500 Mr. S, Bid well. Changes produced by [May 19, 



Table III. — Iron Wire, diameter 1*05 mm. 



Magnetic field due 

 to coil. 

 C.G-.S. units. 



Elongation in ten-millionths of length. 



Witli no current 

 through wire. 



With 2 amperes 

 through wire. 



7 



1 



2-5 



16 



6-5 



11 



25 





15 



34 



i3 



18 



40 



14 



18 



ou 



12-5 



18-5 



62 



12 



18 



87 



10 



16 



134 



3-5 



8 



213 



-5-5 



-1 



263 



-]0-5 



-8 



338 



-20 



-16-5 



Exp. 5. — A current of 2 amperes was passed through the same 

 wire, resulting in an elongation due to heating of 460 ten-millionths, 

 the temperature of the wire being therefore raised about 3°*3. The 

 former observations were again made with the results given in the 

 last column of Table III and in fig. 2. 



It will be seen that with both specimens of iron wire, the effect of 

 a current is of just the same general character. It acts oppositely to 

 tension, heightening the curve of elongation instead of lowering it. 

 This action is certainly not due either directly or indirectly to mere 

 current heating. It has been shown that the thinner wire even 

 when carrying 2 amperes was only about 10°* 7 warmer than when 

 no current was passing through it. Such a small rise of tempera- 

 ture would be quite incompetent by itself to account for the effect in 

 question, for the elongation curves of a given specimen of iron have 

 been found to be not sensibly altered when taken under widely 

 different conditions of temperature. Nor would it exert any 

 material influence upon the susceptibility of the iron ; and, even if it 

 did, the curves would not be affected in the manner observed. 



It is hardly worth while attempting to frame an explanation until 

 many more phenomena of the same order have been investigated. 



Similar experiments were afterwards made with nickel and 

 cobalt. 



Exp. 6. — A nickel wire was used, the diameter of which was 

 0'65 mm. The retractions which it underwent in fields of gradually 

 increasing strength are given in the second column of Table IV. 



