ELECTRICAL APPARATUS IN USE IN PHYSIOLOGICAL WORK 17 



diminishes the make effect (see Chapter III.). This inequality is, 

 however, reduced by a modification introduced by Helmholtz. In 

 this arrangement (Fig. 24) a wire w connects the terminals t l , t z ; 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 

 current passes by the wire w from the terminal 2 s , directly to t l , 

 thence through the primary coil and through the electro-magnet m, 

 which draws down the iron bar and brings the spring in contact with 

 the screw s 2 . A large part of the current now goes directly back to 

 the cell 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 alteration in strength 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 weakened by short-circuiting. 



The Helmholtz arrangement should always be employed for tetanic 

 stimulation, unless the nature of the experiment contraindicates it. 



Kronecker's induct orium. Most du Bois-Reymond coils are 

 furnished with a scale marked in millimetres 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 millimetres. 

 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. 



