US2024378A - Remote indication of gas pressures, flows, and the like - Google Patents

Description

Dec. 17, 1935.

G. B. LINDERMAN, JR

REMOTE INDICATION OF GAS PRESSURES, FLOWS AND THE LIKE Filed Jan. 29, 1951 2 Sheet s-Sheet 1 WE Q...

G. B. LINDERMAN, JR 2,024,378 REMOTE INDICATION 0F GAS PRESSHURES, FLOWS AND THE LIKE Filed Jan. 29, 1931 2 Sheets-Sheet 2 5' if 51 i 7&5?

Patented Dec. 17, 1935 PATENT OFFICE REMOTE Garrett B. Linderman,

INDICATION OF GAS PRESSURES,

FLOWS, AND THE LIKE Jr., Pittsburgh, Pa., as-

signor to Pittsburgh Equitable Meter Company, Pittsburgh, Pa., a corporation of Pennsylvania Application January 29, 1931, Serial No 512,170

14 Claims.

The present invention relates to remote metering and indicating systems especially adapted to the remote indication of gas pressures and flows, but the principles thereof are applicable to a wide range of remote control and metering operations.

The remote indication of gas pressures and flows is highly desirable, but presents special problems, primarily because of the fact that the transmitting devices must usually be installed in locations such as gas main vaults and the like, where there is danger of gas leakage, and consequent danger of explosion in event sparks are discharged. While various types of prior telemetric systems have been proposed for the remote indication of gas pressures, such prior systems have been more or less dangerous due to the use of substantial electrical current flows, comparatively high voltages, or alternating cur- 'rents to control the receiving instruments, with resultant danger of sparks and ignition of leak-' ing gas.

The currents and voltages used for operation of the prior proposedsystems have, furthermore, been of such a nature as to prevent the utilization of existing telephone lines for the transmission of the remote indication. The telephone companies will not allow the transmission over telephone lines of direct current potentials of over 90 volts, or over a few volts of alternating current, nor currents over a few milliamperes. These requirements of the telephone companies precliide the use of telephone transmissionlines for} the prior proposed telemetric systems, with the result that special lines must be installed, rendering the cost of such systems too expensive for use by gas companies, particularly in the larger cities where the need for such systems is greatest.

Accordingly, a primary object of the present inventionis to provide telemetric systems that will operate with low voltages, and substantially no appreciable line current flow eliminating the dan er of sparks and explosion caused thereb due to gas leaks.

' Another object ofthe present invention is to provide novel telemetric systems adapted for operation over telephone lines, and which will operate with voltages and current approved by the telephone companies for operation over telephone lines so that leased telephone lines may be u ed therefor, eliminating the necessity for provid ng separate transmission lines and interconnecting switching mechanisms.

Still another object of the invention is to provide novel telemetric systems in which the length and resistance of the transmission line has no appreciable effect on the operation and indica- I tions of the receiving instruments, thereby permitting the standardized production and calibration of receiving instruments without the need of compensating on calibrating for varying lengths and resistances of transmission lines.

Another object of the invention is to provide novel telemetric systems operable over telephone 10 lines, while permitting simultaneous telephone conversation to proceed without interference.

A further object of the invention is to provide novel methods and apparatus for checking the adjustment and accurracy of my improved tele- 15 metric transmitters and receivers at the receiving station, and without the need for attendants at the transmitting stations.

Further objects of the invention will appear from the following detailed description of the 20 preferred embodiments thereof, and are such as may be attained by a utilization of the various combinations, subcombinations, and principles defined by the terms of the appended claims.

Referring to the drawings:

Figure 1 is a diagrammatic illustration of a simplified system embodying my invention.

Figure 2 is a diagram of the plate circuit current of a receiving tube utilized in one practical embodiment of the system shown in Figure l, 30 showing the plate circuit current in milliamperes for the various grid potentials utilized in the transmission of gas pressure indications over a telephone line.

Figure 3 is a diagrammatic showing of a modi- 35 fled receiver embodying my invention.

Figures 4 and 5 are diagrammatic illustrations of circuit arrangements permitting the transmission of telemetric' indications in accordance with the present invention over telephone lines 40 simultaneously with the transmission of telephone conversations and without interference.

