When anything is said before the mouthpiece _a_, the waves of sound encountering the edges of the plate M place it in vibration, and, continuing their passage inside the tubular magnet, they cause the plate M′ to vibrate at the same time as M. A double inducing action therefore takes place on the tubular magnet, and this is translated by the induced currents developed in the helix, which have greater energy since each of the plates intensifies the magnetic effects produced at the pole opposite to the one they influence, which is always the case with bar magnets when the inactive pole is provided with an armature. This advantage may even be obtained in the case of ordinary telephones, if the screw which holds the magnet is placed in contact with a mass of soft iron. In M. Trouvé’s arrangement, the induced currents therefore possess greater energy; but he adds that the sounds reproduced will also be strengthened by the multiplicity of vibratory effects, and by the enlargement of the magnetic effects, which results from a better arrangement of the magnets. ‘When the ear is placed at _a_,’ M. Trouvé writes, ‘it perceives immediately the sounds produced by the first plate M, and those of the second plate reach the ear through the interior of the magnet. This new arrangement is well adapted for an experimental comparison of the results produced by a telephone with a single membrane (a Bell telephone), and those produced by a telephone with several membranes. It is in fact enough to listen at the two faces of the telephone alternately, in order to perceive at once the difference of intensity in the sounds produced. Those collected at _a_, on the side of the pierced iron plate, appear manifestly doubled in intensity compared with those collected at _b_ on the side of the simple membrane which forms the ordinary telephone. ‘The difference is still more striking if, in transmitting or receiving a sound of invariable intensity through a multiple telephone, the unbroken membrane M′ is repeatedly prevented from vibrating.’ Before making this arrangement M. Trouvé had planned another, which he presented to the Académie des Sciences, November 26, 1877, and which we have glanced at in the beginning of this chapter. He describes it in these terms:-- ‘In order to increase the intensity of the effects produced in the Bell telephone, I have substituted for the single membrane a cubic chamber, of which each face is, with one exception, formed of a vibrating membrane. Each of these membranes, put in vibration by the same sound, influences a fixed magnet, which is also provided with an electric circuit. In this way, by connecting all the currents generated by the magnets, a single intensity is obtained, which increases in proportion to the number of magnets influenced. The cube might be replaced by a polyhedron, of which the faces might be formed of an indefinite number of vibrating membranes, so as to obtain the desired intensity.’