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POPULAR SCIENCE REVIEW. 
duced. The effect was improved by building the nails log-hut fashion into 
a square figure, using ten or twenty nails. A piece of steel watch-chain 
acted well. Up to this point, the coarser vibrations alone were produced, the 
finer inflections were missing ; in other words, the timbre of the voice was 
wanting, but in the following experiment it became more and more perfect,, 
until it reached a perfection leaving nothing to be desired.” 
Mr. Hughes tried all forms of pressure and modes of contact. All metals 
could be made to produce identical results, provided the division was small 
enough, and that the material used did not oxidize by contact with the air 
filtering through the mass. A mass of shot is peculiarly sensitive to sound 
while clear, but as the shot soon becomes coated with oxide, this sensitive- 
ness ceases. Carbon, from its surface being entirely free from oxidation, is 
excellent, but the best results at first attained were from mercury in a finely 
divided state. He used the willow-charcoal employed by artists for sketching,, 
heating it gradually to a white heat, and then plunging it suddenly into 
mercury. The pores thus being filled with innumerable minute globules,, 
held it in a state of fine division. Tin and zinc answered the same purpose. 
Pine charcoal thus treated with iron developed high conductive powers 
though of itself a non-conductor. 
The substances were at first confined in a glass tube or box provided with 
wires for introducing them into the circuit. 
The conductors were found to be affected by sounds absolutely inaudible 
except by this means, and hence the instrument was termed the Microphone. 
In its later form it consists of a lozenge-shaped piece of gas-carbon 1 
inch long, | inch wide at the middle, and £ inch thick, the lower pointed 
end pivots upon a similar block ; all these pieces of carbon being tempered 
in mercury. 
The tube-transmitter exhibited to the Royal Society consisted of a glass- 
tube 2 inches long and \ inch in diameter, containing four separate pieces of 
willow-charcoal pressed together till the resistance was about a third of 
that of the line in which it was to be employed. 
Since then many and various forms have been adopted with success. A 
sphere of carbon, resting loosely between two hemispherical depressions in 
blocks of the same material answers well ; and a pretty as well as a sensi- 
tive modification is made by Mr. Yeates of King Street, Oovent Garden, 
consisting of a short cylinder of battery-carbon 1 inch long and £ inch 
diameter, pivoted loosely at its central point and resting by one end on a 
small carbon anvil, the pressure on which can be regulated, either by a 
superimposed weight or by a small spiral spring. All modifications are 
mounted on thin resonance-boards of pine, by which the aerial vibrations are 
caught and intensified as by the sound board of a violin. 
By the kindness of Professor Hughes, the writer has been enabled to 
examine the last shape adopted. It consists of a small resonance box of 
light wood, like that of a tuning-fork, within which is a vibrating plate of 
iron about inch long, and \ inch broad, pivoted, as in the previous case,, 
about its centre, and held down on the contact-anvil by a light spring, 
capable of opposing a weight of from 5 to 20 grains. The contact pieces 
are of battery-carbon. If two of these microphones and two ordinary 
telephones be included in the same battery circuit, a curious fact comes tO' 
