such cases, regulated by a special code of electric bell signals, but this plan was very unsatisfactory, and the difficulty was at length met by the introduction of the "Tell-tale" instrument. This is really a form of block telegraph, worked under "permissive regulations"; but there is a special apparatus, by means of which a reminder is given to the signalman at a station of the number of trains there are in the section in the rear, (See Fig. 12.)
A block instrument of any kind consists of an indicating dial and a commutator or handle. The indicator is attached to an axle, on which a magnet is centred, being suspended either within, or in close proximity to, a coil of silk-covered wire, and deflected to the right or left according to whether a "negative" or "positive" current flows through the coil. The commutator is so arranged as to alter the direction of the current at will. The bells are generally of the single-stroke type, and are used for calling attention, and for giving the complete code of signals descriptive of the nature of trains, as agreed to by all the railway companies.
The normal state of the indicator of the three-wire block instrument is vertical ("Line closed") when no current is flowing; the deflection when a negative current is flowing is to the right, meaning "Line clear," and with a positive current to the left, indicating "Train on line." (A set of the three-wire apparatus is shown in Fig. 13.)
Briefly the modus operandi is as follows:—Attention is first called on the bell, and then station A will give to station b what is called the "Be ready" signal, consisting of a certain number of strokes of the bell, varying so as to indicate the nature of the train. The man at station
