GENERAL ACCOUNT OF MAGNETIC ACTIONS 189 



through which a capillary tube passed. A change in volume 

 of 1 in 4,500,000 would have been shown, but no motion of the 

 end of the liquid in the capillary was observed. 



Many others have worked on this subject, but we owe our most 

 exact knowledge to Bidwell, who showed that as the magnetising 

 force increases, the lengthening reaches a maximum, decreases to 

 zero, and is followed by a shortening. Bidwell worked with both 

 rods* and rings. His method of carrying out the rod experiment 

 will be seen from Fie:. 136. The solenoid was fixed on the rod to 



<i rnp 



Mirror 



Scale 



FIG 136. 



avoid alteration in the pressure of the rod against the base due 

 to the attraction between the solenoid and the rod a source of 

 considerable error when such small changes of length are to be 

 observed. The apparatus for multiplying the change of length 



IOO//Z IO 7 



250/>7l0 7 . 



500 



1OOO 



Mag net ts inp force 



FIG. 137. 



into an observable quantity will be understood from the figure. 

 The results obtained are given in Fig. 137, taken from the paper 

 in the Philosophical Transactions, loc. cit. p. 228. 



* Proc. R.S., xl. (1836), p. 109 ; Phil. Trans. A. (1888), p. 205. 



