5H0 



THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



less than it was in the first minute, and after about 20 minutes the rate of 

 evaporation is so small that d has nearly reached a steady state. To reduce 

 6 still more it is necessary to increase T. At 1600°K nearly all the Ba has 

 evaporated and the emission pattern looks like that of clean W. Subsequent 

 glowing at still higher temperatures produces only small increases in ip 

 and small changes in the emission pattern. 



(10) For e = about .18 and T = 800°K we have observed a marked 

 redistribution of Ba when a high field is applied: some Ba leaves the 211 

 and 111 regions and accumulates near the 100 regions. If the surface is then 

 held at 800°K with zero field, some Ba leaves the 100 region and returns to 



FLUORESCENT 

 SCREEN \ 



AQUADAG 

 ANODE ■" 



r = RADIUS OF CURVATURE 

 = 3 X lO-S CM 



MAGNIFICATION = 3 X 10^ 



V = ANODE VOLTAGES 

 = 6000-9000 



F=FIELD=5000 X V 



FIELD CURRENT =AF^f F 



(b) 



Fig. 1 — (a). Cross-section of tube. 



(b). Enlarged cross-section of tip of W point and values of constants. 



the 211 and 111 regions. This indicates that the adsorption forces can be 

 modified appreciably by high applied fields. 



PART I 



Description of Field Emission Microscope Tube 



Figure 1(a) is a cross-section of the tube. The loop which was used to heat 

 the point consisted of W wire 5.7 cm long and .0165 cm diameter. The W 

 point projected about 1 mm beyond the loop. It was formed by repeatedly 

 heating the W wire in a gas flame to oxidize it and removing the oxide with 

 sodium nitrite. Two tantalum wire loops of 2.0 X 10"^ cm diameter wire 

 are not shown. They were used to evaporate a tantalum film over most of 

 the glass surface in order to adsorb residual gases and thus decrease the 

 pressure. This proved to be very effective and estimated pressures of lO"^'^ 

 to 10"^* mm Hg. were obtained. The tube also contained a source of Ba 

 which could be deposited on part of the W point and loop. It consisted of a 

 coil of .020 tantalum wire heavily coated with BaO + BeO. The coil con- 



