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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).
Technical paper on the advantages and physics of the annular spark gap.

Identifier  WestWitteringFiles\M\2October1924-December1924\  Scan73
Date  16th July 1924
  
COPY. EFCl/T16.7.24.

THE ANNULAR SPARK GAP.

The chief advantages of the annular spark gap are :-

(1) Constancy in operation.
(2) Low impulse ratio.
(3) Ease of manufacture.
(4) Long life.

The ideal spark gap would be one in which the electric stress was uniform and calculable at every point in the active neighbourhood of the electrodes. Unfortunately such a gap is not realisable in practice. An approach to it is found in a spark gap consisting of two balls whose diameter is large compared with their distance apart. In such a gap, however, the distance apart for voltages of the order of 10,000 volts or so is so small that there is great difficulty in extinguishing the arc which follows the spark, and in addition any small particles of dirt on the surfaces of the balls may cause premature discharge.

In the annular gap, electric stress, although not uniform throughout the whole space between the electrodes, is calculable at any point and is the same across every point of each equipotential surface in the working region. In the annular gap, as in the point gap, breakdown is progressive, i.e. the stress is highest at one point near one electrode, and the breakdown starts at this point and spreads across to the other electrode. The greatest stress occurs initially at the surface of the central electrode.

If we imagine an infinitely long cylindrical wire of radius a cm, surrounded by a concentric conducting cylindrical surface at datum potential of larger radius b cm, than that of the wire, and if we imagine the charge per cm. length of the wire to be e, then the induction density and consequently the electric force just off the surface of the wire is

4π e / 2π a = 2e / a

By similar reasoning, the electric force at a radius r (between a and b) is

2e / r

Contd.
  
  


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