ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 15 



In order to multiply the induction effect the two circuits always 

 take the form of closely coiled wires (Fig. 22) (that of the secondary 

 circuit being very fine and having very numerous coils), and to still 

 further increase the effect the primary coil is wrapped round a core 

 formed of a bundle of soft iron wires which are magnetised and de- 

 magnetised on the closing and opening of the primary circuit, thereby 

 enhancing the induction effects. The induced or secondary current 

 thus produced is of very short duration, but has a high electromotive 

 force. 



For physiological purposes the induction coil was arranged by du 



FIG. 22. Induction coil arranged for single shocks. 



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 farther 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 

 inversely as the square of the distance. For producing single make and 

 break induced shocks the primary circuit is closed and opened with a 

 simple mercury key (Fig. 22). For multiple induced shocks most coils 

 are fitted with an apparatus for automatically breaking and making the 

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

 diagram shown in Fig. 23. The current is conveyed from the terminal 

 t 3 to a steel spring sp t having a bar of soft iron at its free end, and 

 the current passes from the spring, which has a plate of platinum 



