) Where f1 and izare the fundamental frequencies area with maximum 1 of the-.two oscillators, 412 is fourth harmonic of f2.3f1 is thirdharmonic of fiL K. is `the audio frequencypassed by the selective K is verysmall compared to either f or f2.1' Y

` i or 1.78L1=L2 Y (4) As described in connection with Figure "l the outputs of the two oscillators are impressedupon the mixer circuit. `In Figure 4 this mixer circuit is indicated as employing a multi l tube 33 `whichisresistancecoupled toeach of the oscillators 50 and 5| through the resistancesl and. 35. By using a Figure 2, it is possible to rei `duce the coupling between theltwo oscillating circuits to a minimum so that they may always act independently of each other.. The `vacuum `tube network. As

` the resistor l and then fed tulc'e45`` for controlling andoperating `the relay `'rite-output pffthe l y Y l e upon: the selective audio `ampliilenwhiclrpcolnprises. a degenerative y vacuum tube circuit` and amplifier and` operates in `the following way.` All audio frequencies are fed to the `grid. of the .pena i `todeVv tube 36 and are amplified. Anetwork is plate of. thistube` and the Vplaced .between the A grid circuit 1 to feedN back in `the `reverse `phase all frequencies except those `frequencies close i tof` thel particular` frequency determined bythelvalueioi' i Y 4| and` the'` adjustable bias resistor 4' The Arectiedsignalis then filtered.;` by-a .network` com;` prising `the capacitors '4| and 43 connected to to the control grid44`of. the power con 4s; `The grid-tf the tube 45 which may `be called .the relay tube is so connectedto thediode In orderto compensate for" any-slow differenl-` V tial `changes in capacitance which mayoecunfit .Y

generally `necessary to have some means'for 1 indicating the ldirection `of compensationl Vari- A lators` Fand 21"' no i ` the side slopes i either `directiorrof the .frequency to points .la-`

element vacuum multi elementvacuum tube 33 may be` of any suitable type, `such for instance l asthe tube commonly designated as SSA?.

" case the greatest totalarea ousmethocls ,having` this `feature may` be lused for' producing the ldesired compensation. One such method is employed inFigure 2. Bythe arrangement of Figure `2 itis satel for ,changes in climatic` conditions 4as' `forinthe least l unbalance effect.

are combinedin a mixer cirof the curve.` `Small changes in belled "highIcornpensation.1and.` q

merely;` operate to put inrnotion the compensatingdeyices 6| frequency l. back to the vline `actuate the alarm` indicator `65. Both compren--A miner circuit isjlimpressed i Zito ground andl the cathode lead' possible to compen-` will be protectedwith/ l I VIn` `this `case 3 oscil-L` v and 62 forbringingthebeat note of normalgenera@ tion.`` Iftsudden changeintthebeat frequency occurs ;of a magnitude greater'thanthat'corre-H spendingv toA the points of high land 1lowicompensating devicesl and 62of bothrelay' alarm cir-r. cuits 63 and `64` are energized through the rectiners se, s1, vist I'dia-gram of Figure 2 isindicated 1n Figure 5; The

`the selective audio and 15, which has an -14 operate the condenser -7 and 69.V VThe circuit oftheblock lead-in wires land Ail) in this case go to the oscillators 1U and 1I. These oscillator circuits may be similar to that described iny connection with Figure 4 with the exception that the 'oscillator 1| is 'provided with the compensating condenser 12 whichisadjustable bywmeans of the compensating motor 13 and M'respectively to 'add or take out capacityin the oscillator circuit 1I 'and thereby vchange its operatingfrequency. 'This ch'angegin frequency ofv theoscillator 1I is adthe normal 'beat `frejustedtd operate when' quency (Figure 6) has varied on account ofthe climatic conditions acting onthe signal network to produce suicicnt variationsin theharmonics of the Vfundamental Ifrecluencies'of the oscillator y and 1l so that the oscillator 'beat' frequency' vis'jn'ear the point of K l correction as illustrated in Figure 6. The v"outhigh or low compensation 10 and 1I are impressed 83 which'is connected to amplifier circuit, tubes 84 operating resonance characteristic curveof that shown in Figure 6. This circuit n'iayfbe" similar puts of" the' oscillators upon the. mixerVV tube `of this thyratron tube controls the operation l of a relay 96 operating a relay armature 91 for closing the 'Contact to the alarm Circuit. The

