The mouth-tube communicates with the inside of the box or air-chamber, and the sound-waves act upon the tympanum from bellow. On the side of the box are seen a telegraph key, e, and sounder, for signalizing between the transmitting and receiving stations. This is the transmitter successfully used, as will be seen, by Yeates, in Dublin, in 1865. So sensitive was this transmitter that it was found unnecessary, in its early use, to speak directly into the mouth-piece; and in practice the speaker talked and sang at a little distance from it. In the reports of experiments with this instrument the rattling noises which were sometimes complained of as heard in the receiver were undoubtedly due to the complete breaking of the circuit by too loud talking or singing in the mouth-piece. The Berliner and Blake transmitters are liable, unless specially guarded, to the same misadventure from the same cause.
The receiver in Fig. 2 is Reis's latest form of the "knitting-needle" instrument. A helix of insulated wire is attached horizontally to a sounding-box. Through the helix a steel wire or knitting-needle is passed without contact, and supported at each end by a bridge. The vibrations of this knitting-needle magnet, corresponding exactly to the vibrations of the tympanum of the transmitter, are converted into sound-waves by the extended surface of the box acting upon the air. On the side of the box is a telegraph-key to communicate back to the transmitter. The code of signals suggested in the accompanying "Prospectus" contains the following:
"One tap sing.
Two taps speak."
An original telegraphic letter alphabet is also suggested, showing how slight was Reis's acquaintance with ordinary telegraphing.
Fig. 3 represents the form of transmitter, figured and described by Reis in his first memoir of 1861. I present it, out of chronological order, on account of its simplicity and close resemblance to the modern transmitters. A conical chamber, a b, is bored through a cubical