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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).
Advertising leaflet comparing the cost and features of the 'Molla Plug' against a generic alternative.

Identifier  ExFiles\Box 33\3\  Scan121
Date  9th March 1921 guessed
  
Fig. 6
Tighten up carefully the top nut and in order to avoid any stiffing of the spark gap adjustment, it is wise to keep one finger of the left hand pressed on the end of the electrode thus keeping it in place.

Fig. 7
VERY IMPORTANT.
After having tightened the top nut, give a few sharp taps on the end of the electrode (as illustrated) without fear of breaking it. It will then be possible to tighten up the top nut at least another half turn more.

AN EXAMPLE OF THE ECONOMY REALISED IN EMPLOYING THE "MOLLA PLUG"

A Chauffeur employing the X... PLUG.
Expenses :
X... Plug. Cheap, made of steel ordinary porcelain, nothing to look at, not able to be taken to pieces is cleaned with difficulty. Cost.. .. .. 6/
A careless tap with the spanner breaks the porcelain and the plug must be thrown away and another purchased.. 6/
The chauffeur in endeavouring to adjust the spark gap with pliers breaks the horns of the electrode. Another plug.. .. 6/
The thread of the base gets worn out or spoilt which renders the plug unusable. Another plug.. .. 6/
Total: 24/

A chauffeur employing the MOLLA PLUG.
Expenses :
MOLLA PLUG, made of brass insolator mica, or steatite able to be taken completely to pieces, and adjusted. Is thoroughly cleaned in one second Cost. 6/6d
MOLLA PLUG. Cost. New porcelain. -/8d
MOLLA PLUG. The spark gap being adjustable by hand such an accident is rendered impossible but in any case the electrode can be changed if it is spoilt. Cost. 2/3d
MOLLA PLUG. If the thread of the base is worn another Costs. 3/-
Total: 12 : 5d

Not only is an economy of 50% realised But the X... Y... Z... plugs at cannot compare in quality with the MOLLA PLUG
5, Rue Jean-Daudin -- PARIS (15e)

Handwritten text:
British Molla Plug Coy., 111/113 Gt Portland St London. W.1
  
  


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