No.7076 Circuits of Model Telegraph Line - Open Circuit System
using Polarised Relays.
Two Stations

[Diagram of Circuit]

Explanation.  Signals are made by short and long closing of a key according to the Morse Code.

If the Key at A be depressed, current flows from its main battery to Line, thence by the back contact of B's Key through the coils of his relay to Earth and back to A. The cores of B's relay are magnetized and attract the armature on to its front stop closing the red or local circuit in which current then flows from the local battery working the Sounder. Similar action results at A when B closes his key. A Key does not affect its own relay.

The function of the Lightning Arrester is to divert atmospheric electricity to Earth which is effected by the use of two brass plates with paper between. The discharge from the line punctures the paper and jumps to Earth.

Intermediate stations may be included in the circuit if required.

The Polarised Relays are biassed [sic] so that the attraction of one pole-piece acts as a spring to draw the armature back when no current is passing. Non-polarised relays may be used.

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