lOO PHYSIOLOGY OF MUSCLES AND NERVES. 



copper to the zinc, and through the liqnid from the zinc 

 to the copper. A magnetic needle is used to indicate 

 the presence of such a current. An electric current, if 

 made to pass parallel to 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 flows, 

 and according to 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 be 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 when 

 the current is comparatively strong, for the magnetic 

 needle is retained in its position by the attraction of 

 the earth, and the magnetic current must overcome 

 this before it can deflect the needle. In order to detect 

 weak currents, the wire through which the current flows 

 is wound in several coils round the needle. As each 

 coil exercises a force tending to cause the deflection 

 of the needle, the deflecting force is increased ; and an 

 instrument of this sort is, therefore, called a multiplier? 

 In order to increase the sensitiveness of this still further, 

 the attraction of the earth must be annihilated as far 

 as possible, so that even weak currents are al)le to cause 

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

 ranging a fixed magnet above or below the magnetic 

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



' If attention is paid to certain circumstances, which cannot be 

 mentioned in detail here, the same instrument can also be used to 

 measure the strength of currents ; it is, therefore, also called a gaU 

 vauometer. 



