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).
Graph showing amplitude tests for a Wraith Spring Drive operating without balance weights.
| Identifier | ExFiles\Box 124\2\ scan0136 | |
| Date | 2nd April 1940 | |
| B. 60 No. 2 WRAITH SPRING DRIVE WITHOUT BALANCE WEIGHTS AMPLITUDE DEGREES .2 .4 .6 .8 1000 2000 3000 4000 R.M. / J.R.S 2 . 4 . 40 | ||