US2099349A - Vacuum tube potentiometer - Google Patents

Description

Nov. 16, 1937. F. RosEBURY 2,099,349

VACUUM TUBE POTENTIOME TER I Filed Oct. 3, 1934 W U0 w Patented Nov. 16, 1931 VACUUM TUBE POTENTIOMETER Fred Bombay/New York, N. Y., assignor to University Patents, Inc., New York, N. Y., a corporation of New York Application October 3, 1934, Serial No. 746,678-

'2 Claims. -'(Cl. 175-189) The invention relates to apparatus for measuring a condition or property of a substance, as, for example, its acidity or alkalinity (hydrogenio'n'concentration, or pH"), its state of ionization or dissociation, or its state of electric charge, or mechanical stress giving rise to electric charge; each of these conditions being manifested as an electric potential or electromotive force, but incapable of supplying appreciable energy to an electric measuring circuit without undergoing some change in its condition. Hydrogen-ion concentration is measured with glass electrodes,,a modification of which is described, or other types of electrodes.

The invention consists in the use of athermionic vacuum tube, the control grid circuit of which carries but a vanishingly small current; but said control grid'circuit, when influenced by an electric charge or potential, has the ability to influence the anode or plate current of the vac-' uum tube, such that a galvanometer placed in the anode circuit is caused to deflect to a degree depending on the value of charge or electric potential applied to the control grid circuit.

The invention resides in the manner in which, when there is no change in the electrical potential applied to the control grid of the vacuum tube, the galvanometer in the anode circuit is made to remain substantially at zero, or at some other fixed position, independently of fluctuations in the current supplying the various vacuum tube elements and all other parts of the circuit. This object is accomplishedby making the vacuum tube itself function as part of a balanced bridge circuit: the galvanometer, protected by a variable shunting resistance, being in the position to which it is conventionally assigned in such bridge circuits.

The invention further resides in features hereinafter described and claimed. Reference is had to the drawing, in which Fig. 1 shows the circuit with arrangement for the measurement of hy- ,drogen-ion concentration by means of a glass electrode; Fig. 2 shows in perspective one form of switch device suitable for use in the novel together, with the flne control 4, so as to exactly equal and oppose the E. M. F. of the system to be measured. ,The switching mechanism (shown in detail in Fig. 2) by which adjustments are made on 3, is designed (in a novel. manner) so that the current flowing through resistor 3 is the same regardless of the position of the controls 3a and 3b. This is accomplished by having the two contactors 3a and 31) make contact always on alternate taps of resistor 3, as shown, by a special switch, Fig. 2. A single shaft 99 of some tough insulating material such as a phenolic resinoid carries two contacting arms 30., 3b, and

also some means (not shown) of positioning these arms exactly in contact with the studs lill l l4. These studs are mounted on two insulating plates or discs 93 and 94, which are fixed in relation to each other, one of which may be appropriately slotted for the positioning device. The arms 3a and 9b also make continuous contact (at their opposite ends) with the metal sectors and 96, to which external electrical connections are made. The studs l9|-ll4 are provided with soldering lugs, and the switch is connected in the circuit as shown in Fig. 3, in which the dotted line 98 indicates the common shaft? The knob 91 which is to be provided with a pointer or dial, actuates the switch.

The resistance 4, with contactor 4a, is shunted across 3, and provides a means of obtaining continuous variation of E. M. F. by allowing any fraction of the voltage across the two sections tapped oil by 30!. and 3b, at any of the positions which the contacts 3a and 3b may take, to be included in the circuit. The voltmeter 5 is connected so as to read the voltage tapped oif on 3 and 4, in either a negative or positive direction, the tap 6 on resistance element 3 being the neutral or zero volts point. Binding posts I, 8, and 9 are arranged so that either an external standard potentiometer may be connected for use instead of the voltmeter, or so that the voltmeter may be re-calibrated from time to time by means of an externally connected standard potentiometer. The former isaccomplished by removing thelink Ia. and connecting the potentiometer to posts I and 8, previously having set the voltmeter 5 to exact zero by means of the controls 312-31) and 4a. The latter is done by removing the link Ia and connecting the potentiometer to posts I and 9.

The battery current next passes through resistance element 10 which supplies a fixed grid bias for the vacuum tube, following which the current passes through the filament I1 01 the vacuum tube. The value of current through the series of resistance elements is adjusted to the proper operating valu'efor the tube filament II by previous calculation. The current next passes through resistance element H which supplies a fixed voltage for the space-charge grid l8 of the vacuum tube, and thence through resistance element I2 which supplies a fixed anode or plate voltage to the tube. The resistance element I3 is a balancing resistor and has a value equal to the sum of the resistances of 11, II and I2. The variable resistance element l4 serves as' a compensator for battery drain, and as a sensitivity control, since decreasing its value of resistance raises all the operating voltages and currents on the vacuum tube, and thereby increases its mutual conductance. Control I4 also operates as an adjustment for equalizing the slight variations to be expected in the characteristics of different tubes of the sarrie type.

