174 



PRACTICAL PHYSIOLOGY. 



XXVIII. 



(b.) Test (a) either by electrodes applied to the tongue, or (ft) 

 by means of a nerve-muscle preparation (ft 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. 

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



wire " Helmholtz's side wire " 

 from a to /, thus bridging or 

 " short - circuiting " the inter- 

 rupter. Elevate the screw (/) 

 out of reach of the spring (c), 

 but raise the screw (d) until it 

 touches the spring at every 

 vibration. By this means the 

 make and break shocks are nearly 

 equalised. Test this on the 

 tongue. Both shocks, however, 

 are weaker, so that it is necessary 

 to use a stronger battery. The 

 primary circuit is never entirely 

 broken, it is merely weakened. 

 B is . <*** advantageous, 



when USing faradic shocks f Or 



physiological purposes, to use 



springs back again, and thus the process make and break shocks of 

 goes on. A new wire is introduced to , . . .. 



connect a with/. K. Battery. nearly equal intensity, ^.e. 1 use 



Helmholtz's side wire. Why? 



Because any " polarisation " produced by the one current is 

 neutralised 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 

 effects of the 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 



contact with d, g h remains magnetic ; 



