180 



COMPARATIVE PHYSIOLOGY. 



Fig. 166. — Du Boie-Reymond's mductorium (after Roeenthal). i, secondary coil; c, 

 primary coil; 6, electro-magnet; A,armature of hammer; /, email movable screw. 

 The current from battery, ascending metal pillar, passes along hammer, and by 

 Bcrew gets into primary coil, thus inducing current in secondary coil. By con- 

 nection between primary coil and wires around soft iron of 6, iron becomes a maig- 

 net, hammer is attracted from screw/, and current thus broken; but when this 

 occurs, soft iron ceases to be a magnet necessarily, and, hammer springing back, 

 the whole course of events is repeated. This may occur several hundred times in 

 a second. The above may be clearer from diagram, Fig. 167. By sliding second- 

 ai7 coil up and down, strength of induced cuiTent can be graduated. 



may be effected. The apparatus consists essentially of a pri- 

 mary coil, secondary coil, magnetic interrupter, and a scale 



PiQ. 167.— Diagrammatic representation of the working of rig. 166 (after Hosenthal). 



