Referring first to Fig. 23, AB is part of a conducting circuit which includes the battery or other source of current, such as the electric light mains of a ship, connected through a suitable resistance, as shown in the cases of the helm indicator, steering telegraph and engine telegraph, in Figs. 1, 7, 10 and II. T is a galvanometer which is connected in shunt with AB, one terminal being fixed at the point A, and the other terminal, or contact, C, being movable along the conductor AB. It will be clear that if the contact C is moved along the conductor AB, the difference of electric resistance, and, therefore, of electric potential or pressure between the points A and C will be varied; and consequently the extent of deflection of the needle or index of the galvanometer T may be controlled as desired, so as to cause said index to point to any scale-division or other mark or marks inscribed along its path. Connected in multiple arc with the galvanometer T are the two conductors + and -. Obviously these conductors will also be affected by the movement of the traveling contact C along the line AB, and will assume a difference in potential depending on the position of C on AB; so that if galvanometers I be connected to these conductors in the manner shown in Fig. 23, these galvanometers will respond to the movements of the contact C, for the same reason as does the galvanometer T. Furthermore, if all the galvanometers T and I be exactly similar, the needle deflections in all will be the same; and thus any indication caused in the galvanometer T will be repeated in the galvanometers I. Therefore, if T be the transmitting galvanometer, and if the instruments I be located at distant stations, it is plain that an operator at T, by adjusting the traveling contact C, can produce in his instrument a deflection which will instantly be repeated and shown at the distant stations in the receiving galvanometers I. It will be apparent, however, that if the galvanometers I are differently located with respect to each other and to the galvanometer T, so that in the circuit of one there is a different resistance from that which is in the circuit of the others; or if from any cause, such as rise in temperature, and consequent temporary loss of the magnet's strength, any galvanometer becomes less sensitive, means must be provided whereby said galvanometer may be adjusted or regulated so as to compensate for any such differences or changes, or, in other words, so that each receiving galvano

meter may be so regulated that its deflections or indications will correspond to those of the transmitting galvanometer T. In the practical construction of the system these means are provided, and consist merely of an adjustable resistance placed in series with each galvanometer, and secured in a water-tight iron box R.

Turning now to Fig. 24, which shows in detail the means employed for moving the contact C along the wire AB, we see that the wire AB is wrapped in a spiral groove traced in an insulating cylinder D. The ends of the wire are secured to german silver springs. g, g, secured at the ends of the cylinder near the axis. The binding posts A and B are insulated from the metal box J, and their ends are prolonged about half an inch inside the box into cylindrical pins, or axles, a, a, which fit in the cylinder D, and press tight against the german silver springs, g, g, to which are secured the ends of the resistance wire AB. There is, therefore, a complete circuit from the binding post B to the german silver spring g, through the resistance. wire that is wrapped around the cylinder, to the german silver spring g at the other end of the cylinder, and thence to the binding post A.

The cylinder is revolved on the axle AB by means of the handle H, which turns the screw S, and the gear wheel G, and thence the gear wheel G', which is secured to the cylinder D.

The contact C is secured to a stout piece of rubber that is secured on a nut N, that travels on the screw S. The pitch of the screw S and that of the spiral on the cylinder D are equal, so that, if the handle H be turned, the contact C will move with the nut N, though insulated from it, and press successively on different parts of the wire. The contact C is connected by the flexible wire W to the binding post C'. If the battery be connected to the binding posts AB, and if A and C be connected to the binding posts and of T in Fig. 23, it is clear that the act of revolving H will cause the same effect as if the contact C were directly moved along the straight wire AB, in Fig. 23.

Turning now to Fig. 25, which represents the adjustable resistance in the box R, we see that the construction is nearly the same as that in the transmitting box just described. The resistance wire is, however, connected to the german silver spring

at the center of its cylinder at the left end only, as the right end of the wire is secured to the insulating cylinder at the point P. The contact C" is moved in the same way as is C in the transmitting box, Fig. 24, and is connected by the flexible cord W to the binding post C". The current enters at the binding post A', passes thence to the german silver spring at the left end of the cylinder, to the resistance wire wrapped on the cylinder, to the contact C", and thence to the binding post C". Clearly, if this apparatus be placed in series with any galvanometer I, and the handle H' be turned, the resistance in series with I will be varied and the deflection of I correspondingly changed.

The place for the transmitter T is near the reading instrument of the range-finder, so that any distance read from it can at once be made to appear on the transmitter and simultaneously on all the indicators in the ship.

To adjust the apparatus for use, connect the battery or other source of electricity and make the transmitter read, say, 2950, or any other preconcerted number. Then make each receiver read the same number, by turning the thumb wheel H'. The apparatus is now ready; but if there is time, it is best to try several preconcerted numbers in succession. It is better not to use the even hundreds in adjusting, because the graduations half-way between them are finer and give better opportunities for exactness.

The transmitting and indicating galvanometers are all similar, so that, in the event of accident to the transmitter, any receiver may be substituted for it. If this is done, however, it will be necessary, of course, to verify the adjustment of the instruments, in the manner above described.

In case a battery is used, the one found most satisfactory for all this class of work is the "chloride" storage battery. It is extremely uniform in resistance and in electro-motive force, and requires almost no care, provided it be connected to the electric light circuit and charged once a week. To do this is very easy, as it only takes a little more trouble than is required to turn on an incandescent electric light.

The galvanometers can be secured directly to a bulkhead." If desired, they can be graduated to work in a horizontal position.