TEMPERATURE COEFFICIENT OF ICE. JOHNSTONE. 139 



tubing, the air inside escaping throngli the pin-hole. It was 

 found that the water could be frozen with ease in this way 

 without the glass tube being broken. 



The electrometer was then calibrated. In this experiment, 

 the needle was changed to a j)otential of about 200 volts. The 

 electroscope was then calibrated over the range at which it was 

 to be used.-^ 



To obtain a set of readings at different temperatures, the 

 ''U" tube was connected up with the electrometer and electro- 

 scope and source of current, (Fig. 5). In most of the 

 experiments the current was obtained from 10 storage cells, 

 which gave an E. M. F. of about 20 volts. The "U" tube was 

 very carefully packed in the glass vessel with "felt" and so 

 temperature changes were slow. The time, in seconds, for the 

 electrometer needle to pass over 100 scale divisions, was 

 recorded on a stop-watch. The temperature of the ice was then 

 read from the thermo-couple. The potential difference of the 

 two electrodes, c and c^ (Fig. 5), was then determined from the 

 electroscope readings at these points. 



If d ^ the scale divisions passed over per second by the 

 electrometer needle ; 



D ^ the number of divisions per volt, and C ^ the capacity 



of the system in microfarads, then the current, i, passing 



through the ice will be, 



Qd 

 T^Q6 J) amperes. 



If Y ~ the potential diffrvence of the two electrodes c and Ci, 

 then R, the resistance of the electrolyte between c and Ci will 



be, 



V X 10« X D , 



p^— , ohms, 



(J d 



If k denote the cell constant of the apparatus then the 



specific resistance of the ice will be, 



V X 10« X D X Jc 



^ , ohms, 



C d 



1. See Fig. 6. 



