446 



RECENT PROGRESS IN PHYSICS. 



The mean of tliese three results is 



a = 1.1S2, 6 = 0.008810. 



To determine the retarding force of copper, a wire of this metal was 

 placed in the discharger. Its length was 141.6 lines, its radius 

 0.041952 line. Assuming the thickness of the platinum wire, pre- 

 viously examined, as unit}'-, the value of the semi-diameter of the 

 copper wirewas 



/>= 1.0236. 



A series of experiments with this wire gave 



A=1.5lC 

 s 



But, according to the ahove, we have the coefficient 

 1.51= " 



in which a equals the value just found, 1.789, ?. =: 141.6, and p =s 

 1.0236. From this we find for b' the value 



6' = 0.001367. 



Dividing this value hy the value of b found for platinum, we get 



b' 



^-= 0.1552 ; 



that is, the retarding force of copper is 0.155 times as great as that of 

 platinum ; or, taking the retarding force of platinum lor unity, that 

 of copper is 0.1552. 



In like manner Biess determined the retarding force of other metals 

 and found as follows : 



The first column of figures gives the proportion in which wires of 

 the same dimensions, but of different substances, retard the discharge 

 of the electrica ^oattery. The inverse values of the retarding forces 



