NOTES AND ADDITIONS. 



307 



is that the current in the coil c, and at the same time in the 

 electro-magnet, is much weakened ; the latter can no longer 

 attract the hammer, which springs upward, so that the plate 

 is removed from the point f t and the additional closure is 

 interrupted. The current once more passes in full strength 

 through the coil c and the electro-magnet b, the hammer is 

 again attracted, and the whole process is repeated as long as 

 the circuit K endures. If it is required to restore the appa- 



K 



^J 



FIG. 74. HELMIIOLTZ'S APPARATUS. 



ratus to its original condition, it is only necessary to remove 

 the wire g n and to lower the point/. 



7. ACTION OF CURRENTS OF SHORT DURATION (p. 152). 



Either the closing or opening of a continuous current 

 or an inductive current is used to excite the nerve. In the 

 latter case, however, as has already been indicated in Note 



