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HANDBOOK OF PHYSIOLOGY. 



the primary circuit, an instantaneous current of electricity is induced in the 

 secondary coil, g, if it be sufficiently near and in line with the primary coil ; 

 and the nearer it is to c, the stronger is the current induced. The current 

 is only momentary in duration and does not continue during the whole of the 

 period while the primary circuit is complete. When, however, the primary 

 current is broken by opening the key, a second, also momentary, current is 

 induced in g. The former induced current is called the making and the latter 

 the breaking shock ; the former is in the opposite direction to, and the latter in 

 the same as, the primary current. 



The induction coil may be used to produce a rapid series of shocks by means 

 of another and accessory part of the apparatus at the right of the fig. , called 

 the magnetic interrupter. If the wires from a battery are connected with the 

 two pillars by the binding screws, one below c, and the other, a, the course of 



Fig. 334. Diagram of the course of the current in the magnetic interrupter of Du Bois Bey- 

 mond's induction coil. (Helmholz's modification.) 



the current is indicated in fig. 324, the direction being indicated by the arrows. 

 The current passes up the pillar from e, and along the springs if the end of d 

 is close to the spring, the current passes to the primary coil c, and to wires 

 covering two upright pillars of soft iron, from them to the pillar a, and out 

 by the wires to the battery ; in passing along the wire, &, the soft iron is con- 

 verted into a magnet, and so attracts the hammer, /, of the spring, breaks the 

 connection of the spring with d', and so cuts off the current from the primary 

 coil, and also from the electro-magnet. As the pillars, b, are no longer mag- 

 netized the spring is released, and the current passes in the first direction, 

 and is in like manner interrupted. At each make and break of the primary 

 current, currents corresponding are induced in the secondary coil. These cur- 

 rents are opposite in direction, but are not equal in intensity, the break shock 

 being greater. In order that the shocks should be nearly equal at the make 

 and break, a wire (fig. 324, e') connects e andcf, and the screw d' is raised out 

 of reach of the spring, and d is raised (as in fig. 324) , so that part of the cur- 

 rent always passes through the primary coil and electro- magnet. When the 

 spring touches d, the current in 6 is diminished, but never entirely withdrawn, 

 and the primary current is altered in intensity at each contact of the spring 

 with d, but never entirely broken. 



Record of Muscular Contraction under Stimuli. The muscles of the frog are 

 most convenient for the purpose of recording contractions. The frog is pithed, 

 that is to say, its central nervous system is entirely destroyed by the insertion 

 of a stout needle into the spinal cord, and the parts above it. One of its lower 

 extremities is used in the following manner. The large trunk of the sciatic 

 nerve is dissected out at the back of the thigh, and a pair of electrodes is 



