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 electrical properties of insulating materials and the equipment for their measurement.
| Identifier | ExFiles\Box 45\4\ Scan090 | |
| Date | 18th November 1922 | |
| R.R. 199 (250T) (SD970 19-7-17) M/P.18805 Contd. -2- EFCL/T18.11.22. ionization effect being possibly lost by the fact that all the materials you mention of low inductivity have a figure which is at least twice as great as air. (8) In passing we think the term "inductivity" the most satisfactory to use being a scientific parallel to conductivity in that other case. (9) It appears that the only electrical quantities there can be in the case of insulating materials are (a) Conductivity, reckoned in mhos. per cm. cube. (b) Inductivity. (c) Dielectric hysteresis. (d) Breakdown potential gradient. (b) and (c) are connected, (a),(b) & (c) vary with the applied potential difference, and (a),(b) (c) and (d) all vary with temperature. (10) In order to measure (a) it requires as you suggest a source of small current at high direct potential such as a Wimshurst machine and a sensitive milli or micro-ammeter to measure the flow, also some means of maintaining the temperature uniformly at required values. Measurement of (b) requires a source of known high potential, condensers of standard dimensions made up with various materials as dielectrics, and a ballistic galvanometer in order to measure the discharge. Measurement of (c) can be made by the heating effect on the standard condensers by applied high alternating potentials of known values. Measurements of (d) require as usual high alternating potentials of variable but known values, but in this particular class of work does not seem to be of great importance, as a considerable amount of energy is required to effect breakdown which is not available from a coil. (11) We have recently had delivered a transformer up to 20,000 volts on 50 cycles. (12) We think, however, we should now be permitted to obtain an electrostatic voltmeter reading say from 2500 to to 25,000 volts, which would be useful for the measurement of A.C. R.M.S. and D.C. potentials. (13) Using the transformer and a suitable rectifying valve and large capacity condenser, we think we can arrange for a supply of continuous current (milliamperes) at a very nearly uniform high unidirectional potential. This would render unnecessary the use of a Wimshurst machine. Contd. | ||