414 DR. C. H. LEES ON THE THERMAL CONDUCTIVITIES OF SOLIDS 



If the test was to be carried out at a moderate temperature, current was also sent 

 through the coil encircling the air bath for a short time, then stopped. Owing to 

 the heat supplied by the coil to the discs, and given up by them to the bath, the 

 temperature of the bath remains higher than that of the air, and would eventually 

 become constant. The currents through the coil of the discs, and through the coil 

 round the bath, were, however, adjusted from time to time, in order to attain the 

 steady state of temperature as quickly as possible. When the temperatures of the 

 discs and of the air bath had remained constant for 10 or 15 minutes, observations of 

 the wattmetei-, and of the electromotive forces in the various thermo-circuits, were 

 made, an interval of 5 or 10 minutes allowed to elapse, and the observations repeated. 

 If the second set did not show that the steady state had been reached, the tempera- 

 ture of the bath was varied suitably, and observations repeated. In this way it was 

 insured that the observations were made during a state which approximated with 

 sufficient closeness (p. 404) to the steady state. 



If the test was to be carried out at a lower temperature, cold water was allowed 

 to circulate between the inner and outer walls of the air bath, the observations being 

 taken as before. 



If the test was to be carried out at a higher temperature, the current through 

 the heating coil was increased for a time, and current sent through the coil on the air 

 bath till its temperature had been raised to the required amount. The current 

 through the heating coil of the discs was then reduced to its normal amount, and the 

 temperatures throughout allowed to become steady. Observations were taken as 

 before. In cases where there was any suspicion of a change of the material under 

 test having been brought about by the increase of temperature, for additional security 

 the temperature of the bath was again lowered and a second test made at the original 

 temperature. 



The following tables and curves show the substances tested and the results 

 obtained : 



