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. 



Note, In early trial experiments the deflections obtained will 

 probably be too large to be kept well within the scale limits. 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 P.D. 

 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, 



FlG. 57. To illustrate the principle on which a compensator 

 is constructed ; with a battery of I volt, R 1000 ohms and ; I 

 ohm, the P.D. at the ends + and - would be approximately -5-^7; 

 volt; the same P.D. would obtain with a battery of 1.4 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 1.4, 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 e.g., if rio ohms, the voltage obtained .001 ; 

 if r i oo, voltage = .o i , 



