RECENT PROGRESS IN PHYSIOS. 445 



A copper wire, for instance, has to be 6.44 times as long as a plati- 

 num wire of the same thickness to effect an equal retardation ; hence 

 the retarding force of platinum is 6.44 times as great as that of cop- 

 per. Making the retarding force of platinum equal to 1, wo find 

 that of copper to be 0.1552. 



This would be, as I have said, the simplest method for discussion 

 and computation. The prosecution of the experiments, however, 

 would be very troublesome. On this account, Rie.'^s has preferred to 

 make the experiments with wires of determinate length and thick- 

 ness, observing the corresponding depressions, and from these he 

 computed the retarding force by the aid of the law found above. 



In the following experiments the same platinum wire (59.25 lines 

 long and 0.04098 lines in diameter) was retained in the thermometer. 

 A platinum wire of the same thickness, but 34.67 lines long, was 

 placed in the discharger. A series of experiments instituted accord- 

 ing to the method described above, q and s varying, and the corre- 

 sponding depression being observed, gave as the result 



7i==1.37^; 

 s 



a platinum wire of the same thickness, but 87.62 lines long, gave 



s 



a third platinum wire, equal in thickness but 143.5 lines in length, gave 



7i = 0,79^. 



8 



The coefficient of ^has, as we have seen above, (page 440,) the form 



1 -f & A" 



to determine the constants a and h, two series of observations are 

 necessary, that is, two numerical values of these factors must be 

 known, corresponding to two different lengths of -^>. 

 First, we have 



1-f 6.34.67"" 

 then 



= 1.01; 



1-1-6.87.62 



combining these two equations we get 



a =1.787, 6=0.00878. 

 Combining, in like manner, the first and third series of observtv 

 tions, we find ' • 



a = 1.788, 6 = 0.008807; 



combining the second and third series, we get 



a =1.792, 6 = 0.008843. 



