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).
Analysis of annular spark gap design, focusing on wire-to-hole diameter ratios to prevent brush discharge.
| Identifier | WestWitteringFiles\M\2October1924-December1924\ Scan70 | |
| Date | 16th July 1934 | |
| Contd. -4- EFC1/T16.7.34. It appears from the above that in the annular spark gap, the ratio of the wire diameter to the hole diameter should be at least equal to 1/e = .368, in order to prevent the brush discharge occurring. But actually it is found that for small gaps it may be as little as .2 without a brush discharge occurring. This is because the effective breakdown potential gradient of the air in the vicinity of the electrode is somewhat greater than the true breakdown potential gradient of air in a uniform field. It is usual to make h/H = .5, as by so doing we get a considerable increased spark gap for a given voltage as compared with h/H = 1/e, there is less tendency to arc, and accurate centering of the inner electrode is not so important. It will be fairly clear that a gap of this type working in the region indicated, will not be affected to any great extent by dirt or roughness on the wire, as a small variation in a, due to the presence of such, will not affect the electric force materially. Hence these gaps do not require frequent cleaning and attention. Further, since there is a considerable surface subject to the maximum stress, there is much greater probability of the necessary free ions being present to start the breakdown at the moment of application of voltage, than in the case of the point gap. Hence the gap is less affected by wave form, i.e. it has a lower "Impulse Ratio". Contd. EFC. | ||