PRACTICAL PHYSIOLOGY 13 



was arranged by du Bois-Reymond so that the 

 secondary circuit can be made to slide nearer to or 

 farther from the primary circuit ; since with the 

 same strength of battery the nearer or further the 

 coils are from one another the greater or less is 

 the strength of the induced current. The varia- 

 tion is not, however, proportional to the distance, 

 but approximately to the square of the distance. 

 For producing single make and break induced 

 shocks the primary circuit is closed and opened 

 with a simple key. For multiple induced shocks 

 most coils are fitted with an apparatus for auto- 



Secondary 



coil. 



_ -Mercury key 



bnort-cir- \ t*y / / in primary 



cuit key -* * JLJ - 



FIG. 17. INDUCTION COIL ARRANGED FOR SINGLE SHOCKS. 



matically breaking and making the primary circuit 

 (Neef's hammer). This will be understood from 

 the diagram (Fig. 18). The battery current is 

 conveyed from the terminal, T 3 , to a steel spring, 

 Sp, having a bar of soft iron at its free end, and 

 the current passes from the spring, which has a 

 plate of platinum upon it, to the platinum point of 

 a screw, S*, and thence through the primary coil. 

 Before passing back to the battery it is conducted 

 through a small electro-magnet, M; the electro- 



