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).
The basis and development for a dis-proportional speed scale on a magnetic eddy current type speedometer.
| Identifier | ExFiles\Box 59\2\ Scan120 | |
| Date | 23th March 1931 | |
| 23.3.31. A.T. MAGNETIC EDDY CURRENT TYPE SPEEDOMETER. Basis for and development of suitable law of dis-proportionality of speed scale. Each instrument comprises within itself two portions, namely the mile counting and speed indicating portions. Considering an instrument incorporating the speed indicating portion only, it will be shown that there is some argument for artificially dis- proportioning the speed scale according to a certain law developed below. Considering, however, the actual instrument, functioning as it does in both of the ways mentioned above, it will be further shown that there is still more argument for effecting such a dis-proportionality of speed scale; in accordance with a similar law. All instruments so far used by us have been produced with a proportional speed scale. The eddy current torque of a moving magnetic flux on a conducting plate or cup is, on fundamental principles, precisely proportional to speed, other things, including temperature, remaining the same. Since also the angular deflection of a torque spring controlled pointer is proportional to the acting torque, therefore it will be seen that the angular deflection of the pointer in these instruments will, at constant temperature, be quite exactly proportional to the speed of their drive. fore Supposing there, the setting of the instrument is correct in relation to its counter mile indications at any one speed, it is then also correct in this internal relation at all speeds. If in error internally the percentage error of speed to miles is identical at all speeds. Temperature constant. Such internal relation remains correct except as regards :- (a) The effect of temperature change by which the speed indications are increased approximately 2% by a fall of temperature from 30° to 18°C. this effect resulting from the decrease with falling temperature of the electrical resistance of the metal cup. | ||