40 MUSCULAR CONTRACTION. [BOOK i. 



the passage of a single induction-shock, which may be taken as a 

 convenient form of an almost momentary stimulus, will produce no 

 visible change in the nerve, but the muscle will give a short sharp 

 contraction, i.e. will for an instant shorten itself, becoming thicker 

 the while, and then return to its previous condition. If one end of 

 the muscle be attached to a lever, while the other is fixed, the 



by some conducting material, such as a wire, and the current is said to flow in a 

 circuit or circle, from the zinc or positive element to the copper or negative element 

 inside the battery and then from the copper or negative element back to the zinc or 

 positive element through the wire outside the battery. If the conducting wire be cut 

 through, the current ceases to flow, but if the cut ends be brought into contact, 

 the current is re-established and continues to flow so long as the contact is good. 

 The wires or the ends of the wires, which may be fashioned in various ways, are 

 called electrodes. When the electrodes are brought into contact or are connected 

 by some conducting material, galvanic action is set up, and the current flows 

 through the battery and wires; this is spoken of as "making the current" or 

 "completing or closing the circuit." When the electrodes are drawn apart from each 

 other, or when some non-conducting material is interposed between them, the 

 galvanic action is arrested; this is spoken of as "breaking the current" or "opening 

 the circuit." The current passes from the electrode connected with the negative 

 (copper) element in the battery to the electrode connected with the positive (zinc) 

 element in the battery ; hence the electrode connected with the copper (negative) 

 element is called the positive electrode, and that connected with the zinc (positive) 

 element is called the negative electrode. 



In an "induction machine" the wire connecting the two elements of a battery 

 is twisted at some part of its course into a close spiral, called the primary coil. 

 Thus in Fig. 1 -the wire x"' 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. (In Fig. 9. p. 51) 

 the direction of the current from x to y through the primary coil P is shewn by 

 arrows; but in this figure complications are introduced which will be explained 

 hereafter.) 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 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 

 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. 1 when by moving the "key" F, y'" and x'" previously not in con- 

 nection 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 in the making shock is opposed 

 to that of the primary current; thus in the figure while the primary current flows 

 from of" to i/'", 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 y, and is therefore in direction the reverse of the making shock. 



When the primary current is repeatedly and rapidly made and broken, the 

 secondary current being developed with each make and with each break, a rapidly 

 recurring series of alternating currents is developed in the secondary coil and passes 

 through its electrodes. We shall frequently speak of this as the interrupted induction 

 current, or more briefly the interrupted current. 



