60 THE CONTRACTILE TISSUES. 



key ; hence, this is called "short-circuiting." When the bridge is raised the cur- 

 rent passes through the 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 elec- 

 trodes 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. 1 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 shown in the 

 figure, one wire from the battery being brought to the binding screw 6, while the 

 binding screw a is connected with the other wire, putting down the handle makes 

 connection between a and &, 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. 13 and 14, the wire connecting the two elements 

 of a battery is twisted at some part of its course into a close spiral, called the 

 jtriiiHinj coil. Thus, in Fig. 13, the wire cc //x , connected with the copper or nega- 

 tive plate c. p. of the battery, E, joins the primary coil, pr. c. , and then passes on 

 as ?/ x/ , 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 

 secoiidart/ coil ,s\ c. ; the ends of the wire forming this coil, T/ X/ and x x/ , are con- 

 tinued on in the arrangement illustrated in the figure as y' and ?/, and as x / and 

 .-c, and terminate in electrodes. If these electrodes are in contact or connected 

 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 cir- 

 cuit 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. 13, when, by moving the "key" f] y /// and x 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, ?/", x", etc., but almost imme- 

 diately disappears. A similar almost instantaneous current is also developed when 

 the primary current is "broken," but not till then. So long as the primary cur- 

 rent 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 cur- 

 rent 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 in the making 

 shock is opposed to that of the primary current ; thus, in the figure, while, the 

 primary current flows from x /// to y'" , the induced making shock flows from y to 

 x. The current of the breaking shock, on the other hand, flows in the same 

 direction as the primary current from x to ?/, and is therefore in direction the 

 reverse of the making shock. Compare Fig. 14, where arrangement is shown in 

 a diagrammatic manner. 



The current from the battery, upon its first entrance into the primary coil, as it 

 passes along each twist of that coil, gives rise in the neighboring twists of the 

 same coil to a momentary induced current having a direction opposite to its own, 

 and therefore tending to weaken itself. It is not until this "self-induction" has 

 passed off that the current in the primary coil is established in its full strength. 

 Owing to this delay in the full establishment of the current in the primary coil, 

 the induced current in the secondary coil is developed more slowly than it would 

 be were no such " self-induction " present. On the other hand, when the current 

 from the battery is "broken " or "shut off" from the primary coil, no such delay 

 is offered to its disappearance, and consequently the induced current in the second- 



