The positive ion currents from pure tungsten filaments in saturated caesium vapor at bulb temperatures from -35°C to +73°C were measured for filament temperatures from 1000° to 1800°K. The results were corrected for the cooling effect of the leads and for photoelectric emission from the caesium film on the platinum deposited on the bulb which was used as an ion collector. The vapor pressures in mm of Hg are given for solid Cs ($T<\mathrm{}302\mathrm{}$°K) by ${log}_{10}{p}_S=10.5460-1.00\mathrm{}{log}_{10}T-\frac{4150}{T}$ and for liquid Cs ($T>\mathrm{}302\mathrm{}$°K) by ${log}_{10}{P}_L=11.0531-1.35\mathrm{}{log}_{10}T-\frac{4041}{T}.$

The vapor pressures given by these equations are believed to be accurate to within one percent from 220° to 350°K, within 3 percent up to 600° and within about 8 percent at 1000°K.

• Received 23 February 1937

DOI:https://doi.org/10.1103/PhysRev.51.753