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).
Specification table for different types of nickel wire ballast resistances.
| Identifier | ExFiles\Box 168a\5\ img032 | |
| Date | 4th January 1929 | |
| X7720 4.1.29. NICKEL WIRE BALLAST RESISTANCES. Type. Length (Inches) Gauge. S.W.G. Cold Resistance. (Ohms) Standard double. 39 33 1.76 One and a half. 30 32 1.30 Old standard single. 25 31 .82 EFC. | ||