92 THE ROYAL SOCIETY OF CANADA 



In the practical form the instrument is composed of two boxes. 

 The main dial AB consists of 101 coils of wire in series, each of 1000 

 ohms resistance. The ends of each coil are connected to studs tipped 

 with platinum-iridium or with gold, which are set in the top of the 

 box, usually an ebonite slab. A double revolving arm makes contact 

 with a pair of studs including between them two of the 1000 ohm 

 coils. The second or ' ' vernier ' ' dial E'F' connected through negligible 

 resistance to E and F respectively, consists of 100 coils of 20 ohms 

 each, so that the equivalent resistance from A to B is reduced to 

 100,000 ohms. The ends of each of the 20 ohm coils of the vernier dial 

 are also connected to platinized studs arranged in dial form. A single 

 revolving contact spring can make contact with these one at a time 

 and is connected to the galvanometer terminal. 



If n is the reading of the first half of the double contact spring on 

 the main dial and if m is the reading of the vernier spring for a given 

 balance point in an arrangement similar to that shown in the diagram, 

 then we have from equation (2), that. 



n_ = 1000m + 10m 



r 2 ~ 100,000- 1000m- 10m 



If the instrument is used as a potentiometer, then 1000m + 10w corre- 

 sponds to the usual potentiometer reading. 



The same slide as that discussed by Prof. Callendar was used by 

 the writer. The corrections, however, which he determined in 1894, 

 were found to have changed by varying amounts. It has been found 

 that the continual slow change in the resistance of any of the coils 

 measured relatively to the rest of the coils, may amount to several 

 parts in a hundred thousand within a few months. The ratio of the 

 two halves of the box should be frequently tested in order to give 

 warning of any accidental flaw. This is not, however, a sufficient test 



