former preventing, by the elongation of the wire, the pulling of the signal "Off," and the latter, by shortening the wire, preventing the signal from getting properly "On." It is essential that the signalman should know the exact state of his signal, although, by reason of curves or other circumstances, it may be out of his sight, and, in order that he may be so informed, electric signal repeaters have been introduced, the function of which is to send currents from the signal-post, by means of
Fig. 16.
Electric Signal Repeater and Light Indicator
electric contacts, to a miniature signal-post fitted in the repeating instrument at the signal-box. Up to about three years ago the arm was only shown "On" by current, and "Off" by gravity, but since that time it has been found possible to show "On" and "Off" by current and "Wrong" by gravity; and, in addition to this, the apparatus now in use shows "Light in" and "Light out," one wire, only, being required for the five indications. (See Fig. 16.)
