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).
Comparison table of material properties for Le Carbone and Morganite grades.
| Identifier | ExFiles\Box 50\5\ Scan118 | |
| Date | 24th July 1922 | |
| EFC {E. Fowler Clarke - Electrical Engineer} 4/T24.7.22. Maker. Grade. Specific Resis: in ohms per in. cube. Coefficient of friction. Normal Current density amps. per sq. inch. Contact Drop in volts at normal current density. Le Carbone. | Z | .0012 | Not given. | 50 | - " | EG30 | .0014 | " | 50 | - Morganite. | EG2 | .0012 | .22 | 60 | .85 " | EGO | .0016 | .18 | 65 | .85 EFC. {E. Fowler Clarke - Electrical Engineer} | ||