This diagram shows a Morse telegraph circuit connected between to distant points. The point at which telegraph messages are sent and received is called an "office". In this case, each office is equipped with three instruments and two batteries. The purpose of each instrument is explained below. For a detailed explanation, follow the link to "Tools of Telegraphy".
Telegraph key- This is the basic "transmitter". A telegraph key is nothing more than a simple electrical switch. This switch has a pair of contacts that makes and breaks the circuit. The key is ergonomically designed to allow the operator to rapidly make and break the contacts in order to transmit symbols of the Morse code.
Morse Relay- The Morse relay is a simple electromechanical amplifier. The Morse relay is designed to be extremely sensitive to electric current. A horseshoe electromagnet is arranged to attract a lightly balanced iron armature. A contact on the armature and on a stationary post make contact when current flows through the coils of the electromagnet. These contacts make and break the "local circuit".
Local Sounder- The "sounder" is the receiving instrument. This instrument is designed to produce an audible "click" when its electromagnet is energized. It also produces a second "click" with a slightly different tone upon cessation of the current. Thus each current pulse produces a "click-clack" sound. The local sounder is designed for maximum loudness, and it is not very sensitive to electrical current. It takes as much as 20 times as much current to activate a local sounder compared to a relay.
Main Battery- This is an battery made of several electrochemical cells connected in series. Up to several hundred volts may be required depending on the length of the line and the number of instruments cut in to the wire. These batteries were replaced with dynamos somewhere around the turn of the century.
Local Battery- This was usally a single cell with a voltage of 1 to 1.5 volts. Its only purpose was as a current source for the local sounder. The local battery continued in use well into the 20th century in the many depots and way stations without AC power.
Telegraph Line- Usually a very heavy iron wire was used. The wire was galvanized to reduce corrosion. The telegraph line was suspended from wooden poles using specially designed glass insulators. The design of the insulators was critical in reducing leakage paths to ground. These leakage paths were known as "escapes", and they could render a telegraph line useless in wet weather.
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