Inside a Crossbar Office


Principal relay types used



A wide range of electromechanical relays was employed in crossbar offices. Most of the control relays in crossbar offices built in the 1940's and 1950's were of the 'U-Y' type, also known as flat-spring.

In the 1960's the wire-spring relay, with faster actuation and improved contact life was introduced. 

Large versions ('multi-contact') of both flat- and wire-spring relays were used to momentarily connect the markers with other equipment in the office.

Reed relays were used for digit storage and assembled into packages of five relays each. One of these can store a single digit. They used a 2-out-of-5 code: 0,1,2,4,7. The digit '0' is stored as 4+7. 

All transmission paths were switched on crossbar switches, which generally came in 200-point or 100-point varieties, and in 3-wire or 6-wire versions. These formed the switching fabric of the network.

click images for better views

a relay flat P6120083.jpg (86321 bytes)
Flat-spring, or 'U-Y' type relays


a relay flat multi P6120030.jpg (29102 bytes)
Flat-spring multi-contact relays


a relay wirespring P6120119.jpg (28134 bytes)
Wire-spring relays


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Multi-contact wire-spring relays



a relay reed P6120104.jpg (25340 bytes)
Reed relay packs. Each contain 5 reed relays, 
often used for digit storage in a '2 out of 5' code.


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A partial view of a crossbar switch


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The 'mini-switch' variant crossbar switch