THERMO-ELECTRIC EFFECTS OF LONG. TENSION IN DIFF. METALS. 



55 



The current from a good sized gravity Daniell cell was passed 

 through a resistance of 10000 ohms and the potential difference of 

 1/1000 of the cell's E. M. F. established at the terminals of the 

 galvanometer, in whose circuit there was farther inserted a resistance 

 of 300 ohms. It was easy to so adjust the controlling magnet that 

 when the feehle current thus produced was allowed to flow through 

 the galvanometer the range of motion of the light-spot over the scale 

 was some 70 or 80 divisions, so that a deflection of one scale division 

 corresponded to 6 or 7 x 10~ 8 ampères. In the later experiments of 

 the present series I used in place of the Daniell a calomel cell which 

 was found to be very constant and whose E. M. F. I assumed in cal- 

 culations to be 1±0-01 volt. 



Three small mercury commutators arranged in a row were found 

 to be very convenient for standardising and for resistance measure- 

 ment. The mode of action can 

 be easily seen from the accom- 

 panying figure. The wires a a 

 served for throwing the experi- 

 mental and leading wires, with- 

 out moving them from their 

 positions, into the fourth arm 

 of the resistance box, and the wires b b for putting the galvanometer 

 (of course much reduced in sensibility) into one of the diagonals of 

 the box. 



§ 31. The following table contains one of the experiments made 

 on annealed soft iron wires 0*77 m.m. in diameter (from the same 

 bundle as the wire of § '25). This is graphically represented in 

 Fig. 1, PI. IV. 



