1892.] 



Magnetisation in Wires carrying Currents. 



497 



of the demagnetising effect of the wire. The results are also plotted 

 as a curve in fi^. 1. It will be seen that the maximum increment of 

 length, attained in a field of about 40, was 11 '5 ten-millionths ; the 

 decrement of length in a field of 315 was 22*5, while the original 

 length of the wire was unchanged in a field of 130. 



Table I. — Iron Wire, diameter 0"75 mm. 



Magnetic field 

 due to coil. 

 C.G-.S. units. 



13 

 16 

 23 

 34 

 40 

 50 

 61 

 81 

 97 

 130 

 171 

 202 

 244 

 250 

 315 

 319 

 323 



Elongations in ten-millionths of lengths. 



With no current 

 through wire. 



3 

 6 



7-5 

 10 



11-5 

 10 



9-5 



6 



4 





 -4 

 -9 

 13 5 



22-5 



With 1 ampere 

 through wire. 



12 



14-5 

 14 

 14 

 12 



9-5 



8 



3 -5 

 

 -4 



-9 



■185 



With 2 amperes 

 through wire. 



11 -5 



20 



20 



16 

 8 

 -1 

 -5 



-13 



JExp. 2, — A current of 1 ampere was then passed through the wire. 

 The current, which was derived from a Grove's cell, was measured 

 by a tangent galvanometer and regulated by a rheostat, which had 

 been approximately adjusted on the previous day. As soon as the 

 circuit was closed, the index of the measuring instrument began to 

 move, rapidly at first and afterwards more slowly, in the direction 

 indicating elongation of the iron wire. In about two minutes the 

 index had come to rest again, the number of scale divisions over 

 which it had passed showing that the original length of the wire 

 had increased by 310 ten-millionths. Assuming the coefficient of ex- 

 pansion of the iron to be 122 ten-millionths per degree Centigrade 

 this elongation denoted a rise of temperature (due to current heating) 

 of about 2°*5. The experiment described in the last paragraph was 

 then repeated, the several magnetising forces employed being made 

 as nearly as possible the same as before by inserting the same resist- 

 ances successively in the circuit.* The results appear in the third 



* Independent readings of the ampere meter were taken in the two experiments, 

 and the readings corresponding to the same resistance in both series all agreed 



