Prof. Thomson, Electrification produced hy heating Salts. 105 
Electrification produced hy heating Salts. By J. J. Thomson, 
F.R.S., Trinity College, Cavendish Professor of Experimental 
Physics. 
[Read 12 November, 1906.] 
Beattie found that when sodium chloride mixed with iodine is 
sprinkled on a metal plate, large quantities of positive electricity and 
some negative electricity are given off when the plate is heated 
to a temperature somewhat over 300° C. This and similar effects 
have been investigated by Garrett and Willows, who find that the 
electrification is due to the formation and subsequent dissociation 
of the iodide of the metal of which the plate is composed. 
Wehnelt has shown that when an oxide of calcium or barium is 
heated large quantities of negative electricity are given off. The 
following experiments were made with the object of seeing 
whether any light could be thrown on the phenomenon by in- 
vestigating whether there was any relation between the sign and 
amount of the electricity given off when the salt is heated and 
the chemical nature of the salt. 
Method. 
The salt to be investigated was placed on a porcelain tube, 
this tube was filled with a badly conducting powder called 
‘ Kryptol,’ and the heating was effected by passing a current of 
electricity through the powder. This method was found to be 
very convenient, as by altering the current wide variations could 
be produced in the temperature, and, what is more important for 
these investigations, the temperature can be kept very uniform. 
As the porcelain when heated was found to give off considerable 
quantities of electricity, the part surrounding the kryptol was 
Fig. l. 
wrapped with thin mica which gives off very little electricity, and 
the salt to be tested was spread thinly and uniformly over the 
