An Electronic Recording Differential Potentiometer Cyril Solomons and George J. Janz, Department of Chemistry, Rensselaer Polytechnic Institute, Troy,
the course of investigations into the physicochemical properties of solutions in molten salts, a thermometric device was needed, analogous to the \vel1 known mercury-in-glass Beckman thermometer, capable of accurate measurement of small changes of temperature and of being used over a wide temperature range (0" to 600" C.). Furthermore, automatic recording of the temperature-time changes was desired t o increase the usefulness of the instrument.
N. Y.
DURING
DESIGN OF APPARATUS
I n the present apparatus the component units are of commercial manufacture: a potentiometer (Rubicon Co. high precision Type B), a microvolt amplifier (Leeds 6: Nortlirup Model 9835-B stabilized D.C.), and a stripchart recorder (Brown Instrunients Co. ElectroniK Type 14, 10 mv.). The principle of operation is illustrated in Figure 1. The e.m.f. from the thermocouple, A , is opposed by a nearly equal e.m.f. from the potentiometer, B. The small out-of-balance e.m.f. of 50 p v . to 2 mv., depending o!i the range setting of the amplifier, is converted to a 60-c.p.s. alternating current signal in the input stages, C', of the microvolt amplifier and is amplified in a five-stage high gain (about 30,000,000) alternating current amplifier stage, D. The amplified signal is reconverted to direct current, by the converter, E, and part of this is fed back so as to oppose about 99.9% of the input signal. The amplified direct current signal is led tlirougli a sis-range attenuator network, F , and is measured as the output across a 100 to 1 potential divider, G, being recorded on a 10-mv. strip-chart potentiometer recorder, H . Because of the very high degree of negative feedback, the instrument is extremely stable. Furthermore, the input voltage drop is so small that there is practically an open circuit condition across the input, as is essential for accurate nullbalance potential measurements. The attenuator network is such that an input of I, 2, 4, 10, 20 or 40 times full scale reading (50 &v.) is required to give the full output of 10 mv. to the recorder. At all times, the key of the potentiometer must be kept locked in the "operate" position. Further details of the design of the 9835-B type amplifier may be obtained from the manufacturer (Leeds 6: Xorthrup Co.). On the most sensitive range ( X 1) of the amplifier, full scale deflection of tlie 10-mv. recorder corresponds to a temperature change for a platinumplatinum-lO% rhodium thermocouple of about 5' C., and for a copper-Constantaii thermocouple of about 1.3' C.
Figure 1. Schematic diagram of temperature-measuring and recording apparatus A.
Thermocouple Bucking potentiometer Input filter ond converter stage of d.c. niicrovolt amplifier 5-stage high gain 0.c. amplifier stage a.c.-to-d.c. output converter 6-range ( X 1, 2, 4, 10, 20, 40) attenuiitor network G. 100 to 1 potenticl divider H. 10 mv. strip-chart potentiometer recorde'
B. C. D. E. F.
By the use of a 1-mv. recorder, temperature changes of one tenth of the values quoted would give full scale deflection. CALIBRATION OF APPARATUS
The actual e.m.f. being measured is the sum of the "bucking" e.m.f. delivered by the potentiometer and of tlie e m f . which is being amp!ified. The best may of ascertaining this latter term is t o feed to the amplifier a series of known e.m.f.'s the appropriate range to give a calibration chart. The potentiometer connected into the circuit may be used for the calibration by shunting across the "test e.m.f." terminals of the potentiometer in place of the thermocouple. The polarities of the leads from the storage battery and the standard cell to the potentiometer are also reversed, because now it is being used as the source itself. After the recorder and the potentiometer have both been standardized in the usual manner (the null point for the latter case being detected on the 50-0-50 pv. meter built into the amplifier) the desired potentials may be fed successively to the amplifier. This method of calibration is particularly efficient, because no extra equipment is required and the minimum of interference to the circuit is required. The leads from the potentiometer to the amplifier and those from the amplifier to the recorder being untouched, stray e.ni.f.'s in these parts of the circuit are automatically allowed for in the calibration. DISCUSSION
The present design possesses the advantage of greater simplicity, sensitivity, and versatility over earlier designs, particularly those ivhich incorporate galvanometers and photo-
cells or which employ photographic methods of recording. The use of thermocouples as temperature sensing elements is recommended, being less subject to change with use or age than circuits )employing thermistors. The apparatus lias been uscd in this laboratory, mainly for the recording of cooling curves during cryoscopic experiments on molten salt solutions in the tempimture range 200" to 400" C. When the apparatus is used with a platinum - platinum - 10% rhodium couple, on range X 1, the full scale deflection of the 10-mv. recorder used corresponis to a temperature change of about 5" C., and temperatures may be readily read off t o the nearest 0.01 " C. or better The chart speed used, 8 inches per hour, has been found suitable for the present series of experiments, th: final 3" of the cooling curve being spiead over 30 minutes, or 4 inches of chart. The arparatus has also been used to nieasui'e the rise in temperature of a calorimeter in the measurement of the heats of fusion of inorganic salts. In these experiments, a copper-Constantan ccluple is used, so that full scale deflection:, on range X 1 may be obtained with temperature changes of about 1.3" C. Temperature changes of this magnitude may be readily measured with an accuracy to better than 0.005" C. ACKNOWLEDGMENT
The authors wish to thank A. G. Keenan, Cniversity of Miami, Fla., for helpful discussions during the design of this apparatus. RESEARCH supported by U. S.Air Force, Air Office of Scientific Research, Air Research and Development Command, Contract No. P.F (638)-50. VOL. 31, NO. 4, APRIL 1959
623
