624 A MANUAL OF PHYSIOLOGY 



series of shocks, sometimes called an interrupted or faradic current. 

 Such a series of stimuli can also be got by making and breaking a 

 voltaic current at any given rate. 



A ' self-induced ' current can also be obtained from a single coil ; 

 for instance, from the primary coil alone of the induction apparatus. 

 The reason of this is, that when a current begins to flow through any 

 turn of a coil of wire, it induces in all the other turns a current in 

 the opposite direction, and, when it ceases to flow, a current in the 

 same direction as itself. The former current, ' the make extra shock,' 

 being in the opposite direction to the inducing current, is retarded in 

 its development, and reaches its maximum more slowly than * the 

 break extra shock.' But, as we shall see, the suddenness with which 

 an electrical change is brought about is one of the most important 

 factors in electrical stimulation, and therefore the break extra shock 

 is a much more powerful stimulus than the make. Owing to these 

 self-induced currents, the stimulating power of a voltaic stream may 

 be much increased by putting into the circuit a coil of wire of not 

 too great resistance. 



The self-induction of the primary also affects the stimulating 

 power of the currents induced in the secondary ; the shock induced 

 in the secondary by break of the primary current is a stronger 

 stimulus than that caused at make of the primary. The reason is 

 that with a given distance of primary and secondary, and a given 

 intensity of the voltaic current in the primary, the abruptness with 

 which the induced current in the secondary is developed depends 

 upon the rapidity with which the primary current reaches its maxi- 

 mum 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. 212), 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, 

 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. 

 215), 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- 



