From approximations based on Hiske’s theories of Tandem‐Volume Sources, expressions are derived for the H−currents obtained from JAERI’s 1 and 5 A sources, and estimates made of the temperature of the neutral gas molecules in the source.The calculated currents, normalized with respect to their maxima at optimum source pressures, are compared with data extrapolated from JAERI’s graphs, showing H−ion currents vs. source pressures at several discharge currents. At pressures less than or equal to optimum, the observed and calculated currents are very close.