394 Profs. Fleming and Dewar. Dielectric Constants 



The following tables illustrate the immense change in con- 

 ductivity taking place in one or two cases within a range of from 

 10° to 100° :— 



Resistance Measurements of Dielectrics at Low Temperatures. 



Approximate 



Temperature of Galvanometer resistance of 



condenser in deflection the condenser 



platinum in dielectric in 



degrees. centimetres. megohms. 



Sodic Chloride 



(23*6 per cent, solution in water). 



■ 203*0 0-2 2500-0 



-125'0 0-4 1250-0 



• 90-5 26-0 02 



Potassium Iodide 

 (52 per cent, solution in water). 



■200-0 01 5000-0 



-120-0 4-0 1-0 



Potassium Hydrate (KHO) 

 (5 per cent, solution in alcohol). 



-187-0 



01 



5000-0 



-185-7 



0-5 



1000-0 



-183-9 



5-0 



100-0 



-1821 



10-0 



50-0 



-181-5 



4-5 



11-0 



-1791 



120 



4-0 



-178-5 



20-0 



3-0 



-1761 



4-0 



0-9 



-175-2 



7-0 



0-5 



Charging volts changed. 

 Charging volts reduced. 



The observations in the last table show the rapid rate at which a 

 frozen alcoholic solution of potash increases in conductivity between 

 very narrow limits of temperature, whilst the experiments with the 

 frozen solutions of sodic chloride and potassic iodide show the same 

 kind of change, but less rapid, in the case of aqueous electrolytes. 



In each case we have very considerable conductivity in a solid con- 

 dition far below the melting point of the electrolyte, but, at the 

 same time, exceedingly high resistance at the temperature of liquid 

 air. 



On looking at the chart of curves it will be seen that the general 



