Chap. ii.J THE CONTRACTILE TISSUES. 59 



nerve on the electrodes. Thus ' putting down ' and ' raising or ' opening ' 

 the key have contrary effects in A and B. In B, it will be observed, 

 the battery is always at work, the current is always flowing either 

 through the electrodes (key up) or through the key (key down); in 

 A, the battery is not at work until the circuit is made by putting 

 down the key. And in many cases it is desirable to take so to speak 

 a sample of the current while the battery is in full swing rather than 

 just as it begins to work. Moreover in B the electrodes are, when the 

 key is down, wholly shut off from the current ; whereas in A, when 

 the key is up, one electrode is still in direct connection with the battery ; 

 and this connection, leading to what is known as unipolar action, may 

 give rise to stimulation of the nerve. Hence the use of the key in 

 the form B. 



Other forms of key may be used. Thus in the Morse key (F, Fig. 

 3) contact is made by pressing down a lever handle (ha); when the 

 pressure is removed, the handle, driven up by a spring, breaks contact. 

 In the arrangement shewn in the figure one wire from the battery 

 being brought to the binding screw b, while the binding screw a is 

 connected with the other wire, putting down the handle makes connec- 

 tion between a and b, and thus makes a current. By arranging the wires 

 in the several binding screws in a different way, the making contact by 

 depressing the handle may be used to short circuit. 



In an " induction coil," Figs. 3 and 4, the wire connecting the two 

 elements of a battery is twisted at some part of its course irto a close 

 spiral, called the primary coil. Thus in Fig. 3 the wire x n> connected 

 with the copper or negative plate c.p. of the battery, E ) joins the 

 primary coil pr. c, and then passes on as y'", through the • key " F, 

 to the positive (zinc) plate z.p. of the battery. Over this primary coil, 

 but quite unconnected with it, slides another coil, — the secondary coil, s.c. ; 

 the ends of the wire forming this coil, y" and x", are continued on in 

 the arrangement illustrated in the figure as y' and y, and as x' and x, 

 and terminate in electrodes. If these electrodes are in contact or con- 

 nected with conducting material, the circuit of the secondary coil is said 

 to be closed ; otherwise it is open. 



In such an arrangement it is found that at the moment when 

 the primary circuit is closed, — i. e. when the primary current is " made," 

 a secondary " induced " current is, for an exceedingly brief period of 

 time, set up in the secondary coil. Thus in Fig. 2 when, by moving 

 the "key " F, y'" and x"' (previously not in connection with' each other) 

 are put into connection and the primary current thus made, at that 

 instant a current appears in the wires y" x" &c, but almost immediately 

 disappears. A similar almost instantaneous current is also developed 

 when the primary current is " broken," but not till then. So' long as 

 the primary current flows with uniform intensity, no current is induced 

 in the secondary coil. It is only when the primary current is either 

 made or broken, or suddenly varies in intensity, that a current appears 

 in the secondary coil. In each case the current is of very brief 

 duration, gone in an instant almost, and may therefore be spoken of as 

 "a shock," an induction shock, — being called a '"'making shock " when 

 it is caused by the making, and a " breaking shock " when it is caused 

 by the breaking, of the primary circuit. The direction of the current 