As shown in Figure l, A is atransmitting station at which a gas main I is located which is connected through conduit 3 to pressure gauge float chamber 4. Chamber 4 is connectedby U-tube 5 to gauge chamber 6 which in turn communicates through conduit 1 with atmosphere.

As is well known, variations of pressure in main lv cause variations in the level of mercury 5 8, actuating float 9 in accordance therewith.

Float 9 is providedwith an arm ll pivotally connected at l2 to actuating arm l3. Arm I3 is rigidly connected to a spindle l4 journaled in. suitable bearings provided in chamber 4. Se- 5 '"diiferences of pressure between chambers and ,6 on scale I8. Scale 18 may be calibrated to indicate the pressure in gas main in well known manner. v

Pivotally connected at I!) to arm I5 is one end of electrically conductive link 2|, in the opposite end of which contact 22 is journaled. Contact 22 is preferably a sliding contact of silver or other suitable metal and contacts with the turns of potentiometer resistance 23 as the level of float 9 is varied in accordance with the gas flow in main I. Connected in series with resistance 23 by means of wires 24 is a battery 25 or another source of substantially constant voltage direct current supply. The positive pole of the potentiometer is connected to line conductor 26 and the negative pole is connected through resistance 23, contact 22, and conducting link.2| to line conductor 21.

As the level of the mercury infioat'chamber 5.varies, the amount of resistance 23 connected between the terminals of line conductors 26 and 21, and accordingly, the potential I impressed upon lines 26 and 21 at station A will be varied in accordance with the variations of pressure in gas main The use of the mercury operated float to actuate the contact 22 provides a powerful, rugged,

sensitive, and accurate operating mechanism which follows the gas pressure variations closely and. is especially adapted for the transmission of gas pressure variations.

Excellent results have been secured in practice, in full compliance with the regulations of the telephone companies for electrical transmission of gasmain pressures over telephone lines, by providing a 22% volt battery 25 and providing a total resistance of 10,000 ohms at 23, with the transmitting gauge operating over a range of from zero pressure to a pressure corresponding to-a pressure. of 20 inches of water.

It will, however, be understood that the values are illustrative only and are given as a specific example for remote indication of gas pressure readings, The resistance and potential used, and the nature thereof may obviously be varied widely to meet varying conditions of practical operation.

The line conductors 26 and 21, preferably telephone lines in practice, extend to a receiving or recording station B. At the receiving station, I

conductor 21'is connected through a single pole switch 28 to grid .29 of amplifying tube 3|, and a conductor 26 is connected'through lead 32- to filament 33 of tube 3|.

Filament 33 is connectedby conductor 34 to one terminal of resistance 35 forming part of a constant voltage current supply source 36 for the plate circuit of tube 3|. The opposite terminal of resistance 35 is connected through conductors 36', milliammeter 31, conductors 33,1oading resistance 39, to plate 4| of tube 3|. Resistance 39 is provided in the plate circuit to give as closelyas possible a straight line charac-:

teristic to the platecircuit current of tube 3|.

' In the specific embodiment of the transmitter hereinbefore described, approximately three thousand ohms in resistance 39 with a Western Electric Company 205D vacuum tube at 3|, give excellent results, the plate circuit current flow upon the grid of tube 31, allowing a normal 10 plate circuit current of 2.7, milliamperes to flow in the plate circuit of tube 3|.

To balance theefiect of such a normal plate circuit current on milliammeter 31 and to permit adjustment of the milliammeter to zero, compensating battery 42, and a variable resistance 43 are connected in a shunt circuit around the terminals of milliammeter 31, in such manner that the potential of battery 42 will oppose the flow of platev circuit current through milliammeter 31. In the specific example of the invention hereinbefore referred to a 1% volt battery 42, a 550 ohm resistance 43, and a resistance of 5 ohms in milliammeter 31 have given excellent results.

The application of a high negative potential difference to line conductors 26 and 21 by the transmitting potentiometer for a zero pressure condition, and decreasing the negative potential difference asthe pressure in main rises permits the use of the small readings oh milliammeter 31 to indicate small pressures and is desirable for gas pressure measurements. In' ad-- dition, this arrangement permits the use of the transmitting potentiometer battery to maintain a negative bias on the grid 29. of receiving tube 3|. By reversing'the transmitter scale l8 and the scale of milliammeter 31, and arranging the transmitting mechanism to impress a zero potential on the line for a zero gauge indication and to increase the line potentials with increasing gauge indications, it will be noted that satisfactory operation will also be secured.

