the Thomson Effect. 443 



netisrn on the Thomson Effect. Nothing but negative results, 

 however, were obtained. 



The strip of nickel was placed horizontally, with its flat sur- 

 face perpendicular to the axis of a large electromagnet, the 

 strip being between the two poles of the magnet. One surface 

 of the nickel was pressed against one pole; on the other sur- 

 face was placed one face of the thermopile, while the opposite 

 face was in contact with the second pole of the magnet. Mica 

 was used, as in the previous experiments, to protect the faces 

 of the pile. The whole was wedged and pressed tightly toge- 

 ther, and clamped by means of wire, the object being to pre- 

 vent any motion of the nickel when the magnet was made. 

 One end of the nickel was heated by a Bunsen burner ; the 

 other was at the temperature of the air. Six hours were re- 

 quired for the apparatus to reach a condition of thermal equi- 

 librium. The electromagnet was connected with thirty-eight 

 freshly set-up bichromate-of-potash cells, with plates of large 

 size. A current from eight Grove cells was now passed along 

 the nickel, with and without the circuit of the magnet being 

 made. The deflections of the galvanometer were exactly the 

 same in each case, showing that in a magnetic field (at least 

 of the strength in the experiment) the Thomson Effect was 

 unaltered. 



It is unfortunate that the strength of the field could not be 

 accurately obtained, as the batteries had been running about 

 thirty minutes by the time the experiment was completed. 

 The field, however, was verv much stronger (as shown by 

 rough tests) than in another experiment, where the minimum 

 value was found to be 184 times the vertical intensity of the 

 earth's magnetism. 



The determination of the relative value of the Thomson 

 Effect in nickel by the following method gives of course but 

 approximate results. The value, however, is probably as accu- 

 rate as those given by Le Roux for other metals. 



A strip of copper, of about the same dimensions as the nickel 

 used before, was arranged exactly as the nickel had been. The 

 thermopile was insulated from the strip by the same piece of 

 mica, and the same weights were placed on the upper surface. 

 One end of the copper was heated in boiling water; and 

 when the apparatus had reached a condition of equilibrium, 

 the deflection of the galvanometer was 35 centim. A current 

 from four amalgamated Grove cells was now passed alternately 

 in opposite directions along the bar, the deflections of the spot 

 being taken, in each case, after one minute. The results are 

 given in the left-hand table : — 

 2G2 



