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 the variation of maximum gap with the number of secondary turns for special battery ignition coils.
| Identifier | WestWitteringFiles\D\April1920\ Scan9 | |
| Date | 13th April 1920 | |
| X.3775. VARIATION OF MAXIMUM GAP AT 50 S.P.S. WITH NUMBER OF SECONDARY TURNS FOR SPECIAL R.R. BATTERY IGNITION COILS WITH STANDARD PRIMARY CONDITIONS. MAXIMUM GAP (Inches) AT 500 R.P.M. (for coils nos. S10, S12, E14, E18 & Standard) No. OF SECONDARY TURNS. COIL NO. S.10. x BARE DIA. OF SECONDARY WIRE .006 " " S.12. O " " " " .0035 " " E14. ⨂ " " " " .004 " " E18. ● " " " " .0035 " " 149. ⮾ * " " " " .0035 (* STANDARD COIL.) RCS / 31/3/20 E>C 13/4/20 | ||