The source of current supply 36 for the plate45 circuit comprises a voltage regulator tube 45 of any well known and suitable construction connected across conductors 46 and 41 leading from the terminals of resistance 35. Conductor 46 is connected in series through resistance 48 to 50 the midpoint of thesecondary transformer winding 4,9\ the terminals of which are connected through conductors 5| to the terminals of a rectifier tube 52.

'Connected to filament 53 of rectifier tube 52 65 I vention above referred to, the current supply source 36 is arranged to deliver 90 volts across the terminals of resistance 35 to the plate circuit Of tube 3|.

Since in practice variations of as high as ten percent in commercial alternating currents may occur and such variations may cause variations2z1 in the plate circuit voltage sufflcient in amount to affect the accuracy of the readings of milliammeter 31, a volt meter 58 is provided to indicate the voltage in the plate circuit. Since the resistance of milliammeter 31 is negligible, volt meter 58 will indicate substantially the voltage across resistance 35. Accordingly, if necessary, suitable corrections in the readings of milliammeter 31, or adjustments of source 36, may be made to compensate for undue voltage variations indicated by volt meter 58.

While source 36 has been disclosed as a direct current supply source, it is to be understood that direct current is not necessary if a suitably constant voltage alternating current source is available.

To permit checking the adjustments and accuracy of the receiver, and checking the voltage of the transmitting potentiometer battery 25 without sending a man to the transmitting sta-' tion A, a potentiometer 59 is provided at station E comprising a battery 6| and a resistance coil 62 corresponding in voltage and resistance, respectively, to the transmitting voltage battery 25 and resistance 23. The positive terminal of potentiometer 59 is connected by conductor 63 to the filament 33 of tube 3|. A movable contact member and pointer 64, connected by conductor 65 to point 66 of the single pole double throw switch 28, is arranged to contact with resistance 62. Potentiometer 59 is provided with a scale 61, the graduations of which correspond to the graduations of scale |8 of the transmitter A and tothe graduations of the scale of milliammeter 31. Since the constants of potientiometer 59 are the same as the constants of the transmitting potentiometer, it will be seen that for corresponding positions of contact members .64

and 22 of the receivers, similar indications will be given on scales l8 and 61 at the transmitting and receiving stations, and like potentials should be applied to the grid of tube 3| resulting in like indications for similar contact positions on milliammeter 31.

Operation In operation the transmitting apparatus at station A is adjusted so that with a zero gas pressure on the gauge pointer I! will indicate a z are reading on scale l8, and contact 22 will be so positioned on resistance 23 that the maximum negative potential will be applied to conductor 21. With switch 28 in the position shown in Figure 1, this negative potential will be applied to grid 29 of receivingtube 3| resulting in a substantial negative bias of grid 29 with the gauge registering zero.

Since, as is Well known, substantially no. current will under such conditions flow between filament 33 and grid. 29 of tube 3|, there will be no appreciable loss 'in voltage due to line resistance drop over conductors 26 and 21, and the transmitting potentiometer voltages will accordingly be applied across the filament and grid of tube 3| regardless of the length of lines 26 and 21.

As above pointed out, when the transmitting instrument A is in zero position a small current will flow in the plate circuit of tube 3|, the efiect of which is neutralized on milliammeter 31 by current from battery 42 which is varied by adjustment of resistance 43 to bring the the receiving apparatus, as the pressure variations in the gas main cause the level of mercury in chamber 4 and the float 9 to rise and fall, arm l3 rotates spindle |4 causing the arm |5 to swing about spindle M as a pivot. Movement of arm l5 actuates link 2| to vary the position of contact 22 on potentiometer resistance 23 in such manner that as the gas pressure on the gauge increases, the negative potential applied to conductor 21 decreases proportionately, causing a like decrease in negative potential applied to grid 29 of tube 3|. Decrease of the negative potential of grid 29-, as will be seen by reference to Figure 2, causesa corresponding increase of current through plate 4|, resistance 39, conductors 38, milliammeter 31, conductors 36', direct current source 36, conductors 34 and filament 33, causing the pointer of milliammeter 31 to follow the movement of contact roller 22 and pointer l1. Milliammeter 31 will indicate the position of float 9 accurately at all times. The scale of milliammeter 31 may therefore be calibrated to indicate directly the pressures in gas mainl in well known manner. It will be the full range of movement of float' 9 that is transmitted to and indicated on milliammeter 31 with substantially no current flow over conductors 26 and 21.

