THE president's addbess. 339 



membrane is in a state of vibration. This membrane can be 

 tightened like a drum by three mill-headed screws. The ends of 

 the coil surrounding the magnet terminate in two binding screws, 

 by which the instrument is put in circuit with the receiving instru- 

 ment. This instrument is nothing more than one of the tubular 

 electro-magnets invented by M. Nicies in 1852. It consists of a 

 vertical bar electro -magnet enclosed in a tub of soft iron, by 

 which its magnetic field is condensed, and its attractive power 

 within that are increased. Over this is fixed, attached by a screw 

 at a point near its circumference, a thin sheet-iron armature, of 

 the thickness of a sheet of cartridge paper, and this when under 

 the infiuence of the transmitted currents, acts partly as a vibrator 

 and partly as a resonator. The magnet with its armature is 

 mounted on a little bridge, which is attached to a mahogany stand, 

 similar to that of the transmit ting instrument. The action of 

 the apparatus is as follows : when a note or a word is sounded into 

 the mouthpiece of the transmitter, its membrane vibrates in unison 

 with the sound, and in doing so carries the soft iron inductor 

 attached to it backwards and forwards in presence of the electro- 

 magnet, inducing a series of magneto-electric currents in its 

 surrounding helix, which are transmitted by the conducting wire 

 to the receiving instrument, and a corresponding vibration is 

 therefore set up in the thin iron armature sufficient to produce 

 sonorous vibrations, by which articulated words can be distinctly 

 and clearly recognized. 



In all previous attempts at producing this result the vibrations 

 were produced by a make-and-break arrangement, so that while 

 the number of vibrations per second as well as the time measures 

 were correctly transmitted, there was no variation in the strength 

 of the current whereby the quality of tone was also recorded. 

 This defect did not prevent the transmission of pure musical 

 notes, nor even the discord produced by a mixture of them, but 

 the complicated variations of tone, of quality and of modulation 

 which make up the human voice required something more than 

 a mere isoehronism of vibratory impulses. In Mr. Bell's instru- 

 ment not only are the vibrations in the receiving instrument iso- 

 chronous with those of the transmitting membrane, but they are 

 at the same time similar in quality to the sound producing them, 

 for the currents being induced by an inductor vibrating with the 

 voice, differences of amplitude of vibrations cause difl:erences in 



