The Ratio DT/µ for Electrons in Water Vapour at 294 K

Abstract

The ratio DT /µ for electrons in water vapour (294 K) has been measured by the TownsendHuxley method as a function of E/N (where E is the electric field strength and N the gas number density) at vapour pressures ranging from 0 ·103 to 0·413 kPa. For E/N <30 Td, where attachment and ionisation may be neglected, the values are found to be independent of vapour pressure and of the current ratio relation used to derive DT/ J.l values from the measured current ratios. The uncertainty of these DT /J.l values is estimated to be <±2%. Values of DT / J.l measured at E/N > 30 Td were found to be strongly pressure dependent, the strength and sign of the dependence depending on E/N and the current ratio relation used. Since extrapolation to infinite pressure at each E/N value did not give the same value of DT/ u, it has not been possible to derive reliable DT/ J.l values for this higher E/N range. Possible causes of the observed pressure dependences are discussed. The present data are in good agreement with the values predicted by Ness and Robson for values of E/N ~ 24 Td.