tinium bridge voltage valves for obtaining maximum response from Type A l l l thermistors as shonn in Figure 6 are not directly applicable to a different bridge design or to different thermistors. I n order to establish optimum operating conditions for any type of thermistor in any giren bridge design, the following simple method can be used. The method involves plotting a bridge voltage-response curve or a bridge current-response curve obtained by injecting repetitive samples into the gas chromatography apparatus. Air is frequently a good standard to use. 4 curve having a maximum is obtained (similar to the ones in Figure 6 a t 41’ and 80’ C.) if sufficiently high current or voltage values are used. The curves obtained in Figure 6 a t 126’ and 155’C. did not shoIv this maximum, because of the limited bridge voltage applied. The maximum becomes less pronounced a t higher temperatures. The optimum operating bridge current or voltage value for maximum signal-noise ratio is at the top of the response curve. This value shifts to increased voltages at increased temperatures. -4s the absolute bridge voltage or current value for maximum signal from a given thermistor varies with the bridge design, a universal method for evaluating optimum thermistor response is needed. One such method involves the plotting of power dissipation in the thermistor against response in a manner similar to the bridge voltage response curves. Again a maximum is obtained corresponding to the optimum operating condition for the thermistor. The power dissipation is equal to the product of the current through the thermistor and the voltage drop across the thermistor. These current and voltage values are easily determined by placing a milliammeter in series with the thermistor and a high impedance voltmeter across the thermistor. A constant power dissipation value for maximum signal is obtained for the same thermistor in any bridge dfsign. The value also remains approximately constant a t various operating temperatures. I n addition, t h r optiniuni power dissipation value remains constant for other thermistors having similar temperature coefficients and physical size (similar dissipation constants). Different optimuiii values
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resistance thermistors n ithout as much amplification. Unfortunately, the noise level of the thermistor increases proportionally to the increased signal. Hon ever, a t higher temperatures the higher resistance thermistors are useful, as the high amplification required t o obtain parts per million sensitivity for low resistance thermistors causes noise due to soldered connections, etc., t o become significant.
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ACKNOWLEDGMENT
The authors wish to acknowledge the extensive contribution made by Richard A. Parkinson, who performed a large part of the experimental Ivork described. LITERATURE CITED IO 20 30 POWER DISSIPATION, MILLIWATTS
Figure 7. Power dissipation-response curves for 2000-, 8000-, and 80,000ohm thermistors
are obtained with thermistors having different physical size. Power dissipation-response curves for 2000-, 8000-, and 80,000-ohm thermistors (Types A l l l . IX1039, and ilX1040, respectively, obtained from T7ictory Engineering Gorp.) are shon n in Figure 7 . The points on the curves nere obtained from repetitive injections of nhexane in the gas chromatography apparatus a t 64’ C. The bridge design used with each thermistor was similar to the one described in Figure 2, except that the fived resistors were changed to provide a n approximately equal arm bridge a t the operating temperature. I n addition, the attenuator n-as eliminated and the signals measured on a 500- to 1600-mv. variable range recorder. The same optimum power dissipation value n-as obtained for each type of thermistor as shown in Figure 7 . This constant value holds true only as long as the thermistors have similar dissipation constants. The signal from thermistors having similar characteristics increases approximately as the square root of the resistance. The data in Figure 7 show this to be true, as the signals obtained from 2000-, 8000-, and 80,000-ohm thermistors are in the ratio of 1:2:6. This increased signal from the higher resistance thermistors is helpful in obtaining equivalent sensitivity of Ion er
Boer, H., “Comparison of Detecting Nethods for Gas Chromatography Including Detection by Beta Ray Ionization,” Symposium on Vapor Phase Chromatography, Hydrocarbon Research Group, Institute of Petroleum, London, June 1966. Deal, C H., Otvos, J. W., Smith, Y. S . , Zucco, P. S.,A\.IL. CHEX.28, 1958 (1956). Evans, J. B., Killard, J. E., J . -4m. Chetn. SOC.78, 2908 (1956). Griffithe, J., James, D., Phillips, C., .Inalyst 77, 897 (1952). James. A . T.. 1Iartin. A. J. P.. Bio&em. J . 50, 679 (1952). ( 6 ) Kirkland, J. J., Grasselli Department, E. I. du Pont de Semours 8: Co., Inc.,,Ki!mington, Del., private communication. ( i )AIartin, .4. J. P., Brit. M e d . Bull. 10, 170 (1954). (8) Scott, R. P. W., .Ynlure 176,793 (1965). RECEIVEDfor review March 20, 1957. Accepted December 7 , 1957. 9th Annual Delan-are Chemical Symposium, X’eTyark, Del.. Februarv 16. 1957. and Division of Anaiytical Chem’istry, ’ 131st Meeting ACS, AIiami, Fla., April 1957.
Alternating Current Polarography. Determination of Transfer Coefficient of Electrochemical Processes-Correction T n o n-ords have been transposed in footnotesd and e to Figure 1 of the paper on “Alternating Current Polarography” [AXAL.CHEN.30, 312 (195S)l. The footnote. should read: dmplificntion control (rarbon potrntiometcr. ctc.) e Frcqwnc>- control (ganged c a r h i , etc.) P. J. ELYIXG
