4 ANNIVERSARY ADDRESS. 
stretched membrane of gold-beater’s skin. He supposed that 
upon speaking in the neighbourhood of the membrane it would 
be thrown into vibration, and cause the steel rod to make a 
corresponding motion, thereby occasioning undulations in the 
electrical current that would correspond to the vibrations in the. 
density of the air during the production of sound; and further, 
that the change in the intensity of the current at the receiving 
end would cause the magnet there to attract its rod so as to 
copy the motion imparted to that at the sending end. 
Mr. Bell’s first experiments were not altogether successful, 
but, persevering in his efforts, he at last produced a model which 
consisted of a permanent magnet with a coil of wire round it, 
and an iron plate in front, which produced audible results. The 
vibration of the voice caused the vibration of the iron plate, that 
vibration produced a current of electricity, the current of elec- 
tricity caused a variation of power in the magnet in the distant 
instrument ; the variation of power in the distant magnet caused 
the iron plate in front of the magnet to vibrate, and that vibration 
produced a sound. ‘Thus the voice was converted into electricity 
at one end, and electricity became voice at the other end. 
Such was the instrument that Mr. Bell sent to the Centennial 
Exhibition at Philadelphia, and the following is the official report 
of Sir William Thomson upon it :—* Mr. Alexander Graham Bell 
exhibits apparatus by which he has achieved a result of trans- 
cendent scientific interest—the transmission of spoken words by 
electric currents through a telegraph wire. To obtain this result, 
Mr. Bell perceived that he must produce a variation of strength 
of current in the telegraph wire as nearly as may be in exact 
proportion to the velocity of a particle of air moved by the 
sound, and he invented a method of doing so—a piece of iron 
attached to a membrane, and thus moved to and fro in the 
neighbourhood of an electric-magnet—which has proved pertectly 
successful. The battery and wire of this electro-magnet are in 
circuit with the telegraph wire and the wire of another electro- 
magnet at the receiving-station. This second electro-magnet has. _ 
