ON PROFESSOR BELl's ARTICULATING TKLEPHONE. iSj 



Unlike ir.ost telegraphic instruments, the transmitting and 

 receiving instruments are identical in construction, and each may 

 be used for either purpose. It consists essentially of three parts, 

 (i) A steel bar magnet 3 (2) a coil of thin insulated wire wound on 

 a bobbin, placed over one end of the magnet 5 (3) a thin iron disc 

 or diaphragm, placed very close to, bat not actually in contact with, 

 the same pole of the magnet. The whole is enclosed in a wooden 

 case, with a trumpet-shaped mouthpiece, and the ends of the coil 

 of wire terminate in binding-screws, to which are attached the 

 conducting wires connected with the Instrument at the distant 

 station. The usual dimensions of the parts are, — magnet, 4 

 inches long, § to -^ inch diameter ; disc, 2J to 2^ inches diameter : 

 the kind of iron known as '' Ferrotype" by photographers, answers 

 best. The wire should be No. ^6 guage, silk covered, and about 

 i^ to 2 oz. should be wound on the bobbin. 



The explanation of the mode of action of the telephone is very 

 simple. When speech is uttered into the mouthpiece, the iron 

 disc or diaphragm is thrown into vibration, (the sound waves being 

 carried from the larynx through the air and striking upon the disc) 

 and alternately approaches closer to and recedes from the end of 

 the magnet. Each of these movements causes a change in the 

 magnetic field, and consequently the induction of a feeble current 

 of electricity in the coil of wire. This current travels along the 

 conducting wire to the receiving station, where it enters the other 

 telephone, and travels round its coil. The magnetisation of this 

 magnet is thereby disturbed, and its attraction for its diaphragm 

 increased or diminished, causing the diaphragm to move. It will 

 be readily seen, then, that the succession of currents thus arriving 

 will cause the receiving diaphragm to vibrate exactly as the trans- 

 mitting one does. The air in contact with the receiving diaphragm 

 will receive precisely the same succession of iinpulses as those 

 which fell upon the transmitter 3 and consequently an ear applied 

 to the receiving telephone will receive these vibrations of the air 

 in the form of sound, or articulate speech. 



