Ignition coil insulation materials, wire quality, and spark plug gap issues.
| Identifier | ExFiles\Box 164\5\ img026 | |
| Date | 28th January 1931 | |
| EFG1/AD28.1.31 contd. -3- The layers of winding used to be normally insulated with oil varnished paper though some improvement has been provided by using paraffin wax paper of similar thickness and now both papers are used on a 50-50 basis. It is agreed that it would be expected that with coils of our present length one layer of such paper would be ample as an insulation against the voltage of two layers of secondary winding but difficulty has been experienced in getting the right paper free from holes and dielectrically good. Paper of U.S.A. origin supplied by the L.P.S. Co. has so far been found the best. It has, in consequence, not always proved to be the case that the insulation is sufficient especially if the base at which the winding is started has not been sufficiently regular and the winding of the wire itself has not also been sufficiently regular (i.e. without gaps) a feature which appears to be somewhat difficult consistently to rely upon owing to some amount of variability in the diameter of the wire used. We have found on electrical test and in regard to consistency of wire diameter that the best wire available is wire of Swiss origin supplied by Messrs. McLeods this having been recommended to us by Messrs. North. British wire appears to be unreliable. Messrs. Lucas use wire of French origin and L.P.S. paper. Three turns of paraffin wax paper per layer is now standard through the use of this quantity of insulation increases the diameter of the coil secondary and consequently reduces the effectiveness of the interlinking. The explanations contained in your EL/M15.1.31 of the various reasons why it would appear that our ignition coils do not do so well as others, namely :- (a) The use of larger spark plug gaps. (b) The greater tendency to oxidation owing to the use of weaker mixture strengths. are very interesting and it may be the case that they sum up the situation. Disadvantages of using small plug gaps are that the plug oxidation and burning away are more rapid than with larger gaps and that therefore the variation of gaps is more critical with the result that they need more continued attention. | ||
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).