This part is moveable upon a hinge on the lower little box, A A. If the cover, _x u_, is laid back, one sees that a small circle of 3·9 centimetres diameter has been cut out in the same. Into this hole passes a brass collar with a flange 8 millimetres broad, which is furnished at one side with a groove like a pulley. Over the collar there is stretched the membrane, _m m_, by means of a silk thread lying in the shoulder of the same. This circular membrane is surrounded by a wider circular aperture, _b b_, = 8·5 centimetres. A shovel-shaped little strip of platinum, _n s_, lies (over it) leading to the brass binding-screw, _d_, with the circular part, _s_, falling upon the centre of the membrane. By means of some sealing-wax this circular part is fastened to the membrane, and thereby compelled to take part in the vibrations of the same. The further transmission of the galvanic current from the centre takes place by means of a platinum or steel point resting in a cup of mercury, which is extended in a screw, which transmits the current farther. The point _a_ serves as a support for the angular hook, _a s b_, which in general is supported like a tripod, in order that the point of contact, _s_, may remain as constant as possible. The hook, _a s b_, is simply struck with a hole at _a_ upon a projecting point, and lies upon a broader under part. From _b_ the galvanic circuit proceeds by means of an overspun wire to the brass key _e_ (A, Fig. 33), and from there farther in the direction represented by the arrow. The lower part A A of the transmitter is put together of thin wood and forms a parallelepiped, whose height = 6·8 cm., and whose width = 7·7 cm. An inclined mouthpiece of tin with funnel-shaped opening serves to receive the tones. The longer side of this mouthpiece measures 6·7 cm., the shorter 4·7 cm.; the longer diameter of the widening measures 7·15 cm., the shorter diameter 7·5 cm., and finally the diameter of the narrow tube 3·9 cm. It is clear that, if necessary, the platinum strip can be replaced by a strip of thin sheet-brass, the platinum or steel points by iron. Only in this case the points of contact must be oftener cleaned to a metallic polish.