Previous part (The Strowger switch on an industrial scale)

Other things in an automatic exchange

When was it in use? 1912-1995 (UK)

As well as two-motion selectors there is other equipment in an automatic Strowger exchange.

Ringing Machines are generators driven by an electric motor. A series of mechanical cams attached to the motor shaft cause various electrical contacts to be opened and closed at various speeds. The resulting signals are at various frequencies and are applied to the lines by the two-motion selectors to indicate the call status. These tones include the dial tone (a purring noise indicating you can start dialling), ringing tone, engaged tone, congestion tone (more accurately known as Equipment Engaged Tone - used to indicate a path through the exchange could not be found as all are in use) and Number Unobtainable (indicating call can not be connected due to a fault or because the number dialled is not in use). The video on the right shows a ringing machine generating both ringing tone (sent the caller) and ring current (sent to the called). Telephone exchanges run off Direct Current. Ringing the bell requires Alternating Current which is also generated by the Ringing Machine

Another essential piece of equipment is the interface between the line and the exchange. This is called a Line Circuit and consists of 3 relays and a uniselector. The uniselector works a bit like a two-motion selector but instead of stepping up and then rotating, it just rotates. You might call it a 'One-motion selector'. The job of the uniselector (which is controlled by the 3 line relays) is to connect the caller to a free two-motion selector when they lift the handset. Two-motion selectors were fairly big and expensive pieces of kit, so having one for each line (which most of the time would be on-hook and unused) was inefficient. Instead there was one for each call anticipated during the busy period of the day. This idea of 'subscriber concentration' i.e only having enough switching capacity for the anticipated traffic NOT every customer is still used in modern exchanges using solid state digital switches.