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 to Messrs. North & Sons.Ltd concerning the incorrect safety gap length on IS.12 type aero magnetos.
| Identifier | ExFiles\Box 43\4\ Scan229 | |
| Date | 14th February 1930 | |
| X4021 EFC8/AD. 14th February, 1930. Messrs. North & Sons.Ltd. Whippendell Rd. Watford. Dear Sirs, With further reference to the sparking performance of the IS.12. type aero magnetos, you may remember that we were inclined to suspect the possibility of sparking taking place over the safety gap. With this in view we have looked into the matter of the length of gap and made an observation of the breakdown potential difference of this gap on capacity. We find that the gap is not in accordance with our general specification of performance for these machines dated 5.4.28, copy of which was sent to you, in which the second item reads as follows :- "The length of the H.T. safety gap shall not be less than the equivalent of a sparking over voltage of 12000 steadily applied. This, in ordinary forms of safety gaps will correspond to certainly not less than 8.5 mm. and probably somewhat more". Your safety gap only measures 7.7 mm. as against the 8.5 mm. specified, and the sparking over voltage is only in the neighbourhood of 10,000 instead of being 12000. We feel now more convinced that failure on engine test is sometimes occurring by the spark taking the easier route across the safety gap. We shall be pleased to have your remarks, but particularly we would like this | ||