P c. 



THE INDUCTION COIL 7 



because the current is in the reverse direction to that of A b. The line 

 R p s indicates the current induced in the secondary by the sudden 

 change fg in the primary : it is above the line K r because it is in 

 the same direction as the inducing current, and is of greater height 

 than that representing the current induced on make. Von Helmholtz 

 showed how we might approximately equalise the two induced 

 shocks by the introduction of a deriving circuit into that through 

 the primary. Fig. 8 shows how to arrange the apparatus to demon- 

 strate this. A battery is connected 

 to the two terminals of the primary 

 coil, and to these are two further 

 wires connected to a key and 

 forming the derived circuit. It is 

 seen that there is always some 

 current passing through the 

 primary both when the key is Fig. 8.— Arrangement or Apparatus 

 open and closed. When the key F0R Equalising the Make and 

 .11,1 j. ,i Break Shocks. 



is closed the current from the 



battery on reaching the first terminal of the coil divides into two 

 parts, one passing through the coil, the other through the deriving 

 circuit. The amount of current passing through either circuit is 

 inversely proportional to the total resistance in that circuit. If then 

 the resistance of the deriving circuit be small in comparison with that 

 of the coil, only a small proportion of the total current passes through 

 the coil. On opening the key, the whole of the current is thrown 

 through the coil and, as previously explained, an extra-current 

 is produced which for a time delays the establishment of the 

 current to its full intensity. On closing the key, there is a fall of 

 current which produces an extra-current running in the same 

 direction as that of the main current ; and as the circuit through the 

 primary is still closed, this extra-current can act in delaying the fall 

 of strength of the current. The result is that the current induced in 

 the secondary is considerably diminished and made approximately 

 equal to that of the make. These results are indicated in the diagrams 

 of fig. 7. The current passing through the primary when the key of 

 the derived circuit is closed is indicated by a d. On opening the key 

 the current rises in value to a c, but its course is delayed and takes 

 the course represented by the dotted line de. If the key be opened 

 at f, the fall in strength to the line d h is not instantaneous, but takes 

 time and is represented by the curved line f h. The effects on the 

 induced currents in the secondary circuit are represented by the 

 interrupted lines kno and rtv respectively. 



For very many purposes it is essential to have a rapid series of 



