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).
Continued discussion on instrument error calculations and the working principle of speed indicators.
| Identifier | ExFiles\Box 59\2\ Scan054 | |
| Date | 19th August 1930 | |
| EFC/AD19.8.30. contd. -3- have a negative error. In our view of the matter the makers of the instrument should be concerned only to suit the instrument on the lines indicated to the figure of drive revolutions per mile that is given to them i.e. any consideration of road wheel sizes or circumstances of drive gear ratios should (except in the very initial stages) be withheld from them. In arranging matters in the manner described it would be unlikely that the indicated miles would ever have a positive error greater than perhaps 2%. As long as the error were within this amount it would probably not be considered objectionable, but in special circumstances e.g. the very universal use of smaller tyres, it might become necessary to ask for instruments arranged for the next highest figure of spindle revolutions per counter mile. Speed Indicator. Principle. The speed indicator, which is driven at a speed proportional to the rate at which the miles are counted, works upon the principle of a revolving magnetic field produced by a revolving magnet inducing eddy currents in an inductor drum which is free to revolve against a control spring and in so doing move the needle over the scale. | ||