To adjust the zero position of milliammeter 31 at the receiving station, switch 28 is positioned on its contact 66, and contact pointer 64 of potentiometer 59 is positioned at the zero indication on scale 61. The potential applied to grid 29 will then correspond substantially to the grid potential when float 9 and pointer 11' of the transmitting station A are in zero position and controlling the receiving station B. Resistance 43 is then adjusted to bring the pointer of milliammeter 31 to zero position. After the adjustment is completed, switch 28 is positioned as shown in Figure 1, connecting line conductor 21 to tube 3| and the system will then operate in proper adjustment. It will, accordingly, be seen that the receiving milliammeter may be adjusted to zero reading as frequently as desired by the receiving attendant. If the voltage applied to the plate circuit across channels of resistance 35gby the direct current source 36 varies materially, the variations will be indicated on volt meter 58, and suitable corrections or adjustments to compensate therefor may also be made by the receiving attendant. With these convenient adjustments provided at the receiver it will be seen that a high degree of accuracy may be maintained in operation.

When ordinary dryfcell storage batteries are transmission station I the battery graduallydeteriorates and the voltage supplied thereby gradually falls ofi, causing a lowering of the voltage applied to conductors 26 and 21, by the transmitting potentiometer. Since too great a variation in the transmitting potentiometer voltage will result in inaccurate indications on milliammeter 31, and the transmitting stations are usually at remote points, potentiometer 59 at the receiving station is prefrably utilized to check the potential of the transmitter battery 25. This accomplished by installing like batteries 25 and 6| at the same time. Since the transmitting and receiver electrical circuits are substantially alike, deterioration of batteries 25 and 6| will normally proc d at the same rate. When it is desired to check for deterioration of the batteries, switch 28 is ceiving potentiometer 59 across grid 29 and filament 33 of tube 3|. Contact 64 of potentiometer '59 is then moved along resistance 62 and the teries of proper" voltage. If' desired, corrections.

indications on scale 61 are compared with the readings of milliammeter 31. If the potential of battery 25 has not deteriorated materially, the

position of contact 64 on scale 61 will correspond to the indications on-the milliammeter 31. If the batteries have run down until they are delivering a substantially lower voltage than that for which the system is adjusted, the reading of the milliammeter 31 will be less than the reading indicated on scale 61 of receiving potentiometer 59, and the difference in the two readings will indicate the amount of deterioration of the batteries. When the deterioration of battery 6| has proceeded sufliciently to materially afiect the accuracy of the readings of the milliammeter 31, as an approximately equal deterioration should have occurred in battery 25, the batteries 25 and 6| are again replaced by like batfor the indicated deterioration may be made in the readings of milliammeter 31. L

Instead of utilizing a milliammeter 31 it will be apparent to those skilled in the art that a well-known type of recording instrument may be utilized to make a record of the pressure variations in gas main I. It will also be apparent to those skilled in the art that byconnecting gauge chambers 4 and 6 to the opposite sides of an orifice plate disposed in main I, float 9 will be operated in accordance with, and will indicate the flow of gas in main I. Scales l6 and 61 and the milliammeterscale-may then be graduated to indicate the flow rather than pressures in main In gas distribution systems it may be desirable to locate transmitting gauges A to indicate main pressures of flows at various remote points, and to read on a single receiving set B the indications from a plurality of remote gauges.

Since my invention is adapted for operation over telephone lines, any desired number of transmitters A may be connected to ordinary subscribers telephone lines terminating in a switching exchange, and the receiver B may be connected over a telephone subscribers lines to the exchange. Connection may then be established through the operation of telephone switching apparatus in the usual manner by the receiving attendant between his receiving set and any desired one of the remotely located transmitters A with which connection is desired. If desired, at station B-a telephone head set may be provided to permit the receiving attendant to communicate with a central station operator to establish the desired connection, or a telephone switching dial to. control automatic switching apparatus may be included in lines 26 and 21 in the manner usual in automatic telephone switching systems. After a connection has been established, the telephone apparatus should be disconnected from lines 26 and 21, and receiving apparatus B connected to the line through suitable switches provided for this purpose.

