382 Trowbridge and Penrose — The Thomson Effect. 



accurately obtained, as the batteries had been running about 

 thirty minutes by the time the experiment was completed. 

 The field, however, was very much stronger — as shown bv 

 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 ac- 

 curate as those given by Le Koux 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 equilibrum, the de- 

 flection of the galvanometer was 35 centimeters. 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. 



< ;| i§s <~ ' 



^^\JB;^L 



| 1 | : | | ! 



13-3 »| 1-0 



Mean difference — 0-97 i Mean difiereuce=l"13 



The strip of nickel was now substituted for the copper, 

 everything else remaining exactly the same. One end of the 

 nickel was heated, and the thermopile was placed on such a 

 spot that the galvanometer gave a deflection of 35 centimeters 



The same current was passed as above. The results are 

 given in the right hand table. Let d= the mean difference 

 in first table, and d'= that in the second, d and d' arc then pro- 

 portional to the elevation ol temperatnre of the part of the bars 

 under the pile, on account of the Thomson Kilect. Let a= co- 

 efficient of Thomson Effect, that is, a is such a quantity that 

 add represents the heat, absorbed per unit current per unit time 

 in passing from section at temperature 6 to section at tempera- 

 ture d+dd. The heat evolved in unit section when the tem- 

 perature is increased by id. K is £Kd SD. Where K i ~ ~ 



slant ctepencting on the galvanometer, S is the specific heat, 

 and D the density of the metal. It" we consider the Thomson 

 Effect to be constant under the pile, and 6 and 6' to represent 



