PRELIM IN A RY DATA 705 



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 readies 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. 229), and the other with A'. Join A 

 and A' by a short, thick wire. With this arrangement the primary cir- 

 cuit is never opened, but the current is alternately 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 con- 

 nected with the terminals 

 of the secondary coil, 

 and leading to electrodes, 

 a nerve or muscle may Fig 23 o. Unpolarizable Electrodes. A, hook- 

 be stimulated. It is usual shaped; B, U -tubes; C, straight; D, clay in 

 to connect the wires to contact with tissue; S, saturated zinc sulphate 

 a short-circuiting key solution; Z, amalgamated zinc wire. 

 (Fig. 232), by opening 



which the induced current is thrown into the tissue to be stimu- 

 lated. 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 decomposition products of 

 the electrolysis of the tissues deposited on them. And as any slight 

 chemical difference, or even perhaps a difference of physical state, be- 

 tween the two electrodes will cause them and the tissues to form a 

 battery evolving a continuous current, it is often desirable to use un- 

 polarizable electrodes. 



Unpolarizable Electrodes. Some convenient forms of these are 

 represented in Fig. 230. A piece of amalgamated zinc wire dips into 

 saturated zinc sulphate solution contained in the upper part of a glass 

 tube. The lower end of the tube may be straight, but drawn out so 

 as to terminate in a not very large opening, or it may be bent into a 

 hook, in the bend of which a hole is made. Before the tube is filled 

 with the zinc sulphate solution, the lower part of it is plugged with 

 china clay made up with physiological salt solution. The clay just 

 projects through the opening, and thus comes in contact with the 

 tissue. When these electrodes are properly set up, there is very little 

 polarization for several hours, but for long experiments, U-shaped 

 tubes, filled with saturated zinc sulphate solution, are better. The 

 amalgamated zinc dips into one limb, and a small glass tube filled with 

 clay, on which the tissue is laid, into the other. 



45 