Modified receiving station therefore be had to the foregoing description.

In this form of the invention increased .power for the purpose of securing sufficient power to operate recording instruments at 31 requiring substantial power to operate, is secured byincreasing the number of amplifying tubes 3|. In this way, more current is secured. This will be obvious from an inspection of the circuit diagram. From this disclosure it will furthermore be apparent that any desired amplification may be secured by increasing the number of tubes 3|, 10 in a manner that will be obvious to those skilled in the art.

In this form of invention the compensating battery 42 in series with variable resistance 43 in the shunt circuit around milliammeter or recording instrument 31 is eliminated, and the compensating current for this circuit is derived from the direct current source 36, by adding re-v sistance 68 between the junction of conductor 36 with resistance- 35 and the positive terminal of the direct current source 36, and connecting one terminal of variable resistance 43 to the terminal of resistance 68 as indicated in Figure 3. With the value of resistance 35 approximate- 1y 1;000 ohms, a resistance of approximately 100 ohms at 68 has given excellent results in practice. In this way a potential equivalent to the battery potential '42. will be impressed on the shunt cir cuit around the instrument 31, opposing the normal flow of plate circuit current through instrument 31 as above set forth in detail in connection with the form of invention shown in Figure 1. a

To permit adjustment to compensate for changes in the characteristics of tubes 3| with 35 age or use of the tubes as frequently occurs in practice, resistance 39 is made variable, and a shunt circuit embodying variable resistance 10 is, connected around instrument 31 to permit variation of its sensitivity. a 40 With this arrangement, as the characteristics of tube or tubes 3| change in use, the receiving circuit may be connected to the receiving potentiometer 59, contact 64 adlusted to zero reading on scale 61, and variable resistance 43 is then 45 adjusted until a zero reading is indicated by instrument 31. Contact 64 is then moved to indicate a high reading on scale 61, preferably a maximum reading and variable resistances 39 and 10 are then adjusted until the reading on instrument 31 is'the same as the reading indicated by contact 64 on scale 61.

Such adjustments, it has been found in practice, effectively compensate for substantial changes in the characteristics of the amplifying tubes in operation. After the adjustments have been completed the receivermay again be disconnected from the receiving potentiometer 59 and connected to the lines 26 and 21 for normal indicating operations. 0

To permit a determination of the reading of the transmitter from the receiving station independently of the receiving instrument 31, for checking purposes, a selector switch is provided in place of the switch 28 used in the form of invention shown in Figure 1, comprising sectors 1|, 12, and 13 electrically insulated from each other, which may be electrically connected together by inserting a conducting plug in proper hole 14. Sector 1| is connected to the grids of tubes 3|, sector 12 is connected to line conductor 21,

and sector 13 is connected through a protective resistance 14, a galvanometer 15 and conductor 65 to contact 64 of :receiving potentiometer 59.

In normal operation a conducting plug is inserted between sectors 1| and 12 connecting the receiving instrument 31 to the transmitting station over conductors 26 and 21, and leaving ing potentiometer 59, and the receiving circuit may be checked andadjusted in the manner above set forth.

When it is desired to check the readings of the transmitting instrument independently of the receiver, the plug is inserted between sectors 12 and 13 only, connecting the receiving potentiometer 59 and galvanometer 15 to the line. Contact 64 is then adjusted until the galvanometer reaches zero, at which point the transmitted potential will balance the potential of the receiving potentiometer, and the reading on the scale 61 of the receiving potentiometer will then correspond to the reading on the scale I8 of the transmitting potentiometer. Then by removing the plug from between sectors 12 and- 13 and inserting it between sectors 11 and 12 to disconnect the receiving potentiometer and to connect the receiving instrument circuit to the line, a comparison of the reading of instrument 31 can be made and the accuracy of the receiving instrument accurately determined. Thus an accurate method of checking the readings of the transmitting and receiving instruments of the receiving station at will, and with out the necessity for sending an attendant to the transmitting station, is provided.

