14 EXPERIMENTAL PHYSIOLOGY 



rheochord and its rider, and proportionally more of the battery 

 current passes into the experimental circuit. 



Induction coil. If the wires of two separate circuits are at any 

 point near to and parallel with one another and if, in the first or 

 primary circuit, the current of a battery is either made or broken by 

 the closing or opening of a key, an induced current is set up in the 

 other or secondary circuit at the instant of such closing or opening, 

 but not during the passage of the primary current. The induced or 

 secondary current is always of very short duration, but has a much 

 higher electromotive force than the primary or inducing current. 



FIG. 21. DOUBLE-WIRE RHEOCHORD. 



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 magnetized and de- 

 magnetized on the closing and opening of the primary circuit, thus 

 enhancing the induction effects. 



For physiological purposes the induction coil 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 (fig. 22). For multiple induced shocks most coils are fitted with 

 an apparatus for automatically breaking and making the primary 



