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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 speedometer reading inaccuracies, investigating potential causes such as magnetism and control spring power.

Identifier  ExFiles\Box 59\2\  Scan056
Date  9th August 1930
  
EFCL/ADL9.8.30 contd.

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to set the instruments to correct self-relation at that temperature. At normal atmospheric temperature the speed indication would therefore read round about 2% high in relation to the miles, and if the miles are already reading a little high (see above) the error in the speed indication will be greater by that added amount.

Change of speed indication reading due to change of magnetism.

This effect has been suggested as a possible cause of error and has been carefully looked for. It has been previously stated that with properly aged magnets the effect should be non-existent also the makers have stated that they have not observed it. Though we have now made a number of tests we have not been able to find any change which can definitely be put down to this cause.

Change in power of control spring.

In definite cases where we have observed on the bench the indications of an instrument previous to and subsequent to a 10,000 mile run on a car, there has been, not a falling off of the reading as might be expected but actually an increase of reading. In one case this increase was relatively small, whereas in another case it has been rather unaccountably big suggesting the necessity of further enquiry into the case. As we have definite
  
  


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