THE RATIO D-T MU FOR ELECTRONS IN WATER-VAPOR AT 294 K/

The ratio D-T/mu for electrons in water vapour (294 K) has been measur ed by the Townsend-Huxley method as a function of E/N (where E is the electric field strength and N the gas number density) at vapour pressu res ranging from 0.103 to 0.413 kPa. For E/N less than or equal to 30 Td, where attachment and ionisation may be neglected, the values are f ound to be independent of vapour pressure and of the current ratio rel ation used to derive D-T/mu values from the measured current ratios. T he uncertainty of these D-T/mu values is estimated to be <+/-2%. Value s of D-T/mu measured at E/N > 30 Td were found to be strongly pressure dependent, the strength and sign of the dependence depending on E/N a nd the current ratio relation used. Since extrapolation to infinite pr essure at each E/N value did not give the same value of D-T/mu, it has not been possible to derive reliable D-T/mu values for this higher E/ N range. Possible causes of the observed pressure dependences are disc ussed. The present data are in good agreement with the values predicte d by Ness and Robson for values of E/N less than or equal to 24 Td.