In the form of the invention shown in Figure 4, a circuit arrangement is disclosed to permit transmission of simultaneous telephone conversati'on and telemetric indications without interference. In this form of the invention the usual talking coils 16 are connected by line conductors 11 and 18 which may extend through the usual telephone central station equipment. Talking coils 19 are connected in well known manner to substation or subscribers telephone equipment indicated diagrammatically at 80.

A telemetric transmitter A and receiver B of the type heretofore disclosed in detail, are connected by means of conductors 26 to a ground return and by conductors 21 to the midpoints of coils 16.

Potential variations caused by operation of the transmitter A in the manner heretofore set forth are transmitted over conductors 21 dividing in opposite directions through the halves of coils 16, over line conductors 11, and 18, to receiver B, returning by way of conductors 26 and the ground connections back to transmitter A. Since substantially no current flows over this circuit, potential variations only being transmitted over telemetric circuit as above pointed out, and, furthermore, since the telemetric circuit divides equally through opposite halves of coils 16, the transmission of the telemetric potential variations: has no effect on telephonic currents transmitted through coils 16 and 19 in well known manner. Telephone conversations may, therefore, proceed simultaneously with the transmission of telemetric indications and without interference.

In Figure 5 a further circuit arrangement is shown for the simultaneous transmission of telemetric indications and telephone conversation. In this form of invention condensers 8| of. a suflicient capacity to permit the flow of normal telephone line currents without interference, are connected in the telephone line conductor 18 adjacent coils 16. An improved telemetric transmitter A, and a receiver B of the construction heretofore described, are connected across condensers 8| by conductors 26 and 21 in'the manner indicated.

A complete metallic circuit is thereby formed between transmitter A and receiver B, one leg of which comprises conductors 21, coils 16 and conductor 11, and the other leg of which comprises conductors 26 and 18. Telephone conversations will pass through coils 19 and 16, conductors 11 and 18, and condensers 81, without intereference due to the connection of the trnsmitter A and receiver B to the circuit, while the potential variations caused by operation of transmitter A will be transmitted over conductors 21, coils 16, and conductor 11 to receiver B, returning through conductors 26 and line conductor 18. Since substantially nocurrent will flow in the telemetric system as above pointed out, no interference with telephone conversation will occur, and the telephone conversation will have no effect on the telemetric indications.

While my invention has been disclosed for remote indication of gas pressures and flows, it

may be embodied in other specific forms without departing from the spirit or essential characteristics thereof, and may be applied to a wide range of remote indications and control operations. The present embodiment is, therefore, to be considered in all respects as illustrative and not-restrictive, the scope of the invention being indicated by the appended claims rather than the foregoing description, and all changes which-come within the. meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Accordingly, what is desired to be secured by Letters Patent and is claimed as new is:

1. A telemetric system comprising a potentiometer circuit, the output of which is responsive to variations of conditions to be transmitted; a

transmission circuit upon which potential variations are impressed by said potentiometer circuit; a thermionic amplifier having an input circuit and an output circuit; means at the receiver producing a counter potential to prevent a substantial current flow in said transmission circuit; a source of substantially constant voltage current in the output circuit of said amplifier, and an indicating instrument respons'ive to current variations in the output circuit of said amplifier.

2. A system for the. remote indication of gas pressures comprising a gas main; a pressure gauge comprising a mercury operated float responsive to pressure variations insaid main; an arm actuated by said float; a contact member actuated by said arm; a transmission line; a po-- tentiometer adapted to apply varying negative potentials to said line in accordance with the position of said contact member; a vacuum tube having a grid, filament, and plate; means connecting 'said grid and said filament in said line in" a manner to prevent asubstantial flow of current in said line; an output circuit connected to said plate and said filament; a source of tive potentials to said line in accordance with the position of said contact member; a vacuum tube having a grid, filament, and plate; means connecting said grid and said filament in said line in a manner to maintain a continuous negative bias on said grid and to prevent a substantial fiow of current in said line; an output circuit connected to said plate and said filament; a source of substantially constant voltage direct current connected in said output circuit; and a receiver connected in said output circuit.

