372 BELL SYSTEM TECHNICAL JOURNAL 



emission on the two parameters: T, the temperature, and/, a quantity 

 which is proportional to the amount of thorium on the tungsten surface. 

 At a fixed temperature 1274° K it was found that as the amount of 

 thorium on the tungsten surface was increased, the thermionic emission 

 increased to a maximum, then decreased, and asymptotically ap- 

 proached a constant value. For the maximum, / is defined to be 1.0. 

 The maximum value and the final constant value of the emission 

 current were respectively 5.7 X 10^ and 5.7 X 10^ times the value 

 of emission current characteristic of clean tungsten. Moreover the 

 final constant value of the emission agreed to within a factor of 2 with 

 the value characteristic of clean thorium. From / = 0.0 to / = 0.8 

 the relation between the emission current and / satisfied the following 

 empirical equation 



logW = -3.14 - 6.54e-2-w, 



where i is the emission current in amperes per cm^. For 0.8 < / < 2.0, 

 the values of emission currents are tabulated. For any fixed /, the 

 emission obeys Richardson's equation. All the Richardson lines for 

 < / < 1 intersect in a common point at an extrapolated temperature 

 of 12,500° K, and for/^1 the lines intersect in a common point for 

 which the temperature is 3250° K. These results obtained by de- 

 positing thorium on a tungsten ribbon have been compared with 

 results obtained from thoriated tungsten wire. Thoriated tungsten 

 wire can be activated by diffusion of thorium from the interior to the 

 surface. For a while every atom that diffuses to the surface sticks to 

 it so that / increases linearly with the time; later when evaporation is 

 no longer negligible the rate of accumulation, dfldt, gets less and less; 

 a steady state is reached when the diffusion rate equals the evaporation 

 rate. It is unnecessary to assume "induced evaporation" to explain 

 these results. 



Variation of emission from thoriated tungsten with applied field. — It 

 was found that for both the ribbon and the thoriated tungsten wire the 

 dependence of emission on applied field changed as/ was varied. For 

 the thoriated tungsten wire the dependence of the thermionic constants 

 A and h on applied field was most pronounced for 0.3 </ < 0.6. 



Evaporation and migration of thorium on tungsten surface. — 

 Evaporation and migration of thorium on the tungsten surface were 

 studied. The evaporation rate depends on the temperature and the 

 fraction of the surface covered (/). For 0.2 </< 1.0 the rate of 

 evaporation is approximately an exponential function of/. At 2200° K 

 and/ = 0.2 the rate of evaporation was 10"^ layers/sec. and at/ = 0.8 

 was 31 X 10~* layers/sec. It was found that thorium could be de- 



