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).
Calculation table for ampere-turns needed to produce the required magnetic flux.
| Identifier | ExFiles\Box 64\1\ scan0121 | |
| Date | 30th April 1923 guessed | |
| AMPERE TURNS NECESSARY TO PRODUCE FLUX REQUIRED Part. | Matl. | Length ("). | Virtual section (sq.ins). | Total flux. | Flux density Inch units. | Flux density Cm. units. | H.{Arthur M. Hanbury - Head Complaints} | AT. per inch. | A.T. Armature teeth. | Lamin. iron. | .30 | 5.7 x .130 x 2.75 = 2.04 | 167000 | 82000 | 12700 | 15 | 30 | 9 Armature core. | " | .90 | 2 x .70 x 2.75 = 3.85 | 167000 | 43400 | 6730 | 5 | 10 | 9 Air gap. | Air. | .015 | 1.80 x 2.75 =4.95 | 167000 | 33800 | 5230 | 5230 | 10500 | 158 Pole core. | M.S. | .675 | .95 x 2.75 = 2.61 | 200000 | 76700 | 11900 | 15 | 30 | 20 Yoke. | M.S. | 1.96 | 2 x .275 x 4.0 - 2.2 | 200000 | 91000 | 14100 | 26 | 52 | 102 Total 298 Say 300 | ||