4. A telemetric system comprising a line circuit; a transmitting station; a transmitting potentiometer at said transmitting station con nected to said line in a manner to impress po tential variations thereon; means for operating said transmitting potentiometer in accordance with variations of indications to be transmitted; a receiving station comprising a thermionic amplifier responsive to the transmitted potential variations, and having an input circuit and an -output circuit with its input connected to said line; means at the receiver producing a counterpotential to prevent substantial current fiow thereover over said line; and a receiver connected in the output circuit of said thermionic amplifier to indicate the transmitted potential variations.

5. A telemetric system comprising a line circuit; a transmitting station; a transmitting rotentiometer at said transmitting station connected to said line in a manner to impress potential variations thereon; meanstfor operating said transmitting potentiometer in accordance with variations of indications to be transmitted; a receiving station comprising a thermionic amplifier responsive to the transmitted potential variations; having an input circuit and an output circuit, with. its input circuit connected to said line; means at the receiver producing a counter'potential to prevent substantial current flow thereover; a receiver connected in theoutput circuit of said thermionic amplifier to indicate the transmitted potential variations; and means at said receiving station for checking the accuracy of the indications of said receiver independently of the operation of said transmitting potentiometer.

6. In a telemetric system, a transmission circuit, a transmitter comprising means to impress potential variations on said circuit in'accordance with conditions to be indicated; a receiver connected to said transmission circuit comprising a thermionic tube; aninput circuit for said tube responsive to negative potential variations; an output circuit for said tube comprising a source of substantially constant voltage direct current; a current responsive indicator connected in said outputv circuit; and means at said receiver for checking the accuracy of said indicator independently of the operationof said transmitter.

7. A telemetric system comprising a transmission line; a transmitting potentiometer adapted to apply varying potentials to said line in accordance with variations of conditions to be indicated; a receiver connected to said line comprising a thermionic tube; an input-circuit for said tube responsive to potential variations; 5 an output circuit for said tube comprising a source of substantially constant voltage current; a current responsive indicator connected in said output circuit; and a potentiometer at said receiving station adapted to be connected to, said input circuit. 7

8. A telemetric system comprising means responsive to variations of conditions to be transmitted; a potentiometer responsive to said means;

a transmission line connected to said potentiometer and upon which potential variations are impressed by said potentiometer; a thermionic amplifier having an input circuit and an output circuit, and having its input circuit connected to said line; means at the receiver producing a counter potential to prevent appreciable current fiow in said line; a source of constant voltage current in the output circuit of said amplifier;

a receiving instrument responsive to current variations in the output circuit of said amplifier, and means to compensate for changes in the characteristics 01' said amplifier with use.

9. In a telemetric system, a transmission circuit, a transmitter comprising means to impress potential variations on said circuit in accordance with conditions to be indicated; a receiver' connected to said transmission circuit comprising a thermionic tube; an input circuit for said tube responsive to negative potential variations; an output circuit for said tube comprising a source of substantially constant voltage current;

a current responsive receiving instrumentconnected insaid output circuit; and means at said receiver for determining the position 01 said transmitter independently of the operation .of 4c said receiver.

10. -A telemetric system comprising a transmission line; a transmitting potentiometer adapted to apply varying potentials to said line in accordance with variations of conditions to be indicated; a receiver connected to said line comprising a thermionic tube; an input circuit for said tube responsive to potential variations; an output circuit for said tube comprising a source of substantially constant voltage direct current; a current responsive indicator connected in said output circuit; and means at said receiver for indicating the position of said transmitter independently or the operation'of said receiver. a 11. In a telemetric system for indication of a quantity to be measured; a transmitter comprising a potentiometer and means dependent on the quantity to be measured-to vary the potential thereof; a receiver comprising a thermionic tube; an input circuit for said tube responsive to negative potential variations at the transmitter; an output circuit for said tube comprising a source of substantially constant said transmitting circuit having means responsive to variations in a quantity or magnitude to.

affect the electrical conditions of the connected input circuit to thereby vary the current in the output circuit in accordance with variations of the quantity or magnitude, said means preventing any substantial flow of current in said transmission circuit. i

13. The invention as defined in claim 12 in accuracy of the entire system.

GARRETT B. LINDERMAN, J R.

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