[boyle] 



TENSILE STRESS ON ELECTRICAL RESISTANCE 



173 



it was described by Professor Callendar in Phil Trans,, A, 1902, p. 63. 

 The potentiometer consists of two sets of coils, wliicli are the potentio- 

 meter slide and the vernier slide. The former has one hundred and one 

 lUOO-ohm coils in series, and the latter one hundred 20-ohm coils in 

 series. The terminals of the vernier slide are permahenlly fixed to the 

 poles of a two-pole pointer on the potentiometer slide; by means of this 

 arrangement the whole of the vernier coils is shunted with any two con- 

 eecutive coils of the potenciometer slide, thereby throwing in parallel 

 two resistances of 8000 ohms each, making an equivalent resistance of 



Fig. 1. 



1000 ohms. Thus the resisrance of the whole potentiometer is 100,000 

 ohms. The balance point is effected on the vernier slide by means of 

 a pointer connected with the galvonometer. 'I'he galvonometer used was 

 a Kelvin 100,000-ohm reflecting instrument. 



The standard cell, used in connetion with the potentiometer to test 

 the steadiness of the current, was a cadmium inverted cell with an E.M.F. 

 change of .0058 per cent, per degree of temperature. 



The errors of the various coils of the potentiometer are less than 

 1 part in 10,000; determinations of the errors have agreed in all cases 

 for the 101 coils to a tenth of this amount. 



