88 PROCEEDINGS OF THE AMERICAN ACADEMY 



divided by the resistance, and, where proper units are taken, it may 

 be represented thus : — 



C= -^, where G is the current, E the electro-motive force, and 



R the resistance in the circuit. Now electro-motive force depends 

 upon the origin of the electricity. When batteries are used as the 

 source of electricity, the electro-motive force may be modified by 

 coupling cells in series or sets ; but if a single cell be used, its electro- 

 motive force is a constant quantity which depends upon the chemical 

 relations of the substances employed. To modify electro-motive force 

 from batteries is entirely impracticable. Not so, however, the elec- 

 tricity derived from magneto-electric machines. Here the velocity 

 of the armature modifies the electro-motive force. The mechanical 

 motion of an armature in a magnetic field re-acts upon the magnet in 

 such a way as to develop a current with electro-motive force propor- 

 tional to its velocity ; the essential thing being the motion of an 

 inductive substance like iron or steel. Seeing that a piece of iron 

 may be made to move rapidly by sound-vibrations, it is plain that 

 such vibrations in a magnetic field will originate currents of electricity 

 with a great electro-motive force ; for the rapidity of the vibration of 

 the plate for ordinary speech will be for a man's voice in the neigh- 

 borhood of 125 per second for the fundamental sound, to say nothing 

 of the overtones. Professor Bell started with the right principle, 

 and, however faulty his first instrument was, it involved the generic 

 idea, as the subsequent development so fully corroborates. Now 

 there are two distinct methods by which the vibratory currents may 

 be set up ; namely, by electro-magnets and by permanent magnets. 

 "With the first, the action of the armature in the origination of the 

 currents is part of the time to send a wave in the same direction as 

 the current already in the line, and a part of the time to send one in 

 the opposite direction ; in other words, it will alternately increase and 

 decrease the current on the line, and this not only by its inductive 

 action upon the magnet, but by its inductive action upon the current 

 circulating in the bobbin, whereas with the permanent magnet there 

 is the inductive action of the armature upon the magnet and coils, 

 while the only electricity in the circuit originates in the coils. The 

 former method originated with Professor Bell, the latter with myself. 

 Some there are who think the two to be identical. I think they 

 hardly can be identical ; for in the case of an electro-magnet, when 

 induction is thus utilized, there is not only magnetic induction upon 

 the magnetic core, but there is also electric induction, — that is, indue- 



