This is the Bulgin 'Indicator' Lightning Switch, catalogue number S.99, made by the A. F. Bulgin & Co. Ltd., Barking & London, c. 1936. The device has a built-in spark gap and fuse and was intended to protect a wireless set from the otherwise terminal (!) effects of a lightning strike.

Here is an extract from a Bulgin publication showing the way in which the device was intended to be wired up.

Recommended connection of the S.99 Lightning Switch

Reproduced from the Bulgin publication 'Bulgin Radio Service Manual' c. 1936

How effective the device was in practice is questionable. Bulgin were confident enough to offer a 100 limit 12 month guarantee against damage to the purchaser's wireless. I wonder what the chances are of having one's aerial struck by lightning within 12 months of buying this little switch. More chance of winning the lottery I imagine. The original box, abbreviated fitting instructions and 100 guarantee are reproduced below, along with a photograph of the original Bulgin factory where this switch would have been made. 

 

 

 

The original Bulgin factory in Barking

(photo reproduced with the kind permission of Bulgin Components PLC who retain copyright)

 

Copyright 2001 - 2002 Lorne Clark