84 



PRACTICAL PHYSIOLOGY 



Note that, when the mercury key is closed, the current passes 

 round the electro-magnet, which pulls down the hammer and 

 spring, and thus breaks the contact at the screw in the middle of 

 the spring. The current is interrupted, the magnet demagnetised 



FIG. 91. Single Induced Shocks at a given Instant. 

 1. Induction Coil. 2. Drum. 4. Mercury Key. 5. Friction Key. 



and the spring bounds back. The number of interruptions depends 

 on the length of the spring. 



Open the friction key and pull the secondary coil well away 

 from the primary and apply the wires to the tongue. Close 



FIG. 92. A Commutator on Pohl's Reverser. 



A and B the two side cups ; C, D, E and F the four corner cups ; 

 S, the handle made of glass or vulcanite. 



the mercury key, and if no sensation is experienced push the 

 secondary coil nearer to the primary. Record the resulting 

 sensation. 



4. A Single Induced Shock at a Given Instant. Insert the recording 

 drum into the primary circuit fitted up as in Fig. 86 (Fig. 91). To 



