. Eight lectures on the signs of life from their electrical aspect. Electrophysiology. APPENDIX 153 voltage from the compensator is allowed to flow in the circuit, which must be of known resistance (Fig. 59). It is convenient to use a carbon megohm for calibration {vide infra p. 169). In those cases where an alteration of resistance actually occurs during the course of the experiment, the value of the standard voltage must be taken before and after. JSlote.—In early trial experiments the deflections obtained will probably be too large to be kept well within the scale hmits. The best way to obt
. Eight lectures on the signs of life from their electrical aspect. Electrophysiology. APPENDIX 153 voltage from the compensator is allowed to flow in the circuit, which must be of known resistance (Fig. 59). It is convenient to use a carbon megohm for calibration {vide infra p. 169). In those cases where an alteration of resistance actually occurs during the course of the experiment, the value of the standard voltage must be taken before and after. JSlote.—In early trial experiments the deflections obtained will probably be too large to be kept well within the scale hmits. The best way to obtain a readable deflection is to shunt the galvano- meter, thus sending only a convenient fraction of the total current through the instrument. The compensator or potentiometer is a means (i) of supplying a standard voltage, and (2) of compensating and so measuring currents derived from the 'object under examination. Further, the compensating current affords the quickest means of verifying the integrity of the general circuit and of the galvanometer. In its simplest, and for all ordinary purposes, sufficiently accurate form, the compensating arrangement consists of a Leclanche cell,. Fig. 57.—To illustrate the principle on which a compensator is constructed ; with a battery of i volt, R = looo ohms and ;' = I ohm, the at the ends + and - would be approximately ^^-^ volt; the same would obtain with a battery of volt and R = 1400 ohms. Compensation is established by varying r. joined up with two resistance-boxes, which act as numerator and denominator of any convenient fraction of a volt. Taking the voltage of the cell as , and the resistance (R) of the denominator to be 14,000 ohms, than the resistance (r) of the numerator reckoned in ohms will give a voltage in ten-thousandths at the electrodes—^ if r=\o ohms, the voltage obtained = .001; if r = 100, voltage = .01,. Please note that these images are extracted from scanned page images that may have
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