(In the practical investigations it has been established that the choice of material for this tube is without influence on the use of the apparatus, and moreover a greater length of the same for the certainty [of action] of the apparatus is without effect. A greater width of the cylinder spoils the usefulness of the apparatus; and it is recommended that the interior surface be as smooth as possible.) The narrow hinder aperture of the cylinder is closed by a membrane, _o_, of collodion, and on the middle of the circular surface formed by this membrane rests one end, _c_, of the lever, _c d_, the fulcrum (point of support), _c_, of which, supported on a bearing, remains joined to the metallic conductor. The choice of the length of the two arms of the lever, _c e_ and _e d_, is determined by the laws of force of levers. It is recommended that the arm, _c e_, be constructed longer than the arm _e d_, in order to bring the smallest movement at _c_ into action at _d_ with the greatest possible force; but, on the other hand, it is desirable to make the lever itself as light as possible, in order that it may follow the motions of the membrane. An uncertain following of the lever, _c d_, produces impure tones at the receiving station. In the condition of rest the contact, _d g_, is closed, and a delicate spring, _n_, holds the lever firmly in this position of rest. The second part of this apparatus, the pillar, _f_, consists of a metallic support, which is united with one pole of the battery, C, while the second pole of the battery is carried to the metallic conductor of the other station. Upon the support, _f_, there is a spring, _g_, with a contact, which corresponds to the contact at _d_ of the lever _c d_, and whose position is regulated by a screw, _h_. In order not to weaken the action of the apparatus by the communication of the air-waves which are produced in using the apparatus, against the back of the membrane, it is recommended, in using the apparatus, to place over the tube, _a b_, at right angles to its longitudinal axis, a screen of about 50 centimetres diameter, which fixes tight upon the outer surface of the tube. The receiver, Fig. B, consists of an electro-magnet, _m m_, which reposes upon a sounding-box, _u w_, and whose wire coils are respectively connected with the metallic conductor and with the earth or metallic return-conductor. Opposite the electro-magnet, _m m_, stands an armature, which is connected with a lever, _i_, which is long as possible, but light and broad. The lever, _i_, is fastened, pendulum-wise, to the support, _k_, and its movements are regulated by the screw, _l_, and the spring, _p_. In order to improve the action of the apparatus, this receiver can be placed in one focus of an elliptically arched cavity of corresponding size, in which case, then, the ear of him who is listening to the reproduced tones may be placed at the second focus of this cavity.