Page:Popular Science Monthly Volume 5.djvu/320

306 built as lightly as possible, and weighs when loaded only 14 cwt., and is easily drawn by two horses. On the outside are two insulated brass conductors i (Fig. 2), to which wires can be attached. Inside the windows is a shelf with a drawer d, on which the instrument t can be placed when in use, and opposite to this is a seat or bench s for the

operators, on which a man can sleep at night. Under the seat is a recess in which a spare instrument t is kept, while the batteries are arranged in a box b under the shelf. When a message is to be sent from this movable station the wagon is stopped, and the line-wire is attached to the insulated conductor i. This is connected with the instrument and battery, and in order to complete the circuit the battery is placed in electrical communication with the second insulated conductor, to which another wire is attached which joins it to the earth-conductor or earth-stake (piquet d terre) (Fig, 5). Thus the course of the current, when transmitting a message, is from the battery to the instrument, and by the first insulated conductor e (Fig. 2) to the line of wire, the earth-plate of the receiving-station returning it to the earth-conductor, driven into the ground near the wagon, and thus back by the second insulated conductor to the battery.

The instruments are of the Morse pattern, constructed so as to fit in a very small space, and recording the signals with ink. The battery (of which there are two in each station-wagon) is a simple form of M. Marié Davy's sulphate-of-mercury battery. It consists of ten elements, one of which is shown in section in Fig. 8; c is a charcoal vessel, containing sulphate of mercury moistened with water to the consistency of paste, and in this the zinc plate z is suspended by means of the India-rubber cover I. The whole is placed in the India-rubber vessel i, and a copper collar y is added, to which a connecting wire can be attached. This battery has the advantage of being very portable, while the India-rubber cover prevents the charge of sulphate of mercury from being spilt by the motíon of the wagon.

The line may be either an aërial or a ground wire, or a combination