Rolls-Royce Archives
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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).
Instrument testing, focusing on temperature errors, magnetism, and potential adjustments.

Identifier  ExFiles\Box 59\2\  Scan010
Date  3rd February 1930 guessed
  
-2-

7% high. 20 HP. are 720 and give mileages 4.1% high.

(5) Instrument temperature at which its speed indications are correctly in agreement with its counter miles, and instrument temperature error between 18° and 30°C. Owing to increase of electrical resistance with temperature, the eddy current drag on the inductor drum is less at higher temperature to an extent generally between 2 and 3% between 18° and 30°C. (N.B. For pure copper it would be 4.8%).

It is, or should be the aim of the makers to make correct correspondence at between 30°C. to 35°C. as this is the temperature at which the instrument is largely working.

In regard to scale proportionality this is generally quite good.

In regard to loss of magnetism causing reduced readings, we have so far no clear evidence of this and see no reason that it could occur with properly tempered magnets.

In this laboratory we make tests of these instruments at an exactly known speed (synchronous with town frequency) and subject to the varying temperatures experienced on cars. We are therefore able to give exact recommendations.

In regard to the query concerning the possibility of alteration to the speed indicating part of these instruments, it is possible to effect an adjustment within a limited range. The necessity of this should not normally arise, though it has arisen in some cases in which the makers adjustment has not been accurate for the (internally) correct reading at the specified temperature. In this respect we ought to get the makers to work to closer limits.

EFC.
  
  


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