(This narrative does not relate directly to a specific object. However, Object No. ST 007076 is a model of an open circuit telegraph line using polarised relays with two stations and so the narrative has been linked to that object.)
24. 3. 16
Telegraph Line on Closed Circuit System - Three Stations
[Diagram of Circuit]
1. In the Closed Circuit System, all Keys are kept closed on their front contacts when not in use. It will be seen that the back contacts are not electrically connected. A constant current flows throughout the line and returns by the Earth, keeping all relays closed. When a station desires to speak he first opens his Key, and then calls the station he wants by forming his call letters in dots and dashes. All relays on the line respond; and they, in turn actuate their sounders by a local current, the armature of the relay acting as a Key to the Sounder.
2. The system does not actually require more than one battery, but it is usual to place one at each end of the circuit. Stations can be opened anywhere readily and portable relays and keys can be used in the open. Fifteen or twenty stations are often placed on one circuit. The standard line current is fifteen milliamperes.
3. The Lightning Arrester consists of two brass plates with paper between them.
Lightning discharges entering the Line jump from Line to Earth Plate through the paper and thus are diverted from the Instruments.