Ratio of the Diffusion Coefficient to the Mobility Coefficient for Electrons in He, Ar, ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, CO, and C${\mathrm{O}}_2$ at Low Temperatures and Low $\frac{E}{p}$

$\frac{D}{\mu }$, the ratio of the diffusion coefficient to the mobility coefficient for electrons, has been determined as a function of $\frac{E}{p}$, the ratio of the electric field to pressure, in He, Ar, ${\mathrm{N}}_2$, ${\mathrm{H}}_2$, ${\mathrm{D}}_2$, CO, and C${\mathrm{O}}_2$. Special interest is centered on the low-temperature, low-$\frac{E}{p}$ data which are essential for the determination of low-energy elastic and inelastic cross sections. The lowest temperature of operation is either 77°K or the boiling point of the gas being measured, whichever is higher. The lowest $\frac{E}{p}$ value is well within the thermal range where $\frac{D}{\mu }=\frac{\mathrm{kT}}{e}$. A careful analysis of the operation of the experimental tube is presented along with a method for removing inconsistencies in the results which arise from a conventional interpretation of the data.