Constants of Igneous Rock, 



11 



understood from figure 7, where E and B/ are rheostats,having 

 about 35,000 ohms available, and r is a bridge-rheostat con- 

 taining a duplicate scale of 10, as high as 10,000. The cold 



Fig. 7. — Zero method of thermoelectric measurement. Diagram, 



G 



P 3 



Fig. 8.— Form of 

 double key. 

 Diagram. 



V~y~\ 



ends of the thermocouple communicate with the terminals at 

 P, under petroleum. Two zinc-sulphate Daniells are inserted 

 at E. The commutators A and B are useful in measuring- 

 small electromotive forces, since by simultaneously reversing 

 both of them, all disturbing thermo-currents in the connexions 

 are eliminated from the mean result. K is a duplex key con- 

 structed as shown in figure 8, where two of 

 the mercury cups (conveying the battery 

 current from E) are filled to a higher 

 level than the third. By aid of the keys 

 at C the resistances r can be inserted 

 either in parallel or in series. G is a 

 sensitive Thomson galvanometer. 



The thermoelectric forces are directly expressed in terms 

 of E, which is compared from time to time with the given 

 standard Clark cell, the poles of which are temporarily put 

 in connexion with P. The reduction of observations is 

 facilitated by suitable tables, computed once for all. 



In this way my thermoelectric data have the same high 

 degree of constancy in the lapse of time (years) as the 

 standard cell in question. I have found this method, though 

 it is somewhat more laborious, much more trustworthy than 

 the torsion-galvanometer, with which I have also worked *. 

 Even when the conditions are so chosen that the thermo- 

 electric forces are expressed solely in terms of the torsion of 

 the suspending fibre, it is not to be forgotten that increase of 



* See this Magazine, xxix. p. 146 (1890). 



