It is a curious fact, worth recording here, that Mr. Yates, of Dublin, in 1865, when trying to improve Reiss’s telephone, realised to a certain extent Mr. Gray’s conception of the liquid transmitter; for he introduced into the platinum contacts of Mr. Reiss’s instrument a drop of water which adapted it for the reproduction of articulate sounds. However, no notice was then taken of this result. EXAMINATION INTO THE FUNDAMENTAL PRINCIPLES ON WHICH BELL’S TELEPHONE IS BASED. Although the preceding account would suffice to make the principle of Bell’s telephone intelligible to persons acquainted with electric science, this would not be the case with the majority of our readers, and we therefore think it necessary to enter into some details as to the source of the electric currents which are employed in telephonic transmissions. These details seem to us the more necessary, since many persons still believe that Bell’s telephones are not electric, because they do not require a battery, and they are often confounded with string telephones, so that the difference of price between Bell’s instruments and those hawked in the streets seems astonishing. Without defining what is meant by an electric current, which would be too elementary, we may say that electric currents can be produced by different causes, and that, in addition to those which are due to batteries, strong currents are also produced by the force exerted by magnets on a conducting circuit properly arranged. Such currents are called induction currents, and are used in Bell’s telephone. In order to understand how they are developed under these conditions, it will be enough to examine what takes place when the pole of a magnet is brought near to, and withdrawn from, a closed circuit. To do this, let us suppose a copper wire attached to a galvanometer in the form of a circle, and that one pole of a permanent magnet is directed towards the centre of the circle. Now observe what happens: 1. At the moment when the magnet approaches an electric current arises, causing the galvanometer to deviate to one side. This deviation will be great in proportion to the extent of the movement, and the tension of the current will be great in proportion to the abruptness of the movement. The current will however be only instantaneous. 2. At the moment when the magnet is withdrawn, a fresh current of the same nature will arise, but it will appear in an opposite direction from the former. It will be what is called a direct current, because it is in the same direction as the magnetic current of the magnet which produces it, while the other current is called _inverse_.