44 PHYSIOLOGICAL PHYSICS. [Chap. iv. 



induction, it will be readily understood that the use 

 of a bobbin round which is wound the spirals of a 

 coil introduces an element that would not be found 

 did the circuit pursue a straight course. For as the 

 current flows round one turn of the coil, it induces a 

 current in the neighbouring turns of the same coil. 

 This second current is called the EXTRA CURRENT, and 

 is in a direction tending to oppose the establishment 

 of the ordinary current. The arrangement in Fig. 24 



was devised by Fara- 

 day to show this extra 

 current. It consists of 

 a battery of elements E, 

 from which wires are led 

 to c, a single coil of long 

 Fig. 24. Extra Current, fine wire. At the point 



A in the positive elec- 

 trode a branch wire is led off to the galvanometer G, 

 and a wire from G joins the negative electrode at B. 

 Thus the current from the battery splits at A ; part 

 goes round c, and back to the battery, part goes off at 

 A, passes round the galvanometer, and joins the main 

 circuit at B. The arrows indicate these directions. By 

 the action of the current, the needle of G takes up a 

 position a. By a simple arrangement the needle is 

 brought back to zero, and then contact is broken, so 

 that the current ceases to flow round the coil c. At 

 that instant the needle becomes deflected in a direction 

 opposite to the former, i.e. in the direction a. This 

 indicates a current flowing across from B to A, that is, 

 a current in the coil in the direction A c B A, which 

 was the direction of the battery current. Similarly, 

 when the current is established in the coil C, there will 

 be an inductive action of each turn of the wire on 

 its neighbours, and another extra current will appear. 

 It will, in accordance with the rule already laid down, 

 be in a direction tending to oppose the establishment 



