M. de Lalagade has suggested an analogous mode by employing for the formation of the arc a current of which the tension is increased by inserting a strong electro-magnet into the circuit. This electro-magnet acts on a Hughes magnet in order to produce induction currents capable of making the receiving instrument act. M. de Lalagade says that a Bunsen battery, or one of six cells with bichromate of potash, will be enough to produce a continuous voltaic arc between the vibrating plate of a telephone and a platinum point which is sufficiently remote to avoid contact. It is necessary, however, to begin with a contact, in order to produce the formation of this arc. In M. de Lalagade’s system, the vibrating plate should have in its centre a small platinum plate, in order to obviate the oxidising effects of the spark. The inventor asserts that sounds transmitted in this way, and reproduced in a telephone of which the electro-magnetic system is set upon a sounding-box, will have greater intensity than the sounds transmitted by an ordinary telephone, and the speaker will appear to be close to the ear. _Mercury Telephones._--These systems are based on the physical principle discovered by M. Lippmann, that if a layer of acidulated water is placed above mercury, and connected with it by an electrode and wire, every mechanical action which exerts pressure on the surface of the mercury, and alters the form of its meniscus, will cause an electric reaction, capable of producing a current with a force which corresponds to the mechanical action exerted. Conversely, every electric action produced on the circuit of such a system will occasion a displacement of the meniscus, and consequently its movement, which will be more marked in proportion to the smallness of the tube in which the mercury is placed, and to the greatness of the electric action. This electric action may result from a difference of potential in the electric condition of the two extremities of the circuit, which communicate with the electric source employed, or with some electric generator.[8] [Illustration: FIG. 34.]