the Thomson Efect. 441 



been taken hitherto in experimenting with pure metals. We 

 have thought it would be valuable to test the effect in as pure 

 a metal as we could obtain by electrolysis. We have also 

 extended Le Roux's table by the addition of the effect in 

 nickel, which Thomson was unable to obtain, and also in 

 carbon. An endeavour has been made to ascertain if the 

 effect is reversible, and also to discover if it is modified in a 

 magnetic field. 



The strip of nickel, 45 centim. long, 2*6 wide, and 2 mil- 

 iim. in thickness, was placed with its flat surface horizontal. 

 One face of a thermopile was placed at a fixed point on the 

 surface of the nickel, separated from it by a thin piece of mica. 

 A weight pressed upon the other surface. The thermopile 

 was connected with a Thomson's reflecting galvanometer of 

 six ohms resistance. The two extremities of the strip of 

 nickel were connected with a battery of six Grove cells, the 

 wires first passing through a key so that the direction of the 

 current could be reversed. One end of the nickel was kept 

 at the temperature of the air, 15° 0. ; the other at a constant 

 red heat by means of a Bunsen burner. The metal was heated 

 in this way from 9 a.m. to 3 p.m., until it reached a condition 

 of thermal equilibrium, as shown by the galvanometer. The 

 scale of the galvanometer was then moved until the spot of 

 light came to 0. The current from the Grove cells was then 

 passed for one minute alternately in opposite directions, and 

 the deflections of the galvanometer were read every quarter of 

 a minute. Before the direction of the current was changed, 

 the circuit was each time broken, and the spot of light was 

 allowed to fall to 0. The following table gives the results. 

 The column marked " C-H" gives the deflections when the 

 current was passing from cold to hot. The small numbers 

 show which deflections in each pair were taken first. 



C-H. 



H-O. 



Deflections taken every £ minute. 



Deflections taken every £ minute. 



i 



2 1 



2 



i 



2 



l 



2 



l 



2 



4-1 



4-2 



4-0 



4-3 



44 



3-3 



4-0 



3-6 



3-8 



4-1 



6-3 



64 



6-5 



6-4 



6-4 



5-0 



6-2 



54 



5-9 



6-0 



7-3 



7-2 



7-5 



7-0 



7-2 



5-8 



6-7 



6-2 



6-5 



7-0 



74 



7-6 



7-6 



73 



7-7 



6-1 



7-3 



6-5 



6-8 



7-2 



From this table it is obvious that more heat is evolved by a 

 constant current per unit time in passing from the cold to the 

 hot end of the nickel than in passing in the opposite direction. 

 The Thomson Effect in pure nickel is consequently negative ; 



Phil. Mag. S. 5. Vol. 14. No. 90. Dec. 1882. 2G 