`Vbias associated with the diode rectifier' 9,3vis so adjusted that when the v,signal voltageffrom the audio amplifier is near yits peak, 'sufficient positive voltage with respect to the' negative .bias is de- 'veloped on the'grid of the thyratron A9| so that it fires, l actuating the'r'elayv and operatingA the Y In the 4rectiiier circuit 9| l a diode rectifler `98 similar tothe rectiervtube 93 control'sthe grid I00jof a vacuum vtube 99 vthrough a resistance fand capacity network. vThe diode rectifier tube 98j has a negative bias. "I'he- 'signalzprovidesthe gridywith a positive voltage which is 'obtained by connectingthe grid tjo the cathode ofthe rectifier so that for normal operation .of thefcircuit sullicient'plate current ls passed'through thevac- 0 Y 2 so'that the alarm circuit w1l1 not operate. 4When `the signal decreases, the positive voltage Von the uum tube 99 tofkeep the relay lcontacts `|02 apart grid |00 diminishes and the relay which maybe set for sensitive operation releases, closing .the

to that'described in connectionwith 'Figure' '4.9.' The point of 'operation chosen for the rectifiers operated lfrom itsioutput ho-weveris on one of the vslopes o'f the res- Yonan'ce vcufr've preferably along its steep portion. Operated from the output of the' amplifier tube 15'zare therectier' circuits 16 and`11 respectively. The rectifierl circuit `16 c'ompris'esadiode rectifier 18A theoutput lof Awhich controls the grid, 19 of 'a grid ycor'itrolf ga's'eous c'onductiontube such 'as y Y `a thyr'atrnltube' 89 which ingitsaoutput'circuit has-"a relayjcoil" 8 l fwhich operates .the relay'conitactszf tb close the circuit' to the compensating motor. The thyratron 80 associatedgwitli the "rectifiertube 1Bis"so biasedthat when the sig- V"nailed -fron'iu'th'eY of ythe''rectier''circuit 11 is fedto, athy'ra'tron tube 85"-inithe voutput of vwhich isal second relay 86 controlling the operation ofthe electrical cirjcuits across yits armature switchr81. This'diode Y rectifier l89 has a positive'bias and its anode is Y lconnected through a suitable resistor and con'- 'densernetwork.(simi1arly as inL the rectifier cil.-

lcuit16-l tothe grid 88ffoffthe thyratrontubc '85.' A'The positive-bias' is required because the diode issol'lconnected that the signal voltage `'makesA the* thyratron grid negative' with' resp'ctgto the l*original bias.` 'When thereforethe signal voltage 'i is decreased the positive bias causes the thyr'atron tube 85 to iire operating the relay 86- to 'close thejielay contact 81 Iand commencethe operation of the compensating motor 13. These compensating motors 13 and e `grid `of the thyratron" tube 94. Theoutput contacts to the alarm circuit.

. Figures '1, 8, 9 show the construction of the signal wiresystems ornetwork.

It first may be mentioned that the signal wire should preferably be coated with alwater shedding insulation, as for instance a suitable mica lacquer which will not readily allow water droplets to collect on'the wire from rain.

- The'construction in Figure. 'z is simiia'rt that indicated in Figure?, `for the leadin sectionvand Yin this case lit is assumed .that the wire fence itselffislsupported'by the posts 5' and thatrthe signal wires are on the outside of thewire fence. Y

' The construction indicated in Figure -8 showsV Vthe signal'wire von supports 5 set upon an angle of the barbed wire fence.` e i In Figure 9 the signal wire support 5" is shown as inside. the chain link `fence 6'.- The chief point to be noted in the constructions in Figures 7, 8, and 9vis` that the signalwires are spaced with the signal wire presumed to be .on the inside iapart at some distance from the wire fence. The

spacing between parallels should be approximately three feet or more;

The signal wire systems may be set up to: ex-

vtend over a very considerable distance, sufllcient V.tovsurround power plants, Afactories and other ytypes of plants which it is desired to guard andV warn of the approach of strangers orjstrange objects.

Signal networks or systems of 600 ft. or more are practical and where it is desired more than e one system ,may` be vused as, for instance, one on one side of the plant and another onl the other side of the plant.' 'The systeml may also be used to guard small areas as for instance, entrances andl doors and may also be used indoors in an equally efficient and useful manner Aas it is used out of doors.

y A large area may be coveredby systems one adjacent tothe'other.