The resistance elements H and I2 and the vacuum tube filament ll form arm I of the balanced bridge, the element l3 forms arm II and balances thearm I. The resistance elements l5, 2! and 22 are respectively coarse, intermediate and fine variable controls of the galvanometer (20) deflection, and the total of the resistance value tapped oif on these three elements constitutes arm III of the balanced bridge, which is in.

balance when arm III equals the plate resistance ,of the vacuum tube, that is, the in vacuo resistance between the filament l1 and the plate 69; the galvanometer will indicate zero when this condition is satisfied; It will now be seen that, as in balanced bridgevcircuits, the deflections of the galvanometer will be independent of the supply voltage, but dependent only upon the resistance of the various arms. Since, however, the vacuum tube plate resistance, according to the principles involved in the conduction of electricity in vacuo, and also the resistance of the filament H, which increases slightly with temperature, cannot be considered as following Ohms law exactly, the balance of the bridge will be independent of the supply current only over a limited range; This range is s'uflicient, however,

to cover the drop of supply voltage when dry cells are used at the normal current output, which, in the circuit as described, never exceeds 0.1 ampere. Under the most exacting conditions, the requirements for a device of this kind prescribe a zero galvanometer drift only during the time required to make one measurement. This circuit is unique in that it amply meets this requirement.

In operation (refer to Fig. 1), the switch 2 is closed, and after about five minutes to allow for thermal equilibrium oi. the vacuum tube, with the switch 42 closed on contact 48, which connects the grid 23 to the fixed grid bias supplied by resistance element It and a portion of element 3 (which is permanently adjusted by calculation so as to give a grid bias sufliciently negative to insure minimum grid current, but not negative enough to reduce sensitivity to too low a value, determined from the characteristics of the vacuum tube), .the galvanometer'is de-shunted'by moving shunt switch 34 from contact 39 to'contact 38, thence to contact 31, at the same time in the circuit when switch 34 is on contact 31,

full sensitivity and correct damping characteristics are obtained on the galva'nomete'r. Ihe

galvanometer is thus brought to zero. The leads from 44 and I now being connected to the system whose E. M. F. is to be determined, switch 42 is thrown to contact 44, connecting the system under test in series with the grid bias. If the E. M. F. of the system under test is greater or less' than zero volts, the grid 23 of the vacuum tube will now be subject to its initial charge (as supplied by Hi and part of 3), plus or minus the E. M. F. of the test system. Accordingly, the plate current, and hence the plate resistance will change, throwing the bridge ofl. balance, whereupon the galvanometer will show a deflection, the direction of which will indicate whether the unknown is positive or negative in potential, and in which direction the voltage divider 3 and the fine control 4 must be adjusted so as to oppose and nullify the unknown potential of the test system. This is now done, until the galvanometer needle returns to zero. The voltmeter 5 now automatically indicates the E. M. F; of the system being measured, since this instrument is shunted across the portions of the network 3 and 4 which have been included in the grid circuit in order to bring the grid bias back to its initial value. In other words, if the initial grid bias with the switch 42 on contact 48 is E, the E. M. F. of the system under est E, and the portion of network 3 and 4 E", since the galvanometer will indicate zero with but one value of grid bias- E. M. F., all other conditions being equal,

- 'E'iE -E"=E and In order to check the reading, switch 42 is now moved back to contact 48, the galvanometer adjusted to zero if necessary, with control 22, switch 42 thrown to contact 44, whereupon there should be no deflection of the galvanometer needle; a slight deflection is corrected with control 4 and the voltmeter again read.

Since the measurement of E. M. F. with this device is practically an electrostatic eifect, the

measurement of E. M. F. is eflected (a) without drawing appreciable current or energy from the system being measured, (b) through very high resistances (in excess of 100 megohms) without appreciable error, or (c) on systems whose internal electrical resistance is very high. These features make the invention'eminently applicable to the measurement of, for example, hydrogen-ion concentration or "p with glass or any other type of electrode without the necessity of using a tapping key whereby current is drawn only momentarily. Further, with suitable modifications, extensions, or additions, the essential features of the invention can be incorporated into a mechanism for controlling pH or any other property or condition of a substance which manifests itself as an E. M. F., by the use of amplifiers, photoelectric cells, and/or relays, to operate valves, rheostats, or any other electrical or mechanical controlling device.

