of High Specific Resistances. 461 



as has been pointed out by several observers, there is danger 

 of rapid variation of the magnetization ; but it was thought 

 better to risk this than to force it by artificial " ageing " by 

 heating or otherwise. As will be seen hereafter, the magnets 

 were never exposed even during the experiments to anything 

 but the smallest electromagnetic forces, and the controlling 

 magnet was weakened and introduced from high above the 

 galvanometer, and only lowered sufficiently just to make its 

 influence on the combination really felt. This is a delicate 

 operation with ordinary arrangements, but becomes simple 

 when the construction of the galvanometer is modified in a 

 way to be explained directly. If one desires to keep the 

 astaticism perfect, it is necessary to be mindful not to use the 

 controlling magnet so as to produce demagnetization ; nor must 

 the currents through the galvanometer ever rise to much 

 greater values than those corresponding to the effects to be ob- 

 served. From an examination of the investigation in Maxwell, 

 vol. ii. articles 437 and 438, both Mr. Adair and I came to 

 the conclusion that the disk form of magnet would retain its 

 magnetization pretty well. This has turned out to be the 

 case, for after more than a year's hard use, for all sorts of 

 purposes, the galvanometer has still a sensitiveness of about 

 one division for 10 -9 ampere. The galvanometer is in daily 

 use for testing cells with a view to their application to the 

 resistance measurements at present under discussion. 



This galvanometer, however, never came to be relied on to 

 measure currents of less than 10 ~ 8 to 10 -9 ampere. In the 

 course of reading on the subject I consulted the paper by 

 Messrs. T. and A. Gray in the Proc. Roy. Soc. 1884, vol. xxxvi. 

 p. 287. These gentlemen made use of a new arrangement of 

 magnets and coils, which, however, can hardly be understood 

 without referring to the picture, loc. cit. The coils and magnets 

 were so arranged that the poles of the magnets were normally 

 situated in conical holes containing the axes of the coils. 

 The two horseshoe magnets were suspended from a frame of 

 aluminium wire by one or two silk fibres of considerable 

 length. The coils themselves were composed of very fine 

 wire and had a high resistance. I lost three months' hard 

 work in making and testing this arrangement, which certainly 

 has the advantage of being practically perfectly astatic. The 

 suspended arrangement being rather large was most trouble- 

 some to mount and balance, and had the additional dis- 

 advantage of having so great a moment of inertia that its 

 period of vibration often amounted to 70 or 80 seconds. 

 This sluggishness had the property of making it most difficult 



Phil. Mag. S. 5. Vol. 28. No. 175. Dec. 1889. 2 M 



