64 INDUCTION COIL. [BOOK i. 



FIG. 3. DIAGRAM ILLUSTRATING APPARATUS ARRANGED FOR EXPERIMENTS WITH 



MUSCLE AND NERVE. 



A. The moist chamber containing the muscle-nerve preparation. The muscle 

 m, supported by the clamp cl. , which firmly grasps the end of the femur /, is 

 connected by means of the S hook s and a thread with the lever I, placed below 

 the moist chamber. The nerve n, with the portion of the spinal column ri still 

 attached to it, is placed on the electrode-holder el, in contact with the wires 

 x, y. The whole of the interior of the glass case gl. is kept saturated with 

 moisture, and the electrode-holder is so constructed that a piece of moistened 

 blotting-paper may be placed on it without coming into contact with the 

 nerve. 



B. The revolving cylinder bearing the smoked paper on which the lever writes. 



G. du Bois-Eeymond's key arranged for short-circuiting. The wires x and y of 

 the electrode-holder are connected through binding screws in the floor of the 

 moist chamber with the wires a;', y', and these are secured in the key, one on 

 either side. To the same key are attached the wires x" y" coming from the 

 secondary coils s. c. of the induction-coil D. This secondary coil can be 

 made to slide up and down over the primary coil pr. c., with which are con- 

 nected the two wires x'" and y'". x'" is connected directly with one pole, for 

 instance the copper pole c. p. of the battery E. y'" is carried to a binding 

 screw a of the Morse key F, and is continued as y iv from another binding 

 screw b of the key to the zinc pole z. p. of the battery. 



Supposing everything to be arranged, and the battery charged, on depressing the 

 handle ha, of the Morse key F, a current will be made in the primary coil pr. c. , 

 passing from c. p. through x'" to pr. c., and thence through y'" to a, thence to b, 

 and so through y lv to z. p. On removing the finger from the handle of F, a spring 

 thrusts up the handle, and the primary circuit is in consequence immediately 

 broken. 



At the instant that the primary current is either made or broken, an induced 

 current is for the instant developed in the secondary coil s. c. If the cross bar h in 

 the du Bois-Eeymond's key be raised (as shewn in the thick line in the figure), the 

 wires x", x', x, the nerve between the electrodes and the wires y, y f , y" form the 

 complete secondary circuit, and the nerve consequently experiences a making or 

 breaking induction- shock whenever the primary current is made or broken. If the 

 cross bar of the du Bois-Eeymond key be shut down, as in the dotted line-%' in the 

 figure,, the resistance of the cross bar is so slight compared with that of the nerve 

 and of the wires going from the key to the nerve, that the whole secondary (induced) 

 current passes from x" to y" (or from y" to x"), along the cross bar, and practically 

 none passes into the nerve. The nerve being thus "short-circuited," is not affected 

 by any changes in the current. 



The figure is intended merely to illustrate the general method of studying muscular 

 contraction ; it is not to be supposed that the details here given are universally 

 adopted or indeed the best for all purposes. 



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 y, and is therefore in direction the reverse of the making 

 shock. Compare Fig. 4, where arrangment is shewn 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 

 neighbouring 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 



