Letter from Philip Challis regarding a report on the design features of the Silca Sparking-Plug.
| Identifier | ExFiles\Box 33\3\ Scan178 | |
| Date | 25th September 1921 | |
| [Handwritten at top: X1812 b] [Handwritten at top left: HS {Lord Ernest Hives - Chair} ] Your reference CJ.from HS. {Lord Ernest Hives - Chair} HS {Lord Ernest Hives - Chair} 2/LG14.921 C.Johnson Esq., Messrs Rolls-Royce,Ltd., 15, Conduit Street, W. 1. 30, CARLISLE MANSIONS, VICTORIA, S.W.1 25/9/1921 [STAMP: TELEPHONE VICTORIA 268] [STAMP: RECEIVED SEP 28 1921] Dear Sir, Silca Sparking-Plug. The Report, marked as above, has been handed to me, the Patentee of this Plug. I beg to thank you for the trouble taken and much appreciate being afforded the opportunity of a further test, for which purpose a plug, incorporating the features required, has been put in hand. With reference to the plug on which the present Report was based, I should like to point out:- 1. Over 100 precisely similar plugs have been in use under close observation. As far as can be ascertained, in no other case has oil escaped, through the upper seating of the quartz insulator, in sufficient quantity to coat the leakage length of this insulator from the central electrode stem to the plug body. 2. The quartz insulator of the "production" plug will be glazed over its whole surface. In the plug in question, only the cone -- the lower leakage length-- was glazed; the cylindrical upper end of this insulator had a rough surface, as left by the grinding wheel, which permitted a capillary action to occur, facilitating the creep of the oil, which, in itself, should not have been there. The, interrupted, annular spark-gap was introduced for several reasons, some of which are:- a. {Mr Adams} To reduce the amount of "beading" that occurs. 24 spark-gaps as against 1 or 3. 1 spark from each point as against 8 or 24. "Beads" of only one eighth or one twenty-fourth the size from an equal amount of use. b. To act as a baffle to the passage of oil, in spray or vapour form, to the insulator. c. To cause any oil, which may be thrown on the central disc electrode, to sink into the spaces between the adjacent teeth, from whence it runs down the convex face of the electrode head, without necessarily increasing the diameter of the head and thereby provoking short circuits. Several means for adjusting the annular spark-gap are provided in the Patent Specification, but it was thought that the average car engine would be more kindly treated if the average driver were to be relieved of the responsibility of maintaining correct gaps, by means of a number of spark-gaps. Yours faithfully, Philip Challis. [Handwritten in left margin: As from [signature]] | ||
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).