SINGLE NEEDLE TELEGRAPH

Cooke & Wheatstone's five needle telegraph has the advantage that the transmitted characters are immediately indicated and no operator training is required. However, it is difficult and expensive to implement because it requires five wires.

In 1845 Cooke & Wheatstone also patented a single needle telegraph. I have two of these instruments in my collection. These systems were still used by the British Railways until the 1970s. It is a bipolar telegraph where a needle is deflected depending on the polarity of the signal. The coding is  the same as Morse, but the dots and dashes have equal duration. The movement of the needle follows the rotation of the handle. The needle touches a metal plate where the sound is different for both sides. Therefore the operator does not need to watch the needle movement while he or she is recording the transmitted message. The instruments on a railway line are in series, so all stations can eavesdrop on a message.