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POPULAR SCIENCE REVIEW. 
to which the wires are attached. Articulate sounds were thus heard very- 
loud and distinct, though occasionally marred by the rattling of the cinders 
in the jar. 
lie then filled a shallow box of thin wood with cinders, nailing to its two 
ends tin connections. It proved to be a very sensitive transmitter, and if 
three such boxes were hung like pictures on the walls of a room almost any 
noise made in the room was revealed. A part song by two voices was dis- 
tinctly transmitted. When water was poured into the original can, the 
Leclanche cell could be dispensed with. 
It proved that the jar of cinders would act as a receiver as well as a 
transmitter. Two Grove’s cells were included in circuit, and articulate 
sounds were clearly heard. The magnetic element of the telephone was 
here entirely superseded. 
To demonstrate the actual motion and vibration of the cinders under 
the influence of sound, a strong battery and a glass jar were used. In 
a dark room flashes of electric light were observed between the frag- 
ments. 
In a paper read before the Physical Society, Professor Hughes describes 
the u Physical Action of the Microphone ” as the introduction into the 
circuit of an electrical resistance, which varies in exact accord with 
sonorous vibrations, so as to produce an undulatory current from a con- 
stant source. The undulations in length, height, and form, are an exact 
representation of the sonorous waves. It is essential that the instru- 
ment be so arranged as regards pressure between the touching surfaces 
us to be adapted to the particular vibrations employed. Thus a box 
suitable for a man’s voice is not adapted to observe the tramp of a fly. 
When speaking, a galvanometer should be interposed in the circuit, and 
the pressure between the surfaces gradually increased from a minimum 
until the needle remains stationary, when a maximum loudness will be 
attained. In the experiments illustrating the paper, a small clock being 
placed on a drawing-board carrying a microphone, and made to inter- 
rupt the current passing through a telephone, the tick was audible 
throughout the room. The telephone being provided with a bell-mouthed 
tube, enabled the sounds to be heard at a distance. A second tele- 
phone was then introduced into the circuit, and laid on the board, a 
continuous sound being at once produced. Thus a relay for the human 
voice could be obtained ; for it is only necessary to provide such an 
arrangement at each station for a speech to be received and transmitted 
to any number of succeeding stations. The system is perfectly duplex, 
for if two correspondents speak into microphones, and use telephones in 
receiving, each can hear the other, but his own speech is inaudible, and 
if each sing a different note, no chord is heard. 
Messrs. Edwin Houston and Elihu Thomson, writing from the Central 
High School, Philadelphia, to 11 Nature ” (June 28th), describe a u Tele- 
phone Relay ” or “ Repeater,” which consists in applying the microphone 
directly to the diaphragm of the receiving telephone ; the two fixed 
pieces of carbon being cemented to the diaphragm itself, and the third 
piece placed in cavities near their ends. The microphone forms part of 
the new circuit in which it is desired to repeat the message. 
