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 principles of speed measurement, considering temperature effects on instrument accuracy and calibration.
| Identifier | ExFiles\Box 59\2\ Scan055 | |
| Date | 9th August 1930 | |
| -4- EFC1/AM9.8.30 contd. The reaction between the magnetic field of the eddy currents and that of the magnet produces a drag which, other things being the same, is strictly proportional to the speed. So long therefore, as the control torque on the inductor is proportional to the angular deflection the scale of the instrument will also be definitely proportional to the speed. Temperature Effedt. There is however, a temperature effect arising from increase of electrical resistance with temperature. The induced eddy currents decrease in magnitude as the temperature increases and the reading is thus reduced. In the early instruments this effect was quite considerable and too much to be tolerated. In the instruments now available this effect has, by the use of a more suitable inductor material, been reduced to within quite a reasonable amount which mainly does not now exceed a 2% drop from 18° to 30°C. Relation of speed indications to counter miles. In view of the fact that a temperature effect causes lower speed readings at the higher temperatures, it has been generally agreed that the speed indication should be correct in its relation to the counter mile at the nearly highest temperature, say 35°C, to which the instrument is subjected, and the makers have been asked | ||