HALL. — CONDUCTIVITY OF SOFT IRON. 137 



to add to the description is this, — that in these later experiments the 

 freedom of movement of the covers of the gas-holders, upon which the 

 accuracy of regulation of the gas pressure depends, has been greatly 

 increased by keeping the hammer of an electric bell in brisk action 

 against the side of each gas-holder. When the regulating devices were 

 working well the maximum variation of temperature in either stream 

 during one of the main sets of observations, occupying some 45 min- 

 utes, was usually about one fifth of a degree C. 



Results in Detail for Thermal Conductivity. 



Experiments on conductivity of the iron with the apparatus as de- 

 scribed in this paper were begun July 13, 1899. The results of that 

 day and of July 14 are not here given, as they were obtained before the 

 full importance of some precautions was recognized ; but all the results 

 obtained later are recorded below. The subscript 1 refers to cases in 

 which the warmer stream of water ran above the disk, the subscript 2 to 

 those in which it ran beneath the disk. T, the mean of T x and T 2 , is the 

 mean temperature of the disk during a pair of complementary trials. K, 

 the mean of K L and K 2 , is the mean value of the conductivity given by a 

 pair of complementary trials. The results obtained July 31 and August 2 

 are distinguished from the others by the fact that they were made after a 

 certain change in the position of the thermo-metric spirals (see p. 135). 



0.1560 0.1468 58°.3 0.1514 



