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).
Comparative bench test report on Watford and B.T.H. aero magnetos, with a performance summary table.
| Identifier | ExFiles\Box 43\4\ Scan255 | |
| Date | 26th February 1930 | |
| X4021 WE.8. 26.2.30. X.4021. X.3832. WATFORD AND B.T.H. AERO MAGNETOS. We have now completed comparative bench tests on an up-to-date Watford type IS.12 (now SP.{Mr Spinney}12-2) magneto No.83132 and a B.T.H. type SC.12-7B. magneto No.11C12. The tests were carried out to our general schedule of performance dated 26.6.29 which embraces our "General specification of aero magnetos". Neither of these two magnetos passed this specification entirely the various discrepant points being as follows :- B.T.H. Weight .32 lbs excess, poor leaky gap performance no regular sparking being obtainable, and the minimum regular sparking speeds in excess of our figures particularly at retard. Watford. Weight .375 lbs excess, safety gap only 7.7 mm. (altered to 8.5 mm. by ourselves), very poor utility performance no sparks being obtained, and the minimum regular sparking speeds in excess of our figures at full retard. Generally, the B.T.H. magneto gives the better all round electrical performance and the various points of superiority or otherwise between the two magnetos are summed up in the following table :- | Item. | Watford. | B.T.H. | | :--- | :--- | :--- | | Utility performance. | Very poor. | Poor. | | High speed performance. | Very good. | Good but with gap increased - very critical and cuts out 2/3rd. retard. | | Low speed performance. | Poor. | Fair. | | ||