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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).
Analysis of potential speedometer issues and the application of the law of speed scale contraction.

Identifier  ExFiles\Box 59\2\  Scan121
Date  23th March 1931 guessed
  
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(b) The possibility of change of magnet strength. This effect has been looked for but its existence to any extent has been sufficiently disproved to enable us to ignore it.

(c) The possible reduction in rate of control spring, an effect which has been suspected in some cases as a result of an otherwise unexplained increase in rating after a period of operation of an instrument on a car. The possibility of this effect is denied by the makers but we are still on the lookout for its occurrence. For our present purposes, however, it may be ignored.

Writing now, unless otherwise stated and in-so far as the speed indications only are affected, of the instrument at one fixed temperature say 35°C, which temperature is representative of the temperature of the instrument in normal operation, we give below the necessary explanations of

(1) The practical basis for the law of speed scale contraction.

(2) The development of the mathematical application of this law.

(3) The argument for the application of this law to the case of a speedometer without mile counter portion.

and (4) The further argument for the application of this law to a speedometer which combines the function of mile counter with that of speed indicator.
  
  


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