Page:Popular Science Monthly Volume 3.djvu/418

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railway, with the view of ascertaining whether the rails could be employed as lines of telegraph, made the discovery that the earth would serve instead of a return-wire, and with the advantage of diminished resistance; the earth, in fact, behaving like a return-wire of infinitely great cross-section, and therefore of no resistance.

We are not, however, to suppose that the current really returns from the receiving to the transmitting station through the earth. The duty actually performed by the earth consists in draining off the opposite

Fig. 2. Fig. 3.
PSM V03 D418 Single needle instrument.jpg
Single needle Instrument. Internal Arrangements.

electricities which would otherwise accumulate in the terminals. It keeps the two terminals at the same potential; and, as long as this condition is fulfilled, the current will have the same strength as if the terminals were in actual contact.

One of the best-known telegraphs in England, though little or not at all employed elsewhere, is the single-needle instrument of Wheatstone and Cooke, represented in Figs. 2 and 3, the former showing its external appearance, and the latter its internal arrangements as seen from behind. The needle, which is visible in front, is one of an astatic pair, its fellow being in the centre of the coil (C C). When the handle (H) hangs straight down, the instrument is in the position for receiving signals from another station. The current from the line-wire enters at L, and, after traversing the coil and deflecting the needle, escapes through the earth-wire (E), having taken in its course the two tall contact-springs (t t').