16'0 rilYS'loLOC.Y OF MfSCLES AND NKUVKS. 



ii tin- x.iuc and through the liquid from the zinc 

 to the copper. A magnetic needle is used to indicate 

 tin- prex-nce of such M current. Aii elect ric current, if 

 made t< pa>s parallel 1o a magnetic needle, deflects 



the latter from its normal position, and tends to place 



it at right angles to its original position. According 

 to the direction in which the positive electricity rl<>\\-, 

 and according <<> the position of the conducting -wire 

 relatively to the magnetic needle, the north pole of 

 the needle is deflected either to the east or to the west ; 

 so that not only the actual presence of an electric cur- 

 rent may he shown by means of a magnetic needle, but 

 its direction in the wire may also be determined. This 

 simple means, however, only serves the purpose \\hen 

 the current is comparatively strong, for the magnetic 

 needle is retained in its position by the attracti..n of 

 the earth, and the magnetic current must overcome 

 this before il can deflect the needle. In order to d t. . t 

 weak currents, the wire through which the current flow- 

 is wound in several coils round the needle. As cadi 

 coil exercises a force tending to cause the deflect i,,n 

 of the needle, the deflect ing force is increased ; and an 

 instrument of this sort is, therefore, called a /// ttt/lj>lii /.' 

 In order to increase the sen sit i\ vness of t his st ill further, 

 the attract ion of the earth must be annihilated as far 

 as possible, so that e\eii \\cak currents are able to cause 

 deflection. This is accomplished, for instance, by ar- 

 ran-ing a li\ed magnet above or below the magnetic 

 needle, so that it acts on the latter in a direction 



1 If :it trnl ion is | a :! to crrtnin circumstances, \\ 1 1 idi c:iniii'l !' 

 in ill-tail lii-iv. iln- Miuir i i ist ni inriit can also \n- usc-d to 

 nirasiirr tin- stivnyth of ciirn-nts ; it is, tlu-rrforc, also callnl a <jnl- 

 rniioini ti-r. 



