Fig. 4.
To this apparatus there may be added a second bobbin of coarser wire into which is passed a current from a local pile. This produces a much intenser magnetic field, and, consequently, louder sounds. This modification, however, is really useful only for long distances.
Any arrangement imaginable may be given the transmitter and receiver; but, aside from the fact that the ones just indicated are the simplest, they give results that are at least equal, if not superior, to all others.
We shall insist here only upon the arrangement of the microphone, which is new (at least in practice), and upon the uselessness of having well magnetized steel bars and wires of extreme fineness in the receiver.
We have stated that the nail microphones are the simplest. The nails may be replaced by copper or any other metal, or they may be well nickelized; but common nails answer very well, and do not oxidize much. An apparatus of this kind (Fig. 5) that has been for more than a year in a laboratory filled with acid vapors is yet working very well. These apparatus possess the further advantage of being very strong, and of undergoing violent shocks without breaking or even getting out of order. They may be used either with or without induction coils. We have not yet measured their range, but can cite the following fact:
Fig. 5.
One of these apparatus, quite crudely mounted, was put into a circuit with a resistance of 300 ohms. With a single already exhausted bichromate element, giving scarcely 2 volts, musical sounds and speech reached the receiver without being notably weakened. Such resistance represents a length of eighteen miles of ordinary telegraph wire. After this, 700 ohms were overcome with 3.4 volts. This result was obtained by direct transmission, and without an induction coil, and it is probable that it might be much exceeded without sensibly increasing the electromotive force of the current.—Le Genie Civil.