118 



Prof. J. A. Ewing. 



circuit was 1*36 ohms and, if we assume the electromotive force of 

 the cell to have been 1 volt, the magnetising force was therefore 



A v 660 v lxlO 8 on ., . 



47rX_Xp^--^=20 c.g.s. units very nearly. 



The ideal diagram (fig. 1) will help in describing the directional 

 relation of the transient currents, the magnetisation, and the torsion. 



Fig. 1. 



There ab is the iron wire, with the twisting-arm and dial-plate at h. 

 The two directions of twist are distinguished by the signs + and — , 

 the latter being used when the twist is that of a common screw. The 

 two directions of the magnetising current will be called A and B ; the 

 connexions for A are shown by the full lines, and for B by the dotted 

 lines. A has the effect of .making the dial end (b) of the wire a 

 nominal N. (i.e., a " north-seeking ") pole. A transient current will 

 be called positive when it flows along the wire from a to b. 



§ 3. When the wire was in a state of no torsion the closing of the 

 battery circuit did not of course produce any current in the ballistic 

 galvanometer ; nor, when the wire was sensibly free from magnetisa- 

 tion, did twisting it produce any effect. But when the wire was first 

 twisted negatively (like a common screw) and then magnetised by 

 closing the circuit, A, a transient current flowed along it from b to a, 

 that is, from 1ST. to S. Reversal of the magnetising current from A to 

 B, the wire being still held twisted, caused a transient current of 

 twice the quantity of the first to pass in the opposite direction. 



In like manner, if the magnetising current A was first made and 

 the wire then twisted suddenly like a common screw, a transient 

 current flowed along it from b to a, that is from N. to S. 3 and similar 

 currents were observed when, without maintaining the magnetising 

 force in action, a permanently magnetised wire was suddenly twisted. 



