Magnetism on a permanent Electric Current. 319 



strip was taken to be not 6*05, but 8'S centims. Assuming 

 the specific resistance of the silver in this plate to be '00000165 

 ohm (the value given by Jenkin for " hard -drawn " silver), we 

 obtain as the thickness of the strip '00000407 centim. It will 

 be shown below that this value is probably very much too 

 small'; but I will for the moment give the results obtained on 

 the basis of this estimation of the thickness. 



Passing over a result obtained at quite an early period of the 

 experiments, and which there are excellent reasons for. reject- 

 ing, we have 



££. 



Jan. 30th, 7120 k x tan 43 33 487 x 10 10 



„ „ 19 32 499 x 1 10 



Mean . . =493 xlO 10 



The discrepancy between this result and that obtained with 

 the thicker strip of silver was so great that I determined to try 



Silver Film B. 



I have assumed the thickness of B to be the same as that 

 of A. The other dimensions are about the same ; and the 

 result is 



M. C. **Y. 



May 4th, 6640 k x tan 47° 30' 491 x 10 10 



The agreement of this result with the mean of those just 

 preceding is entirely satisfactory, and the discrepancy above 

 mentioned as existing between the results with plates of dif- 

 ferent kinds is confirmed. This disagreement was so large as 

 to be difficult to account for, without the hypothesis of a spe- 

 cific difference exhibited by different forms of the same metal, 

 under the conditions of the experiment. To be sure, the me- 

 thod of estimating the thickness from the electrical resistance 

 was open to suspicion. Among other probable sources of error, 

 there was the possibility of having assumed a wrong value for 

 the specific resistance of the silver in this condition. It did 

 not appear to me probable that an error of about 400 per cent, 

 could be accounted for in this way; but it seemed worth while 

 to attempt a determination of the thickness of the films by 

 another method. 



Plate A was taken and cleaned with alcohol to remove the 

 particles of cement adhering to the glass and metal. The area 

 of the silver film was roughly determined ; and the plate was 

 dried and, when cool, carefully weighed. The silver was then 

 removed by dissolving in nitric acid, after which the glass was 



2A2 



