When the number of people using teleprinters to contact each other increased, a separate public exchange network called Telex was introduced. It was utilised to give standard communications amenities on a circuit switching basis. By the end of 1974, the telex network had over 700,000 subscribers world wide.
Telex works in a very similar way to the telephone network. The users rent their teleprinters and pay call charges, which are determined by the distance and duration of the call. It enables the operator to dial immediately to the subscriber to whom the communication is addressed. No aural answer comes from the switching equipment, but instead there is a button which illuminates indicating when to dial. As there is no necessity for the operator to be present at the other end, the exchange equipment automatically creates a signal known as "Who are You" in the machine that has been called. This activates another signal called "Answer-Back", which comes from the addressed machine. This is printed on the calling teleprinter, confirming that the correct connection has been made. The operator can then forward the relevant message using their keyboard.
The receiving teleprinter deciphers the signal and activates the printing mechanism. The mechanism itself can be one of many different types including a dot matrix printing head, or a type basket similar to a typewriter. The message is generally printed onto a continuous roll of paper. The transmitting and receiving teleprinters both print the one message. Up to six copies can be printed by utilising interleaved carbons. The transmitted message is frequently printed in red to distinguish it from the message received.
In the last five years the telex machine has been superseded by the facsimile machine, which performs similar functions at a fraction of the cost.
