34 



PflYSIOLOGY OF MUSCLES AND NERVES. 



traction of this sort is called tetanus of the muscle to 

 distinguish it from a series of distinct pulsations. 



Another method of frequently and repeatedly clo- 

 sing and opening the current is by means of a self- 

 acting apparatus which is put in motion by the current 

 itself. This, which is called Wagner's hammer, is re- 

 presented in fig. 12. The current of the chain is con- 

 ducted through the column 

 represented on the right to 

 the German silver spring o o. 

 A small platinum plate c is 

 soldered on to the latter, and 

 is pressed against the point 

 above it by the elastic force 

 of the spring. The current 

 passes from this to the coils 

 of a small electro-magnet, 

 and, after passing through 

 this, back to the chain 

 through the clamp connected with it on the left. An 

 armature of soft iron, n, fastened on to the spring 

 o o, is suspended over the j)oles of the electro-magnet. 

 This iron being attracted by the electro-magnet, the 

 small plate c is forced away from the point and the cur- 

 rent is thus interrupted. In so doing, however, the 

 electro-magnet parts w^ith its magnetism, and conse- 

 quently relinquishes its hold upon the armature ; the 

 plate is thus again pressed by the action of the 

 spring against the point. The current being thus again 

 closed, the electro-magnet recovers its force, again at- 

 tracts the armature, and again interrupts the current ; 

 and these processes are continued as long as the chain 

 remains inserted between the column on the right and 



Fig. 12. Wagner's Hammer. 



