ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



13 



coil to the electro-magnet E, and so to the pillar B. When the circuit 

 is thus made, E becomes an electro-magnet, pulls down the spring H 

 from its contact with S x and breaks the circuit ; consequently E ceases 

 to be a magnet, the spring flies up into contact with S 1} and again 



p.c 



FIG. 18. Diagram to show the action of Wagner's hammer. 



completes the circuit. The number of times the circuit will be thus 

 made and broken per second depends upon the length of the spring H ; 

 in most coils it is of such a length as to give 50 complete vibrations 

 per second. At each make and break of the circuit a current is induced 

 in the secondary coil, just as when the circuit was broken by hand ; 



PIG. 19. Diagram to show the adtion of the Helmholtz side-wire. 



further, the break-shock is stronger than the make-shock, and for the 

 same reason as before. 



Determine the distance necessary between the two coils for the 

 shocks just to be felt on the tongue. 



