A SIMPLE BRIDGE BALANCE INDICATOR FOR CONDUCTANCE MEASUREMENTS E. B. THOMAS and R. J. NOOK John Carroll University, Cleveland, Ohio
FOR detection of the point of balance of alternating current bridges any device with the sensitivity of a good pair of headphones will suffice. The chief advantages offered by visual detection methods are freedom from fatigue for the operator and usability in noisy surroundings. Many bridge balance indicators have been described,' some using meters and others employing electron-ray and oscilloscope tubes as the balance indicating components. In all probability none of the visual detection methods approaches the sensitivity of a tuned audio-amplifier working into a pair of headphones, but the precision with which the point of balance can be read on most bridges seldom warrants such sensitiveness in the detecting device. The device described below and diagrammed in Figure 1,has been used for several years with an impedance bridge of the General Radio type, modified as shown in Figure 2B, for the determination of the conductance of solutions. Lately this same device has been used with a General Radio type 650-A bridge without modification, since it was found to be sufficiently precise for routine conductance measurements and for purposes of instruction. Nothing original is claimed for the individual portions of this circuit,%but its performance as a unit and the simplicity of the operating procedure are such as to warrant recording ithere. The circuit consists of a conventional pentode amplifier, resistance-capacitance coupled to a triode amplifier (the triode section of the 6SQ7), the output being further amplified by a 3: 1 audio transformer and impressed on a diode rectifier. The rectified and filtered output of the diode section of the 6SQ7 is fed to the grid of the indicator tube. Point of balance is indicated by maximum width of the nonfluorescing sector of the 6E5 tube. Personal preference would call for minimum shadow angle rather than maximum a t the balance point; however, the simplicity of the circuit, freedom from adjustments, and slightly greater ratio of change of shadow angle to grid voltage change with the 6E5 tube argue for the maximum. None of the circuit values seems to be critical. Adequate filtering and decoupling are em1 HAGUE, B., "Alternating Current Bridge Metliods," Fifth edition Pitman & Sons, Ltd., 1945,616pp. , M. E. DROZ, "ExperiMBLLER,R. H., R. I,. G m m ~ AND mental Electronics," Prentice-Hall, Inc., New York, 1942, 330 PP. The detector circuit is similar to that of Garman and Kinney (Ind. Eng. Chem., Anal. Ed., 7 , 319 (1935)), and to many other circuits appearing since then, most of which have been used in condenser checkers.
T I POWER TRANSFORMER 220 V. C.T. 40 MA.
F i g 1.
Bridge-Balance Indicator
ployed in the power circuit; all points a t ground potential are connected to the chassis by the shortest possible leads, and the chassis is grounded to the nearest water line during operation. This grounding of one side of the input of the detector (and output of the bridge) necessitates the use of transformer coupling between the oscillator and the bridge, but it allows the point of bridge balance to be determined without the use of a Wagner Ground. The built-in 1000-cycle source of the G-R 650-A Bridge incorporates such a transformer. Sensitivity of the detector may be controlled by the '/s-megohm input potentiometer and preliminary bridge balance adjustments should be made a t reduced sensitivity, final measurements being made a t full sensitivity. This practice is especially convenient, and indeed necessary, when voltages are equal but out of phase a t the point of balance for the resistive elements. Unless compensation is made for the phase shift introduced by the capacitance of the conductance cell complete balance cannot be attained, and the best indication obtained will be a slight hum if headphones are employed, or a nonfluorescing sector of considerably less than 90 degrees with an indicator of this type. In conductance measurements this difficulty is noticed when dealing with solutions of high ohmic resistance, and is likewise more pronounced the higher the frequency employed. The compensation usually employed consists of a variable capacitor across either
JOURNAL OF CHEMICAL EDUCATION
A Bridge; with an external oscillator, compensation was necessary. In these measurements point of balance was always approached with the detector a t reduced sensitivity, measurements being taken a t the highest practical sensitivity. The values of the resistances measured with and without compensation agreed within the precision with which the bridge could be read. In attempts to increase the precision with which the bridge could be read, a vernier rheostat (1100 ohms) and a selector switch controlling a decade of 1000ohm precision, noninductively wound resistors, series connected, were introduced into the conventional bridge as shown in Figure ZB, retaining the 11,000ohm rheostat (variable arm of the bridge) but having this 11,000-ohm rheostat operative only when a nonlocking push-button switch was depressed. By pressing this button and varying the 11,000-ohm rheostat of the arms adjacent to the cell, the numerical value over its course, the point or region of balance may be of the capacitance required being a function of several quickly established. The selector svitch may now be variables such as the frequency of the a. c. employed, set to the nearest value of resistance lower than that the ohmic resistance of the cell, the ratio of the fixed indicated by the 11,000-ohm rheostat, the push button arms of the bridge, and which of the arms adjacent to released, and final adjustment made with the 1100the cell is to be shunted by this capacitance. By ohm rheostat. If the bridge is to yield a reading employing a bridge which is grounded as shown in directly in ohms this vernier rheostat must be calibrated Figure 2A complete compensation may be attained from zero to full scale a t either 10- or 20-ohm intervals, by use of a variable capacitor between point A and the scale being marked from 0 to 1.10 with the smallest ground. In work of a routine nature a t 1000 cycles per division being 0.01 or 0.02 depending on whether the second on solutions whose ohmic resistances varied 10- or 20-ohm intervals were chosen for calibration. from 10 to 50,000 ohms it was found that point of While this vernier arrangement has worked satisfacbalance could be determined without even the above torily on one unit which has been in operation for four mentioned compensation, when the source of a. c. years, i t is not considered of sufficient advantage to was the built-in 1000-cycle hummer of the G-R 650- incorporate it into newer bridges.
