174 



PRACTICAL PHYSIOLOGY. 



[XXVITI. 



(A.) Test (a) either by electrodes applied to the tongiie, or (^) 

 by means of a nerve-muscle preparation {ft to be done after the 

 student has learned how to make a nerve-muscle preparation). 



(r.) Close the key K, thus short-circuiting the coih Open and 

 close key K'. There is very little effect. 



(iL) 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. 

 loo), P' and P", or to a and e (fig. 102). In fig. 102 connect a 



wire — " Helmlioltz's side wire " 

 — from a to /', thus bridging or 

 " short - circuiting " the inter- 

 rupter. Elevate the screw (/) 

 out of reach of the spring (c), 

 hut raise the screw (d) until it 

 touches the spring at every 

 vibration. By this means the 

 make and break shocks are nearly 

 equahsed. Test tliis on the 

 tongue. Both shocks, however, 

 are weaker, so that it is necessary 

 to use a stronger battery. The 

 piimary circuit is never entirely 

 broken, it is merely weakened. 



It is always advantageous, 

 when using faradic shocks for 

 physiological purposes, to use 

 make and break shocks of 

 nearly equal intensity, i.e., use 

 Helmholtz's side wire. Why? 

 Because any " polarisation " produced by the one current is 

 neutrahsed by the other. This is not the case with the ordinary 

 arrangement, where the break shock is stronger than the make, 

 whereby there is a progressive summation of the polarisation 



FlO. 102. — Helmholtz's Mndiflcation 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, e, d, e, is formed ; e then 

 springs back again, and thus the process 

 goes on. A new wire is introduced to 

 connect a with/. K. Battery. 



effects of the 



IS a 

 break shocks. 



5. To Approximately Equalise Single Make and Break 

 Induction Shocks. 



As we have seen, the extra-current is the cause of the greater 

 intensity of the break shock. If, however, the intensity of the 



