Equipotential Lines of an Electric Current. 143 



ably became heated much more slowly than a thermometer 

 which was kept in the bore of the electromagnet. This ther- 

 mometer rose usually about 5° Centigrade during one set of 

 observations on the transverse current when the strongest mag- 

 netic fields were used. If it be assumed that each of the 

 crosses rose in temperature one-half as fast as this thermome- 

 ter, the conclusion is that the mean temperature of the cross 

 during: such a set of observations was not more than one or 

 two centigrade degrees above its temperature at the beginning. 

 This initial temperature was probably known within one or 

 two degrees usually. According to experiments with other 

 specimens of iron and steel,* a change of two degrees in tem- 

 perature should cause a change of about 1 per cent in the 

 transverse effect in the crosses It is improbable that our con- 

 clusions will be seriously affected by the uncertainty as to 

 temperature. 



The sensitiveness of the astatic galvanometer was another 

 variable. This was determined three times in connection with 

 this series of experiments by passing a known current through 

 the galvanometer and noting the deflection produced. 



The results found were : 



Sensitiveness. 

 June 4th ... 100 



June 13th . . . 101-4 



June 20th . . ... 102*6 



In the meantime the galvanometer was not touched, so far 

 as I am aware, by anyone but myself and I made no intentional 

 changes in its condition. In making reductions it will be 

 assumed that its change of sensitiveness was uniform from one 

 date to the next. This may leave room for an error of 1 or 

 2 per cent at some points. 



Rough observations of the temperature of the astatic gal- 

 vanometer circuit were made in order to correct for changes 

 of resistance in this circuit. 



A rough preliminary series of experiments was made, be- 

 ginning with fields of moderate strength and proceeding to 

 stronger and stronger ones, but the troublesome charac- 

 ter of the investigation and the pressure of other work 

 have made it necessary to limit the more careful and thorough 

 study to the very strongest fields and to those of medium 

 strength. Even with this limitation the results reached are 

 not in all respects harmonious and satisfactory. The data are 

 given below in some detail in order to show the scope of the 

 corrections and reductions which have been made. 



In the tables below — 



* This Journal, Feb., 1885. 



