. Biophysical research methods. Biophysics -- Research. V. TEMPERATURE DETERMINATIONS 159 The Wheatstone bridge circuit {25)—as used with the four lead platinum resistance thermometer—is shown in Figure 12. In this circuit: R1/R2 = Rz/R. (6) Two observations are required with this circuit to measure the resist- ance of the terminal element. R\ and R2 are the resistances of the equal ratio arms. 7?3 is the resistance required to balance the bridge. Rx is the resistance between the branch points of the thermometer, the four leads of which are designated as A, B, C, and D. By means of the slide w
. Biophysical research methods. Biophysics -- Research. V. TEMPERATURE DETERMINATIONS 159 The Wheatstone bridge circuit {25)—as used with the four lead platinum resistance thermometer—is shown in Figure 12. In this circuit: R1/R2 = Rz/R. (6) Two observations are required with this circuit to measure the resist- ance of the terminal element. R\ and R2 are the resistances of the equal ratio arms. 7?3 is the resistance required to balance the bridge. Rx is the resistance between the branch points of the thermometer, the four leads of which are designated as A, B, C, and D. By means of the slide wire the ratio of the arms containing Rz and 7?^ may be adjusted to equality within one or two parts in ten million. Once the Fig. 12. Bridge circuit for four lead, precision resistance bridge has been balanced, with the battery connected to thermometer lead C, a commutator with amalgamated contacts is used to switch the battery to lead B. The positions of thermometer leads A and D are interchanged, and the bridge is again balanced. Since it can be shown that: R,= {D + A)/2 (7) the reversal of the leads wall cancel out any effects of resistance in the leads and the true resistance of the thermometer will be obtained. A small variable resistor is sometimes placed in one of the thermom- eter leads to equalize the lead resistance in the two positions of the commutator. Unless it is desirable to determine temperature dif-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Uber, Fred Murray, 1905-. New York : Interscience Publishers
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