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POPULAR SCIENCE REVIEW. 
believes it to be — it is yet capable of rendering distinctly the words, or 
even the cough or sneeze, of a person thirty or forty miles distant. And 
the marvel of all this is greatly enhanced when one sees the extreme sim- 
plicity of the apparatus by which such extraordinary results are brought 
about. A small mahogany cylinder, which can be easily carried in the 
pocket, and is strongly suggestive of a stethoscope, contains all the me- 
chanism; nothing indeed is visible outside except a mouthpiece at one 
end, and a couple of binding-screws at the other. Moreover, this modest 
little bit of apparatus serves equally for the transmitter and for the receiver 
of the sounds. You first speak through the mouthpiece, and then transfer 
it to the ear and listen for the reply. Even the interior of the wooden 
cylinder is simple in the extreme. Near to the mouthpiece there is a small 
disc of thin sheet-iron, which is thrown into vibration by the voice. These 
vibrations are executed in front of a soft-iron core attached to one pole of 
a permanent bar-magnet, and surrounded by a small coil of No. 38 silk- 
covered copper wire, one end of which is connected with the line-wire, 
while the other end communicates with the earth. The permanent magnet 
induces a magnetic field all round it, and attracts the iron diaphragm. 
When this disc vibrates, the condition of the magnetic field is affected, and 
a current is consequently induced in the coil ; the strength of the current 
depending upon the amplitude and form of the vibrations. Each induced 
current traverses the line- wire, and passing through the coil of the receiving 
instrument, alters the magnetism of its core ; thus increasing or diminishing 
its attractive effect on the iron disc, which is therefore thrown into a state 
of vibration. The vibrations of the receiving diaphragm are executed in 
perfect unison with those at the transmitting station, and consequently every 
sound uttered at one end, even to the nicest modulation of the voice, is re- 
produced at the other end. 
Many of those who experimented with the telephone at Plymouth, and 
failed to get very audible replies, may have concluded that it is far from 
being a trustworthy instrument. But it should be borne in mind that 
Bell’s instrument is so extremely sensitive, that when in connection with 
a telegraphic line, the currents along the neighbouring wires induce cur- 
rents in the coils of the telephone, and the oscillating disc then produces a 
hum which almost overpowers the voice. In the United States, however, 
the apparatus is in practical use, instruments on Bell’s principle being 
worked in New York, in Boston, and in Providence. At Plymouth verbal 
messages were sent from one part of the town to another ; and in some of 
the deep Cornish mines conversation has since been carried on between the 
bottom of the shaft and the surface. Mr. Edison, of New York, has effected 
certain improvements in Professor Bell’s apparatus ; and Mr. Preece tells 
us that, with this improved form of telephone, words have been distinctly 
heard through a resistance equivalent to a distance of a thousand miles of 
wire. In the face of these results, no -one will dare to say that oral tele- 
graphy is not an accomplished fact. 
While Bell’s telephone is at present unique as an apparatus for the trans- 
mission of articulate speech, it need hardly be said that there are several 
other forms of telephone capable of transmitting musical notes, though not 
distinct words. Such, for example, is Mr. C. F. Varley’s instrument, which 
