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).
Report investigating magneto safety gap sparking issues, comparing Watford and B.T.H. models.
| Identifier | ExFiles\Box 43\4\ Scan234 | |
| Date | 19th February 1930 | |
| Messrs. North & Sons, Ltd. Whippendell Rd. Watford. 19th February, 1930. -2- not occurring only at high speeds and for this reason it was suspected that it was due to sparking over at the safety gap. As the safety gap is internal and not readily observed on the rig, it was not clear at first whether this was due to faulty distributor timing or too low safety gap spark over voltage. We have therefore investigated both points. The breakdown potential differences of the safety gaps on both the Watford and B.T.H. magnetos have been duly observed, the results coming out as follows :- Magneto. Gap measurement (mm.) Breakdown potential (Kilovolts) Watford 7.7 10 B.T.H. 8.8 13 Reference being made to the terms of our General Specification of aero magneto performance dated 5.4.28, it will be seen that in the Watford case it does not comply with item 2. of that specification which calls for the H.T. safety gap to be the equivalent of sparking over voltage of 12000 steadily applied, and goes on to say that the corresponding safety gap would not be less than 8.5 mm. Since the time these observations were made we have satisfied ourselves that an increase of safety gap to 8.5 mm. cured the particular missing which we have observed on bench test. On later observations we are certain that a spark was passing to safety gap because we found that by suitably sound shielding the working gaps we could distinctly hear the passage of the safety gap spark internally. | ||