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).
Article from 'The Motor' magazine about the construction of modern sparking plugs using mica.
| Identifier | ExFiles\Box 33\3\ Scan091 | |
| Date | 29th December 1920 | |
| X.18/12-br.{T. E. Bellringer - Repair Manager} December 29, 1920. 1117 The Motor THE MARVELS OF MICA. Some Sidelights on the Construction of the Modern Sparking Plug TAKE a piece of flimsy paper and try to split it into two single sheets of the same length and breadth. It is, of course, practically impossible. A sheet of mica of the same thickness as the paper—that is, some 2-1,000ths of an inch—can readily be split into two layers of 1-1,000th of an inch each. This is one of the first processes in modern plug manufacture. Mica in its original form is rock found in the earth. Like slate, it is made up of an innumerable quantity of thin layers. As delivered to a plug manufacturer it is in pieces about 6 ins. long by 4 ins. wide. These pieces are split up by nimble-fingered girls with sharp knives until the thickness of each piece is 1-1,000th or 1 1/2-1,000ths of an inch thick. Each separate film is carefully examined and finally cut to size. It then forms the material from which the insulator of the plug is made. There have at various times appeared heated controversies on the subject of mica versus porcelain plugs. Whatever may be the theoretical considerations, it has been amply proved in practice that mica plugs will and do stand up to severest service in internal-combustion engines of the super-efficiency type. Although there are many other excellent mica plugs on the market, we can take the K.L.G. as an example for the purposes of description by reason of its proven trustworthiness and dependability. The first stage in its making is the wrapping of the central electrode. This is delivered from the machine shop as a slender rod some 2 1/2 ins. long, threaded at the top and with a small collar near its lower end. The rectangular pieces of mica are wrapped round its centre—interleaved in such a manner as to ensure practical homogeneity—until the roll thus formed is about a 3/4 in. in diameter, which size is accurately gauged by the operator. Making the Gastight Joint. A metal collar is then slipped over this roll, and its gastightness is ensured in the following interesting manner: the upper portion of the collar is coned; that is, it is thicker at the bottom than the top. The electrode is put in a press and the flanges of the collar gripped by a nut. A circular punch with parallel internal bore is then brought down on to the cone of the collar with terrific force by a press, and this action causes the inner bore of the collar to cave inwards all round, thus tightly gripping the mica roll, and its freedom from leakage is tested by a device incorporated in the press, which puts air pressure of a 100 lb. per square inch on the top of the collar, any leakage being c3 [Image Captions] Splitting up the mica into strips one-thousandth of an inch in thickness—half the thickness of a human hair. The insulation has to withstand a spark over 3/4-in. long. Wrapping the central electrode with the mica leaves. | ||