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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).
Electrical breakdown voltage tests on various insulating materials.

Identifier  ExFiles\Box 24\2\  Scan234
Date  1st January 1923
  
Contd.
-3-
EFC2/T3 01.23.

The
No. of Sample. Material. Thick-ness. Voltage Reading. Trans. Ratio. Breakdown Voltage. Volts per mil.
1. Oiled silk. .002" 126.0 25.0 3,150 1,575
2. Diamond Fibre. .013" 151.5 25.0 3,790 291
3. Bakelite. .020 118.5 100.0 11,850 593
4. " .044 151.0 100.0 15,100 343
5. Continental Bakelite. .025 124.0 25.0 3,100 124
6. " .047 146.0 75.0 10,930 233
7. Bakelite. .148 179.0 100.0 17,900 121

With the exception of samples 1 and 5, the puncture of the material was accompanied by a distinct snap.

With No.1 there was no noise, the breakdown being notified by lamps and voltmeter only.

No.5 was not a clear breakdown, a small arc being struck through a small crack already existing in the material on a voltage of 3100 being reached.

In nearly every case the puncture was found to be near the circumference of the area covered by the electrodes, which suggests that the surfaces of the electrodes, do not register properly, the contact pressure thus being unequal.

Tests up to No.2 were carried out with the electrodes mounted on their original ebonite base, but on approaching 11,000 volts an arc was set up between the electrode supports. As the base was too short to allow of any adjustment the electrodes were remounted on a thick piece of fibre; the only thing available. This was used for the remainder of the tests.

Specimens Nos 3 & 4 were ordinary bakelite discs.
It will be seen that the thicker disc broke down at a much smaller voltage per mil.

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