144 



EXPERIMENTAL PHYSIOLOGY. 



(Fig. 48). 



Fig. 48. Scheme of the Break Ex- 

 tra-current. B : Battery ; K and 

 K', keys; P, primary coil; N, 

 nerve. 



Notice that both keys and the primary coil 

 of the induction machine 

 are in the primary circuit, 

 both keys being so arranged 

 that either the primary coil, 

 P, or the electrodes attached 

 to key K', can be short-cir- 

 cuited. 



(6.) Test (a) either by elec- 

 trodes applied to the tongue, 

 or (/3) by means of a nerve 

 muscle preparation (/3 to be 

 done after the student has 

 learned how to make a nerve- 

 muscle preparation). 



(c.) Close the key, K, thus short-circuiting the coil. Open 

 and close key, K'. There is very little effect. 



(d.) Open K', the current passes continuously through the 



primary coil. Open key 

 K, a marked sensation is 

 felt, due to the break 

 extra-current. 



4. Helmholtz's Modification. 

 The break shock is stronger 

 than the make, and to equalise 

 them Helmholtz devised the 

 following modification : 



(a,) Connect the battery 

 wires as before to the two 

 pillars (Fig. 46), P' and 

 P", or to a and e (Fig. 

 49). In Fig. 49 connect 

 a wire from a to /, thus 

 bridging or " short-cir- 

 cuiting" the interrupter. 

 Elevate the screw (/) out 

 of reach of the spring (c), 

 but raise the screw (d) 

 until it touches the spring 

 By this means the make and break 



Fig. 49 . Helmholtz's Modification 

 of Neef's Hammer. As long as c is 

 not in contact with d, g h remains 

 magnetic; thus c is attracted to d, 

 and a secondary circuit, a, b, c, d, e 

 is formed ; c then springs back again, 

 aud thus the process goes on. A 

 new wire is introduced to connect a 

 with /. K, battery. 



at every vibration. 



shocks are nearly equalised, but both shocks are weaker. 



