Selecting the dialled number

Dial tone

The uniselector has connected the caller to a free 1st-stage 2-motion selector, which returns dial tone and waits for the first digit.

The 2-motion selector has a bank of 100 outlets, arranged in 10 levels of 10. Group Selectors accept just one digit and step up to the matching level: dialling 1 steps to the 1st level, a 0 to the 10th. The selector then hunts around the outlets on that level for one that's free. In an exchange with a 4-figure numbering scheme, if the first digit is a 5, the 1st Group Selector steps up 5 levels and finds a free outlet on that level, to a 2nd Group Selector handling numbers starting 5xxx.

The 2nd Group Selector accepts the 2nd digit. If this was a 2, it would step up 2 levels and connect through to a Final Selector handling lines starting 52xx.

The Final Selector accepts the last 2 digits — by this stage the choice has narrowed to 100 lines. Dialling 14 steps the Final Selector up to level 1, then round to outlet 4: the call is now through to line 5214, and the Final Selector tests it to check it's free.

The Final Selector returns the appropriate tone to the caller (ring or engaged), and applies ringing current to the called line.