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EXPERIMENTAL PHYSIOLOGY 



upon it, to the platinum point of a screw s l , and thence through the 

 primary coil. Before passing back to the cell it is conducted through 

 a small electro-magnet m ; the electro-magnet being thus set in action, 

 draws down the iron bar and with it the spring, which leaves the screw 

 and breaks contact so that a break induced current is set up in the 

 secondary coil. But, the current being broken, the electro-magnet m 



FIG. 23. Diagram of du Bois-Reymond coil arranged for faradisation. 

 (For description see'text.) 



FIG. 24. Diagram of the arrangements employed in the Helmholtz modifica- 

 tion of du Bois- Raymond's induction coil. (For description see text.) 



is no longer active, the bar springs up again, and contact is re- 

 established between the spring and screw ; this produces a make 

 induced current in the secondary coil. Thus the spring vibrates to 

 and fro, and break and make induced currents are set up in the 

 secondary coil many times a second, according to the rate of vibration 

 of the spring. 



These make and break shocks are unequal owing to the extra 

 current which is self-induced within the primary coil, and which 



