16 



EXPERIMENTAL PHYSIOLOGY 



electro-magnet, m, is no longer active, the bar springs up again, and 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 diminishes the make effect 

 (see Chapter III.). This inequality is, however, overcome by a modification 

 introduced, by Helmholtz. In this arrangement (fig. 24) a wire, w, connects the 

 terminals t 1 , t 3 ; the screw s l is raised altogether away from the spring, and does 

 not come into use ; the screw s 2 is brought nearly up to the spring. The battery 

 current passes by the wire, w, from the terminal, t 3 , directly to t 1 , thence 

 through the primary coil and through the electro-magnet, m, which draws down 



FIG. 24. DIAGRAM OP THE ABRANGEMENTS EMPLOYED IN THE HELMHOLTZ MODIFICATION 

 OP DU BOIS-REYMOND'S INDUCTION COIL. (FOR DESCRIPTION SEE TEXT.) 



the iron bar and brings the spring in contact with the screw, s 2 . A large part 

 of the battery current now goes directly back to the battery through this contact, 

 and is diverted from the primary coil and electro -magnet. This greatly weakens 

 the current through the primary coil, and the equivalent of a break induced 

 shock is obtained in the secondary circuit ; for any sudden variation in the current 

 of the primary coil is effective in producing an induced current in the secondary 

 coil. But the electro-magnet is also weakened, so that the bar and spring fly 

 up. This breaks the short-circuiting contact which was established between 

 the spring and s 2 , and the whole current again passes through the primary coil, 

 producing the equivalent of a make induced shock in the secondary circuit, 

 and so on automatically. It will be observed that the primary circuit is never 

 actually broken, but only short-circuited. 



The Helmholtz arrangement should always be employed for tetanic stimula- 

 tion, unless the nature of the experiment contra-indicates it. 



Kronecker's inductorium. Most du Bois-Reymond coils are furnished with 

 a scale marked in millimeters to indicate the distance of the secondary from the 

 primary coil. But the Kronecker coil is so graduated as to indicate the relative 

 strength of the induced current when a standard battery of constant strength is 

 employed in the primary circuit, and the scale is marked in units as well as milli- 

 meters. Moreover, the contacts are between a platinum wire and mercury, the 

 latter being kept clean by passing water or dilute alcohol over its surface. This 

 gives much more constant make and break effects than the contacts between 

 platinum point and platinum plate which are supplied with ordinary coils. 



