12 PRACTICAL PHYSIOLOGY 



A purely physical proof of the break extra current can be obtained 

 by connecting one pole of a battery to the primary coil, and by touching 

 .with the other wire from the battery the milled head of the other 

 binding-screw of the primary coil. Every time that the battery circuit 

 is broken, the break extra current will pass across from the screw to 

 the wire as a minute spark ; no spark, or a very feeble one, is seen 

 on touching the first terminal, for in this case there is no current in the 

 primary coil. 



Equalisation of Make and Break Induced Currents. From what 

 has been said it is clear that, if the break extra current were provided 

 with a circuit to run in, the strength of the current induced in the 

 secondary coil at break would be reduced to that of the current induced 

 at make ; and so they would be equalised. In order to effect this the 

 battery-circuit is not broken, but is nearly completely short-circuited 

 out of the primary coil by a Du Bois key (Fig. 17). Now again 

 test the relative strengths of the make and break induced currents. 



FIG. 17. Arrangement of apparatus for equalising the make and break induced currents. 



They may be approximately equal, but the original difference is not 

 infrequently overcorrected, and now the break-shock is the weaker. 

 This is caused by the make and break extra currents running in 

 circuits of different resistance. At make the extra current runs not 

 only through the primary coil but also through the resistance of the 

 Daniell cell ; but at break the extra current has to run only through 

 the resistance of the primary coil, hence it is the more effective current 

 of the two, and reduces the effect induced in the secondary coil at 

 break more than the make extra current does on closing the primary 

 circuit. 



Faradic or Tetanising Shocks. Induction-coils are provided with 

 an automatic arrangement for rapidly making and breaking the 

 primary circuit by means of Wagner's hammer. Connect up the 

 battery to screws 5 and 6 of the coil, interposing a spring-key, and 

 follow out the primary circuit (Fig. 18). The current passes up 

 the pillar A along the spring H to the screw Sj, through the primary 



