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From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Letter from a supplier discussing the function of a safety gap in a magneto and issues caused by excessive resistance.

Identifier  ExFiles\Box 65\3\  scan0414
Date  9th July 1930
  
X7791
.SPEEDOMETERS & MAGNETOS.
THE BRITISH MADE Watford
NORTH & SONS, LTD.
WHIPPENDELL ROAD, WATFORD.
TELEPHONES:
LONDON 1346 GERRARD
WORKS:
622 & 623 WATFORD.
CONTRACTORS TO
THE ADMIRALTY, WAR OFFICE, AIR MINISTRY,
CROWN AGENTS FOR THE COLONIES, INDIA OFFICE.
TELEGRAMS:
AEROMAGS WESTCENT
LONDON
AEROMAGS WATFORD.
Ref. AMA/CT.1/DD
Your ref. PN{Mr Northey}4/WT.30.6.30.
9th July, 1930.
SALES DEPARTMENT.
Messrs.Rolls-Royce Ltd.,
Claremont Road,
Cricklewood,
London, N.W.2.
Dear Sirs, For the attention of Mr. Percy Northey.
re Chassis No. 1 WJ.
With reference to your letter of the 1st inst., reference A17/H.W.30. in connection with the above, and further to our letter reference CT7/DD of the 19th ult., We would state that we do imply an excessive resistance in the high tension circuit external to the Magneto.
The object of the safety gap in a Magneto is to take care of an occasional overload, such as a plug fault or a detached high tension cable; a consistently overloaded Magneto will cause a leak across the safety gap in the form of brush discharge; the brush discharge in conjunction with moisture present in the atmosphere, form nitric acid, & a Magneto, under these conditions, must eventually breakdown.
  
  


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