The sender resembles the one represented in fig. 19, except that, instead of liquid, M. Righi employs plumbago mixed with powdered silver, and the platinum needle is replaced by a metallic disk D (fig. 38). The receiver I, which contains the powder, is supported on a spring R, which can be pushed up and down by a regulating screw V, and the whole is fitted into a box C C, and supported on a foot P. The speaker places himself above the mouthpiece E, and the vibrations transmitted to the membrane L L cause the variations of resistance in I which are necessary for the transmission of speech, as in the Edison system. Two Bunsen cells are enough to set the instrument at work, and it will make the sound of a trumpet or flute audible throughout a room. Vocal music, which is less intense, is necessarily transmitted to a rather less distance, and words spoken in the natural voice are heard by those standing about two yards and a half from the instrument. The maximum distance at which the instrument has been worked with the battery only is twenty-eight miles, the distance between Bologna and Ferrara, and for greater distances it is necessary to have recourse to induction coils. In this case, an induction coil is introduced into the circuit at each station, and its primary wire is traversed by a current from the local battery, and so also is the sender, which is elsewhere connected with the receiver by a commutator. The secondary circuit of these coils is completed through the earth and line wire. From this arrangement it follows that the induced current which influences the receiver in correspondence, only produces its effect after a second induction, produced on the primary wire of the local coil, and it appears that this is a sufficient effect; but the advantage of this arrangement is, that it is possible to receive and transmit sounds without the aid of anything but the commutator. Among other arrangements which have been suggested, we may mention one in which, instead of the bar magnet, a horseshoe magnet is used, with a vibrating plate placed between its poles. For this purpose the poles are tipped with iron, and one of them is pierced with a hole which corresponds to the mouthpiece of the instrument. The two branches of the magnet are also furnished with magnetising helices. When anything is spoken before the hole, the vibrating plate causes induced currents in the two helices: these currents would be of opposite direction if the poles were of like nature, but, since the magnetic poles are of contrary nature, they are in the same direction. The vibrating plate then acts like the two plates of M. Trouvé’s instrument, which we have described above.