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).
Proposal for a new electrical speedometer design using an alternator and a moving coil instrument with a metal rectifier.
| Identifier | ExFiles\Box 165\4\ img054 | |
| Date | 25th April 1930 | |
| To E.{Mr Elliott - Chief Engineer} from EFC. c. Sg.{Arthur F. Sidgreaves - MD} Ev.{Ivan Evernden - coachwork} c. PN.{Mr Northey} Wor.{Arthur Wormald - General Works Manager} c. Rg.{Mr Rowledge} HS.{Lord Ernest Hives - Chair} X 6023 W.1097. ELECTRICAL SPEEDOMETER. There are now available on the market, alternating current milliammeters which are rendered possible by adopting a combination of a small bridge type metal rectifier unit incorporated in the case of a moving coil D.C. instrument of otherwise orthodox design. Such instruments, as alternating current milliammeters or as voltmeters for ranges for which the external instrument resistance is a large percentage of the whole resistance, have a uniform scale as a result of being fundamentally D.C. instruments. As we have mentioned in other connections, the metal rectifier units appear to be absolutely constant and permanent. There would appear to be no reason why an accurate speedometer should not be possible by adopting the combination of a pure magneto alternator of pure inductor type or with revolving magnet, and with stationary armature, as the generator, to be run in conjunction with an indicating instrument such as described above, and we seriously suggest that this combination might be made the subject of experimental work preceded by the necessary design work of the small alternator necessary for the purpose. It would be a comparatively simple matter for us to obtain from Messrs. Ferranti, an instrument of this type of suitable rating in full scale alternating milli-amperes and / or voltage. The scale would be uniform in speed calibration because the generator voltage would be in proportion to the speed, and the damping would be of the same order as any high grade moving coil instrument, and therefore of a considerably better order than our present speedometer. It will also have been noted that the alternator, unlike the small R.R. magneto D.C. instruments which we produced some years ago, would have no commutator and brushes and could be very simple mechanically and electrically. We commend the matter for your consideration. | ||