This apparatus consists of the tone-indicator (transmetteur) and the tone-receiver (récepteur). The tone-indicator (Fig. 34, p. 109) consists of a conical tube, _a b_, having a length of about 15 cm., a front aperture of about 10 cm., and a back aperture of about 4 cm., the choice of the material and the greater length of which is said to be indifferent, while a greater width is said to be injurious; the surface of the inner wall should be as smooth as possible. The narrow back aperture of the tube is closed by a membrane, _o_, of collodion, and upon the centre of the circular surface formed by this membrane rests the one end, _c_, of the lever, _c d_, the supporting-point of which, _e_, being held by a support, remains in connection with the metallic circuit. This lever, the arm, _c e_, of which must be considerably longer than _c d_, should be as light as possible, so that it can easily follow the movements of the membrane, because an uncertain following of the lever, _c d_, will produce impure tones at the receiving station. During the state of rest the contact, _d g_, is closed, and a weak spring, _n_, keeps the lever in this state of rest. Upon the metallic support, _f_, which is in connection with one pole of the battery, there is a spring, _g_, with a contact corresponding to the contact of the lever, _c d_, at _d_, the position of which is regulated by means of the screw, _h_. In order that the effect of the apparatus may not be weakened by the produced waves of air communicating themselves towards the back part, a disc “of about 50 (?) cm. diameter, which rests fixedly upon the exterior wall of the tube,” is to be placed above the tube, _a b_, at right angles with its longitudinal axis. [Illustration: Fig. 34.]