In article ,
Harry Bloomfield wrote:
Chris Holmes explained :
How would this have worked? Would all of the indicator current have
been sent through the coil that activated the switch for the extra
contact, and only when the extra bulb was connected would the current
be sufficient to throw the switch?

Yes, through a relay with a low resistance coil winding.

They used the same idea for detecting when a car lamp failed long ago.
It's all done by electronics now.

The bulb failure unit on the old Rover uses reed relays. And a vast amount
of extra cabling.

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*If you must choose between two evils, pick the one you've never tried before

Dave Plowman London SW
To e-mail, change noise into sound.