748 INDUCTION OF CURRENTS. 



tion will be easily understood from the figure without further ex- 

 planation. The silk thread by which the astatic needle is suspended 

 is drawn through two rings in the wire which supports it, and wound 

 round a pin w, which is movable with considerable friction within a 

 hole in the wooden base of the instrument. By turning w one way or 

 the other, the suspension thread may be raised or lowered and the 

 astatic needle adjusted to the proper height. The whole should be 

 protected against currents of air by a bell jar, or a glass bottle from 

 which the bottom has been removed, the binding screws being left 

 outside the cover. 



The larger instruments of this kind, such as are used 

 in many scientific investigations, have coils which con- 

 sist of a great many turns of much finer wire. Such 

 fine wire could not be coiled without support as in the 

 apparatus fig. 377, but it is generally -coiled on two sepa- 

 rate frames placed one on each side of the astatic needle. 

 By means of such instruments, consisting of several 

 thousand turns of wire the presence of currents may be 

 detected which are incomparably weaker than those 

 observed in the preceding experiments. 



53. Induction of currents. Under certain circum- 

 stances the current which traverses a conductor may deve- 

 lop a second current in a conductor near it, through 

 which no current was passing previous to the action of 

 the first current. This class of effects of a current is 

 comprised under the name ' current-induction.' The 

 investigation of the laws of current-induction, and their 

 experimental demonstration, require rather complicated 

 apparatus, and it will be sufficient in this work simply 

 to state these laws, and to describe an apparatus the 

 action of which depends upon them. The laws are the 

 following : 



I. A current which begins, a current which approaches, 

 a current which increases in strength from any cause, in- 



