346 



Mr. S. Bidwell. On an 



[Feb. 21, 



resistance will be changed in a corresponding manner, and the equi- 

 potential line will be rotated in the opposite direction MN. 



The peculiar thermoelectric properties of copper and iron dis- 

 covered by Sir William Thomson are thus seen to be sufficient to 

 account for Hall's phenomena in the case of those metals. 



It has often been said that the best test of the truth of a theory is 

 the power it gives of accurate prediction. If the explanation which 

 I have offered of Hall's phenomenon be the correct one, it is clear 

 that Hall's "positive" metals, cobalt and zinc, should when tested 

 according to Sir William Thomson's method exhibit the same thermo- 

 electric effects as iron; that his "negative" metals should behave 

 thermoelectrically in the same manner as copper ; and that lead might 

 be expected to be thermoelectrically unaffected by strain. It became 



Fig. 5. 



exceedingly interesting to ascertain if this was the case, and I there- 

 fore proceeded to repeat Thomson's experiment upon all the metals 

 mentioned by Hall. When it was conveniently possible to do so, the 

 metal was used in the form both of wire and of sheet or foil, and 

 except in the case of cobalt, the following simple method of operating 

 was adopted. 



A short piece of the wire or strip of the foil to be examined was 

 held at the ends by two fixed brass clamps, about 5 or 6 centims. 

 apart, which were connected with the terminals of a low resistance 

 reflecting galvanometer. A point near the middle of the metal 

 was then gripped by a pair of pincers which had been heated in a 

 Bunsen flame, and one half of the wire was drawn taut, leaving the 

 other half slack; a galvanometer deflection immediately occurred, 

 which ceased when the tension was discontinued. Tbe process was 

 then repeated, the pull being made in the opposite direction, this 



