Why Ringmain?

And then she appeared...on a flat-screen TV in our gleaming ultra-modern new offices, was the girl and the clown and the noughts and the crosses.

I've written before about the Test Card F nostalgia trip, and the fact that although you seldom (if ever?) see it on BBC One or BBC Two nowadays, it is still used in the BBC prior to internal communications broadcasts on the ringmain. Apparently (according to Wikipedia) since 1999 we have used the same image on Test Card J and Test Card W. Yesterday the flat-screens in the Broadcast Centre 5th Floor hubs were displaying Test Card W (albeit in a mangled aspect ratio).

It dawned on me though that despite my four years at the BBC I have no idea why we call the ringmain "the ringmain". It is an internal television channel, used to announce major BBC-wide initiatives, carry the launches of TV seasons throughout the whole organisation, and on days like yesterday deliver the number of job cuts to expect.

I posted a message on talk.gateway, the BBC's internal message board system, to see if anyone knew where the term "ringmain" originated, and so far the best we could come up with collectively is that it is the equivalent of the electric ringmain in a house, i.e. a signal sent in a loop all round a premise.

I'm still not convinced myself - someone out there must be able to tell me when it came into being and how it got to be named.