Under these conditions, the sender consists, as we see (fig. 63), of a bar tuning-fork, _a_, which is grooved for the passage of a runner, heavy enough to tune the fork to the desired note, and it oscillates between two electro-magnets _e_ and _f_ and two contacts I and G. The difference of resistance in the electro-magnets is very great: in the one _f_ the resistance is equal to 2¾ miles of telegraphic wire, in the other it does not exceed 440 yards. When electric communication is established as we see in the figure, the following effect takes place. Since the current of the local battery through the two electro-magnets is broken by the rest-contact of the Morse key H, the plate _a_ is subject to two contrary actions; but since the electro-magnet _f_ has more turns than the electro-magnet _e_, its action is preponderant, and the plate is attracted towards _f_, and produces a contact with the spring G, which opens a way of less resistance for the current. Since the current then passes almost wholly through G, _b_, 1, 2, B, the electro-magnet is now able to act; the plate _a_ is then attracted towards _e_, and, by producing a contact on the spring I, it sends the current of the line B P through the telegraphic line, if the key H is at the same time lowered on the sending contact: if not, there will be no effect in this direction, but since the plate _a_ has left the spring G, the first effect of attraction by the electro-magnet _f_ will be repeated, and this tends to draw the plate again towards _f_. This state of things is repeated indefinitely so as to maintain the vibration of the plate, and to send out signals corresponding with these vibrations whenever the key H is lowered. The elastic nature of the plate makes these vibrations more easy, and it ought also to be put in mechanical vibration at the outset. The receiver, represented fig. 64, consists of an electro-magnet M, mounted on a sounding-box C, and having an armature formed by a tuning-fork L L firmly buttressed on the box by a cross bar T. There is a runner P on the armature, sliding in a groove, which makes it possible to tune the vibrations of the tuning-fork to the fundamental note of the sounding-box C, which is so arranged as to vibrate in unison with it. Under these conditions, the box as well as the tuning-fork will act as an analyser of the vibrations transmitted by the currents, and may set the register at work by itself reacting on a breaker of the local current. To obtain this result, a membrane of gold-beater’s skin or parchment must be stretched before the opening of the box, and a platinum contact must be applied to it, so arranged as to meet a metallic spring connected with any kind of register or a Morse instrument, when the membrane vibrates. As, however, in America the messages are generally received by sound, this addition to the system is not in use. [Illustration: FIG. 64.]