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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).
Table of sparking voltages for various gap sizes using a 3-point gap, and for specific magnetos.

Identifier  ExFiles\Box 4\5\  05-page284
Date  11th June 1924
  
EFC.
WA.{Mr Wallis} 43.

X.430

X.430.
X.918.
X.699.

SPARKING VOLTAGES FOR VARIOUS GAPS
USING A 3-POINT GAP.

Gap in mm. R.M.S. Volts. Peak volts.

2.5 3460 4900
3.0 3880 5500
3.5 4350 6150
4.0 4900 6950
4.5 5200 7350
5.0 5500 7780
5.5 5750 8130
6.0 6000 8500
6.5 6275 8880
7.0 6525 9230
7.5 6730 9520
8.0 6950 9820
9.0 7200 10800
10.0 7500 10600
11.0 7675 10850
12.0 7925 11200

The breakdown voltage of the safety gaps of the R.Bosch and American Bosch magnetos was also ascertained, and they are as follows :-

Magneto. Gap mm. R.M.S. Volts Peak volts.

R.{Sir Henry Royce} Bosch 10.2 8300 11740

U.S.A. Bosch. 8.6 7675 10850

3rd. Point spark 11.0 7675 10850
gap.

E.O.6. Watford. 8.5 7500 10600

RFC/HEW.
  
  


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