430 
PROFESSOR C. H. LEES: THERMAL AND ELECTRICAL 
disappear, the temperature and the values of R for balance were determined 
accurately. 
Liquid air was then poured into the Dewar tube, and when the temperature had 
become steady, the observations of temperature and of values of R were repeated. 
The liquid air was then poured out of the Dewar tube, and the temperature rose 
slowly. During the rise several observations of temperature and of R were made at 
intervals. After three or four observations had been taken the temperature of the 
apparatus was raised 40° or 50° by sending a current for a few minutes through the 
manuanine wire wound on the outside of the brass tube. When the rate of rise of 
temperature was again normal, observations were again taken. These steps were 
repeated till the original temperature of the rod was reached. In no case did the 
value of the resistivity at the end differ appreciably from its value at the beginning 
of an experiment. 
Reduction of the Electrical Conductivity Observations. 
In order to obtain the temperature of the rod from the measurements of the 
resistance of the platinum thermometer and its leads, we require to know the 
resistance of the leads and the constants of the thermometer. The resistance of 
the leads was determined by placing an equal length of the same wire alongside the 
leads and determining its resistance for various temperatures of the rod during one 
of the experiments. The values thus found were subtracted from the resistances 
determined by the bridge in each experiment. 
The constants of the platinum coil were found by comparison with the two coils 
previously used in the heat conductivity measurements. The three coils were 
made in the same way of the same wire, and the new coil agreed throughout 
with the old “ B ” coil. The constants used in the reduction of the resistance 
measurements to temperature in the former experiments were therefore used in the 
present case. 
As it was not the practice to wait till the temperature was quite steady before 
observations of temperature and resistance were made, there may be a slight differ¬ 
ence of temperature between the rod and thermometer. Attempts were made to 
determine the magnitude of this difference by raising or lowering the temperature of 
the zinc rod at about the same rate as was usual in an experiment, i.e., 1° C. per 
minute, and comparing the results. No difference of temperature of the thermometer 
so large as the experimental error, , 2° C., could be detected between observations 
which gave the same value ol the resistance of the rod with rising or falling tempera¬ 
ture. A rough calculation from the dimensions of the mica ring on which the 
thermometer was wound showed that the difference of temperature of rod and 
thermometer could not exceed one-tenth of the change of temperature of the rod in 
one minute. Whenever in an experiment the rise of temperature has exceeded ‘5° C. 