A short summary of the operation of the system' with suitable chosen values is givenbelow;

The capacitance of the two sections .of the signal wire systems is carefully measured atapproxirnatoly 1D0 kc. The two sections of' the signal wire systems are then connected inturn to oscillators 10 and 1|Yfor'instancej'FiXed coni densers as required LVare vconnectedy across each oscillator. The value of these xedc'ondensers for a balanced vsystem will he 'inthe ratioA of ythe,

capacitance of thealarm wires Assume one Y the resonance curve.

section of the signalwire system had a capacitance of 2000 micromicrofarads while the other half had a capacitance of 1800 micromicrofarads; thexed condensers across the `oscillator associatedv with the 2000 micromicrofarads might be 2000 micromicrofarads while in this case the iixed capacitance `across `the oscillator associated with the 1800 micromicrofarads would be` 1300V micromicrofarads. This in general would result in a balanced system provided the two sections of the alarm wire system were symmetrical.

^ However departure from this may lbe-,indicated where non-symmetrical signal wires exist and where the ground conditionsunderneath the signal wire system are diierent on the two sections.` Measurements as to the change in capacitance` with weatherconditions and from dayfto `night on thetwo sections ofthe signal wire system in general enable a more balanced system to result than the empiricalmethod described above.

The inductance of the variable oscillator coil is then adjusted so that the audio signal fed from the selective network `is approximately in the center Vof one of the steep sides ofthelresof nance curve. Thisis indicated practically by the meter. This adjustment should be made `with the compensating condenser plates about midway in mesh which allows for the maximum compensation in either direction@ The bias on the variousdiodes is then adjusted so that the correction motors associated with the variable con-` denser will retain a suitable operating point on The biases on` the alarm relays are adjusted so that they operate at` suitablepoints. The sensitivity of the system` is then checked. If this sensitivity is too great, minorv changes in sensitivity can be accomplished by, adjusting the bias on the two alarm relays so that they fare farther apart on the resonance curve.` However if these minorladjustments do not give the `desired sensitivity, the sensitivity` can be further reduced by additional xed con;

dansers across the two oscillator circuits. A secl corresponding ond operating point is obtained by adjusting the i inductance of Vthevariable oscillator coil and the same procedure as given above is followed. The inductance of the oscillator coil which I employ enables either the second and third, the third and fourth, the fourth and fifth harmonics to be employed. Naturally the higher the harmonics employedjthe greater the sensitivity so that in- Y stead ofchanging the fixed condensers across the oscillator coils it may be possible if the sensitivity is too great for a given installaton'to vary the inductance sufficiently so that a lower set'of harmonies are employed. 'I'he sensitivity may also Y be changed in a minor degree by .relay adjustments on the alarm circuits.

In practice it has been found that condensers and 29 (see Figurefi) in series with the alarm wire system help the stability of the system under various weather conditions. l

It should be noted particularly that the frequencies of both oscillators and therefore their harmonics will vary and that no attempt is made to keep any of these` frequencies of either oscillator constant. In fact it is suflicient for maintaining balance that thel difference between the harmonics, i. e., the beat frequency of the oscillators remain constant.

Having now described my inventionI claim: 1. AnV electrical protective system for guarding against the approach of objects or persons corri` prising dual capacitance to ground elements extending about an area to be protected, means intively connected to said length on` both `said cluding two separate `oscillatory circuits opera-4 tov one otsaid elements,

tively connected, each said circuits with capacitance elements having each `diiierentoscillatory frequencies,` means,

actuated by the approach of an objector person?V `to change" differentially the frequencies ofsaidoscillatory to" one of said capacitance elements circuits, selective electrical circuitmeans operatively connected to said"oscillatorycireuits and` l" selectively tuned to a beat frequency `correspond-` ing `to the difference between different harmonic y Vfrequencies of said oscillators and means opera-V` ,i

selective electrical circuit@` i for `indicating `by a change from' a chosen beat` frequency, the approach of anobjectlor person;

2. `A capacitance operated` signal,ldevicefconjl` i, rising two signal wire systems having oscillatory 1j; i

circuits `of, differentfrequencies, a signal responf ferential capacitance and means. for operating" said signal responsive means when relatively rapid capacitance changes said difference frequency `from` alter materially the selected value.`