Since the galvanometer is in a no-current position in this circuit and the plate load resistance is effectively equal to the internal plate re sistance of the vacuum tube, the full mutual conductance, or sensitivity, of the tube is attained. The vacuum tube used is the General Electric Company's FP-54, which has a mutual conductance of 25 microamperes per volt. The features of this circuit consequently allow a galvanometer deflection equivalent to one microampere for every 40 millivolts change in the voltage applied aoeasco I control grid, space-charge grid, and anode, a

anywhere in the control grid circuit. with a galvanometer having a sensitivity of 0.25 microampere per scale division, one scale division corresponds to a change of 0.01 volt in the control grid circuit, and 0.001 volt may be estimated. A gaivanometer having the sensitivity mentioned is a rugged, compact, pointer type. .Hence the high sensitivity of this circuit obviates the necessity of using an expensive and clumsy reflecting type galvanometer which has been practically a necessity with all previous circuits. This is ments of hydrogen-ion concentration (pH) by.

'a feature-which allows portability. Also, the use of such a galvanometer eliminates completely all eifects due to vibration and to external electrostatic or magnetic disturbances.

This instrument is calibrated for measurethe use of one or more standard bufler solutions. The formula used is:-

Where the subscript s indicates the standard solution, the subscript :1: indicates the unknown solution, E=E. M. F. in volts,

T=a temperature factor (0.0591 at 25 0.),

and

pH=H-ion concentration.

ing small voltages. comprising in combination an electron-discharge device having a filament, a

control grid, and a' plate electrode, a single battery for supplying a heating current to said filament, a plurality of resistances connected in series with'said'filament, "connections from-said resistances to the grid and plate for impressing respectively negative and positive voltages thereon with res t to the filament, a switch for impressing a unknown voltage on-said control grid, an indicating instrument associated with the plate circuit ofsaid electron-discharge device, a pair of resistances across which the in- 1dicating instrument is connected, there being a connection at a point intermediate the said'pair oi resistances to the positive side of the battery, the said pair of resistances constituting two arms of'a Wheatstone bridge of which another arm is constituted by the positive voltage-producing resistance and the filament and a final. arm being constituted by the plate resistanceof the electron-discharge device, together with an adjustable voltage divider, including resistances in series with the battery and the said plurality of resistances, the voltages set up thereby being adapted to be impressed upon the control grid.

2. A vacuum.- tube potentiometer for measur-' ing small voltages, comprising in combination a fourelement vacuum tube having a filament,

single battery for supplying a heating current to said filament, a plurality of resistances con-' nected in series with said filament, connections from said resistances to the control grid and to the.space-charge grid and anode for impressing respectively negative and positive voltages thereon with respect to the filament, a switch for impressing an unknown voltage on said control grid, an indicating instrument associated with the plate circuit of said four-element vacuum tube, a pair of resistances across which the indicating instrument is connected, there'being a connection at a point intermediate the said pair of resistances to the positive side of the battery, the said pair of resistances constituting two arms of a Wheatstone bridge of which another arm is constituted by the positive voltage-producing resistance and the filament and a final arm being constituted by the plate resistance of the fourelement vacuum tube, together with an adjustable voltage divider, including resistances in resistances, the voltages set up thereby being adapted to be impressed upon the control grid.

. series with the battery'and the said plurality of 3. A vacuum tube potentiometer for measuring small voltages, comprising in combination an electron-discharge device having a filament, a control grid, and a plate electrode, a single battery for supplying a heating ,current to said filamerit, a plurality of resistances connected in series with said filament, connections from said resistances to the grid and plate for impressing respectively negative and positive voltages thereon with respect to the filament, a switch for impressing an unknown voltage on said control grid, an indicating instrument associated with the plate circuit of said electron-discharge device, a

, there being a coninstrument is connect diate the said pair of nection at a point inter pair of resistances aorgz; which the indicating d resistances to the positive side of the battery,

the said pair of resistances constituting two arms of a I Wheatstone bridge of which another arm is constituted by the positive voltage-producing resistance and the filain v constituted by the plate resistance of the electrondischarge device, together with an adjustable voltage divider, including resistances in series with the battery and the said plurality, of resistances, the voltages set up thereby being and a voltage measuring instrument connected in circuitwith the voltage divider.

'4; 'A vacuum tube potentiometer-of the character set forth in claim 1, in which one of the pair of resistances constituting the two arms of the Wheatstone-bridge is adjustable.

5'. A vacuum tube potentiometer of the character set forth in claim 1, in which one of the pair of resistances constituting the two arms of the Wheatstone bridge is adjustable, and'-' the other balances the positive voltage-producing re.-'

sistance and filament.

6. In a vacuum tube potentiometer of the-character set forth inclaim 1, the? provision of an adjustable-resistance in series with the battery.

'7. In a vacuum tube potentiometer of the char acter set forth in claim 1, the provision of an adjustable resistance shunting the indicating instrument FREDROSEBURY.

ent and a final arm being adaptedto be impressed upon the control grid,

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