Page:Popular Science Monthly Volume 22.djvu/346

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of jointed vertical bars (B, B', B"), which, except at the times for the signal, are kept in contact by springs with cocks in the circuit of the wire; but at the times for the signal a long metallic bar (C) acting as a cam (better shown in Fig. 7, in front of the vertical bars), is made by clock-work to disconnect all these bars from their instruments. The bar (C) is divided into three parts, corresponding to the long, medium, and short provincial lines, insulated from each other, and connected respectively with the bars of the relays (V, V', V") through the galvanometers (g, g', g", Fig. 8). The left or rest contacts of these relays are in connection with the zinc poles of separate batteries, whose copper poles are grounded, so that, when the bars of these relays are put in connection with the line wires, a zinc or "preliminary" current is ready to be sent out; this current prevents the distant relays from being actuated by contacts or accidental currents, and serves as a warning signal. The right-hand contacts of the relays are connected respectively with the copper poles of separate batteries whose zinc poles are grounded, so that, when the bars are moved over to the right (which is done by the incoming Greenwich current), the outgoing current is reversed, and this constitutes the signal. The relay V" is for distributing the signals only to points in the metropolis, and, as the wires on these lines are under ground, no "preliminary" current is necessary.

The mechanical operation of the apparatus is as follows: On the clock (R, Fig. 9) there is an ebonite wheel (W) in which are two notches (N, N') corresponding to 10 a. m. and 1 p. m. Shortly before 10 a. m. the pin (P) on one arm of the forked lever (L) falls into the notch (N), allowing the end (Q) of the other arm to rest on the ebonite hour wheel (T). About two minutes before the hour, the end (Q) comes against the contact (S), and completes the circuit of the local battery (IT, Fig. 8) through the starting magnet (M, Fig. 9) and sets the clock-train (shown in Fig. 7) in motion, pressing the cam (C) against the vertical bars, disconnecting them from their instruments, and connecting them respectively, in groups as already shown, with the relays (V, V, V"), in readiness to send a "preliminary" current to the line wires. At ten seconds to the hour an insulated pin (i, Fig. 9) on the wheel (T) lifts the lower arm of the forked lever (F),so that its upper arm comes in contact with a small cam on the arbor of the escape wheel (K). This contact closes the circuit of the battery (U) through the coils of the two relays (Z, Z'). The relay (Z) puts on the earth connection at (E), for the four relays (V, V,' V", V'"), so that the current from Greenwich may be received and divided between them, while the relay (Z') disconnects the Westminster clock-wire and connects it with the metropolitan lines to receive the signal from the relay (V"). The relays (V, V, V", V'") have a resistance of 5,000 ohms to allow of the splitting of the current. At precisely ten o'clock the Greenwich signal reverses the current on the lines, and thus gives the exact time. At ten seconds past the hour the contact between H and K is