542 A MANUAL OF PHYSIOLOGY 



with which the primary current reaches its maximum at closing, or 

 its minimum (zero) at opening. Now, the make extra current retards 

 the development of the primary current, while in the opened circuit 

 of the primary coil the current intensity falls at once to zero. 



The inequality between the make and break shocks of the 

 secondary coil can be greatly reduced by means of Helmholtz's wire. 

 Connect one pole of the battery with v (Fig. 155), and the other 

 with A'. Join A and A' by a short, thick wire. With this arrange- 

 ment the primary circuit is never opened, but the current is alter- 

 nately allowed to flow through the primary, and short-circuited 

 when the spring touches v. The 'make' now corresponds to the 

 sudden increase of intensity of the current in the primary when the 

 short-circuit is removed, and the ' break ' to its sudden decrease 

 when the short-circuit is established. In both cases self-induced 

 currents are developed, and therefore both shocks are weakened. 

 But the opening stimulus is now slightly the weaker of the two, 



FIG. 156. UNPOLARIZABLE ELECTRODES. 



A, hook-shaped ; B, U-tubes ; C, straight. D, clay in contact with tissue ; S, 

 saturated zinc sulphate solution ; Z, amalgamated zinc wire. 



because the opening extra shock has to pass through a smaller 

 resistance (the short-circuit) than the closing extra shock (which 

 passes by the battery), and therefore opposes the decline of current 

 intensity on short-circuiting, more than the closing shock opposes 

 the increase of current intensity on long-circuiting through the 

 primary. 



By means of wires connected with the terminals of the secondary 

 coil, and leading to electrodes, a nerve or muscle may be stimulated ; 

 and it is usual to connect the wires to a short-circuiting key (Fig. 

 158), by opening which the induced current is thrown into the tissue 

 to be stimulated. For some purposes the electrodes may be of 

 platinum ; but all metals in contact with moist tissues become 

 polarized when currents pass through them, that is, have decom- 

 position products of the electrolysis of the tissues deposited on them. 

 And as any slight chemical difference, or even perhaps a difference 

 of physical state, between the two electrodes will cause them and the 

 tissues to form a battery evolving a continuous current, it is often 

 desirable to use unpolarizable electrodes. 



Unpolarizable Electrodes. Some convenient forms of these are 

 represented in Fig. 156. A piece of amalgamated zinc wire dips into 



