Dr. Tyndall on the Laws of Magneiism. 373 



tHat'the i]jict,uced,' current is proportional to sin ^0. But the 



magnetism of the core is known to be proportional to the in- 

 duced current which it is able to excite *j and hence we have the 



strength of the magnet proportional to sin ^ ^- The magnet 



with which I experimented was not furnished with two wires 

 such as those described ; but a second helix happened to be at 

 hand, into which a moveable core could be introduced. This ' 

 helix was placed upon the end of the magnet and the core 

 introduced, so that its axis was a continuation of the axis of the 

 magnet. On exciting the latter the core was also excited, and 

 thus a cm-rent was aroused in the wire of the surromiding helix 

 which was connected with an astatic galvanometer. It was ne- 

 cessary that the latter instrument should stand at a distance 

 from the current, and it was also necessary that the current could 

 be inteiTupted or re-established in an instant. A glass with a 

 quantity of mercury was introduced into the circuit ; two stout 

 copper wires connected with the bands of sheet copper before- 

 mentioned dipped into the glass ; one of these tui-ned on a kind 

 of hinge, which permitted of its being lifted out of the mercury. 

 To this wire a string was attached, which passed over a pulley 

 placed above it and thence to the galvanometer. Standing at a 

 distance from the current, it was thus in the observer's power to 

 break it or establish it in a moment. By pulling the string, the 

 wire left the mercuiy and the circuit was broken ; by letting the 

 string go, the wire fell into the mercury again and the circuit 

 was closed. 



13. In this way, not only the indications of the tangent galva- 

 nometer, but also those of the astatic galvanometer were observed 

 for each weight. The strength of the magnet being, as before ob- 



1 . ^^^2*^ 



served, proportional to sin ^ c^y the quotient — _ — ought to be 



'\^^s\\^m'i^\'Y — : Y/nrSfllV/oDot oiLt OJ 



a constant quantity, and hence,. „\ ^.,„v,^, i,„.,> vv.w'^i. ..v.v\vv .^ ^^^^v^ 

 „ wsin-^ 



i' .^')ii7/Wilfn):(()' W/: to Ir •«'> 



where » '\k a' c&ilsikpirVh'fc'h '^jrbi^ekses 'ifcHe, ratio of tan /S'to 



8in^0: ■■ ■ ' ■ I'l. ■'■' "'' ■'■'■' ilV' ■•■■■•.' )ii-.vii; •• ■ ' ' ' 



In the following Bcricsjj w ifl equal to p7i4 j, Ijpth quotients are 

 multiplied by (? for the reason before as^^gijedv ", 



i/Oli| "' /i^»* Pofrgcndorff's /in?(fl?tfn, vol. xlvii. |). 230. '^ "' -Ji' 



