194 PROCEEDINGS OF THE AMERICAN ACADEMY 



of all precautions, however, there were large thermo-electric effects. 

 Sometimes these effects were sufficiently large and sufficiently con- 

 stant to carry the index of the mirror galvanometer completely off the 

 scale and hold it there. In such cases a compensating arrangement 

 was resorted to, which consisted of a short wire inserted in the galva- 

 nometer circuit, which wire formed also part of another circuit contain- 

 ing a Daniell cell and a variable resistance. This arrangement tended 

 to send through the galvanometer a small current, which was made to 

 oppose and nearly neutralize the thermo-electric current from the 

 nickel strip. There were, nevertheless, serious fluctuations of the gal- 

 vanometer index when high temperatures were used, and at the high- 

 est temperatures, where, it will be observed, the Hall effect in nickel 

 was very small, it was necessary to take a long series of readings in 

 order to obtain a result even approximately satisfactory. The follow- 

 ing table shows the results arrived at, the Hall effects being expressed 

 as before, in divisions of the galvanometer scale. 



The observations now recorded for nickel have been used in plotting 

 the curves B and C in Figure 1. The nickel strip was, like the steel 

 strip, so thin that the magnetic induction per square centimeter through 

 it is assumed to have been the same as the strength of the field in 

 which the strip was placed. Accordingly, the value of the magnetic 

 induction is constant for each curve. The abscissas represent tem- 

 peratures and the ordinates represent Hall effects. These ordinates 

 for the nickel curves are reduced to the same scale as those for the 

 steel curve; that is, the curves are an attempt to show how the Ilali 

 effects in nickel, with magnetic inductions 1500 and 3000, would 

 compare in magnitude with those in steel, with magnetic induction 

 1400, if the strips were of the same dimensions and carried the same 

 primary current. 



