THE ELECTRIC STRENGTH AND MOLECULAR STRUCTURE OF HYDROCARBON GASES

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Abstract
Precise measurements of the sparking potentials of a number of hydrocarbon gases over a wide range of pressure and electrode spacing are reported. The range has been extended beyond that reported earlier to include the n-paraffins up to n-octane and the olefins up to hexene-1. The effect of the triple bond in acetylene and conjugation in butadiene as well as the effects of cyclization are included. Careful experimental control involving the observation of prebreakdown pulses is an important feature of the measurements, and all the gases tested were of the highest purity.A theoretical argument is given, relating the sparking potentials Vs to the molecular structure of the gases. This assumes a Townsend criterion for breakdown and a particular expression for the Townsend α coefficient. It is then possible to obtain information concerning the relative electron-scattering cross-sections of these hydrocarbons, which are related not to the molecular bonds as has been suggested earlier, but to individual atomic groups. With such a model, CH3 and CH2 groups and groups involving double or triple bonds have characteristic cross-sections. The theory then becomes closely allied to that for the ultraviolet spectra of these hydrocarbons. It appears that the present system of description is a more natural one than that involving bond cross-sections.It is believed that dissociation occurs readily in many hydrocarbons and that photon action is small. This produces a value of γ (the second Townsend coefficient) which is very small (~10−8) in comparison with the value (~10−4) in more common gases. Such a low value can account for the considerable pulse activity observed in these hydrocarbon gases before breakdown occurs. Few pulses were observed in cyclopentane and cyclohexane, which do not dissociate readily, and it appears that a more usual value of γ = 10−4 occurs in these particular gases.

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