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).
Table of gap measurements and tolerances for a push button switch.
| Identifier | ExFiles\Box 64\3\ scan0289 | |
| Date | 13th November 1928 | |
| X7650 13.11.28. PUSH BUTTON SWITCH TO LEC.1904. (D.70790) TABLE OF GAPS. Position. Carbon gap. Copper gap. Button depression. Off .063 +12/-13 .090 +15/-15 .026" Carbons in contact. O .040 +5/-5 .055 +6/-6 Coppers in contact. O O .078 +9/-9 EFC. | ||