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 limits for a main switch unit in various positions.
| Identifier | ExFiles\Box 168a\2\ img091 | |
| Date | 1st February 1931 | |
| MAIN SWITCH UNIT GAP TABLES. Position of switch. | Air gap at core centre. | Core to Armature stop. | Copper gap (first blade to make). | Carbon gap. Off | .064 +4 | .030 +5 -2 | .062 +4 | .025 +5 Carbons just in contact | .071 +1 -3 | .074 +3 | .040 | 0 Coppers just in contact. | .047 + -2 | .045 | 0 | 0 Full on. | .010 + -2 | 0 | 0 | 0 ---- These limits must not be exceeded. 1.2./'31. | ||