3.1i` capacitance operateddevice,comprising `two` separate electrical oscillatory circuits hav; ing different oscillatory frequenciesincluding signal wire systems connected,. one lto each of said oscillatory circuitsand adapted to cause a dif` ferential change in the oscillatory frequencies by` the approach of an object or persongtoonelof said signal wire systems, a selective audio ifrel.

quency amplifying network `having a resonance characteristic curve with a point' near the {peak toa beat frequency between different harmonic frequencies of said oscillators 1in normal operation, means operative upon the vai'iation of said beat frequency from `its `normal position for operating an alarm` indicator indieating the presence of any object or personal: said signal wire.

4. A `capacitance operated device Vcomltiri/sing. two Aseparate electrical oscillatory circuits having diierent oscillatory frequencies including signal wire systems connected, one to each of said oscillatory circuits and adapted to cause a differential change in the oscillatory frequencies by the approach of an object or person to one of said signal wire systems, a selective audio frequency amplifying network having aresonance characteristic curve with a pointmidway on the slope thereof corresponding toa beat frequency` between different harmonic frequencies of said `oscillators in normal operation, means loperative upon the variation of said beat frequency from its normal positiornfor operating an alarmV indicatorindieating the presence of an object or person at said signal wire, and means `operativelupon variations in the normal beat frequency less than the variation for causing the operation of the alarm to correct one of said oscillatory frequencies to returnthe beat frequency at arslow rate to its nor mal operating position.

5. An alarm system comprising dual capaci"` tance to ground elements arranged to be propor tionally affected by weather changes, compensatingmeans operatively connected to said elements for maintaining a differential balance bey n' changes, electro-responsive means,

alarm (means, operatively connected to said contween said elements during said weather changes and electro-responsive means including an alarm means connected'to said elements `for producing an alarm when said differential balance has been upset by changes of lal character different from said weather changes.

6. An .alarm system comprising two independent electrical conductingv wires extending over an area, theapproach to which is to be electrically energizing said wires providing dual capacitance .to ground elements, arranged and dlmensioned to beV substantially proportionally affected by normal weather `including conducting wire system is affected by the same climatic changes, and connected to said pair of electric circuits for operation thereby by anfunbalance, when the con- A ducting wire systems are ailected'by the approach f of an' object to the area to be guarded.

ducting wires having electrical characteristicsv maintaining said alarm means unoperated during normal weather changes substantially, proportionally aiecting the capacitance to groundv of said wires and operating said alarm means when the capacitance to ground of said wires are nonforeign object -to said Wires.

proportionately 'affected as by the approach `of aV 9. Ina signal system, a double antenna system comprising two separate conductors of substantially equal electrical capacity to ground and extending in different directions from a common point, two oscillators respectively impressing oni said conductors oscillating currents of predeter- Y mined frequencies, a converter tube having grids Y connectedrespectively with the output circuits of said oscillators whereby the output circuit of 'said converter tube has abeat frequency determined A by said predetermined frequencies, an amplifier circuit, means feeding the output of said converterrtube to said amplifier circuit, and a signal 7. An alarn'iv system comprising means prcvid ing independent electric fields extending over an area the approach to which is to be guarded, said means adapted to `have the ncapacitance to ground of each iield substantially proportionally affected by normally vexpected climaticchanges, signal means, means connected between the means providing the independent electric elds and said signal means vfor operating said signal means only under l changes substantially non-proportionally affecting the capacitance to ground of each field.

8. An alarm system comprising two independenty electrical conducting'wire systems extending over 4 an areal the approach to which is to be guarded,

a pairrof electric -circuits each operatively coni.

nected toA onecf'said conducting wirevsystems and each having impedance elements of diierent magnitudes apportioned fo an electrical balance between each conducting wire system andrits operatively connected electric circuit` when each devlceoperable by said amplifier circuit.

l0. The combination cf the preceding claim including lter means connected in circuit between said ampliiler circuit and said signal device for 's selecting the frequency which" will operate saidsignal device.

z" GLENN jH. BROWNING.

, nEFEaENcEs CITED Thefoliowing lreferences are of record in the signal means operatively y Great Britain Apr. 9. 1931

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