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).
Graph showing maximum spark gap and average current versus contact breaker speed for various ignition coils.
| Identifier | WestWitteringFiles\D\April1920\ Scan5 | |
| Date | 5th March 1930 | |
| Average Current (Amperes) Max. Gap. (Inches) Curve A Max. Gap-Speed Curve for S.R.14 coil. Curve B '' '' '' '' '' poor standard coil. Curve C '' '' '' '' '' normal '' '' Curve D Average Current-Speed Curve for all above Coils. x points refer to coil S.R.14. ⊗ '' '' '' '' poor standard coil. ∙ '' '' '' '' normal '' '' present standard production spark gap. Spark length for varying speed of R.R. standard battery (12V) ignition using coils as above, the high tension terminal being connected to platinum point gap in air (no distributor). Length of H.T. cable = 18". R.P.M of Contact Breaker. | ||