Abstracts of Technical Articles from Bell System Sources 



Electron Microscope Studies of Thoriated Tungsten} Arthur J. 

 Ahearn and Joseph A. Becker. Many past experiments have shown 

 that the thermionic activity of a thoriated tungsten filament is deter- 

 mined by the concentration of thorium on its surface. This concentra- 

 tion is in turn determined by the rate of arrival and rate of evaporation 

 of thorium. Typical published values of these rates are given in Fig. 1. 

 An electron microscope used to obtain electron images of thoriated 

 tungsten ribbons is described. Comparison with photomicrographs 

 shows that the active and inactive patches composing an electron image 

 agree in size, shape and number with the exposed grains of the tungsten. 

 The electron microscope shows that thorimn comes to the surface in 

 ''eruptions'' at a relatively small number of randomly located points. 

 From a comparison of photomicrographs showing thoria globules and 

 electron images of thorium eruptions, it is deduced that all the thorium 

 in a globule comes to the surface when an eruption occurs. Factors such 

 as a high temperature flash and sudden heating and cooling of the 

 filament affect the frequency of eruptions. Thorium eruptions are the 

 only observed manner in which thorium arrives at the filament surface. 

 They are repeatedly observed in the early stages of thoriation. Erup- 

 tions are not observed in the later stages of thoriation where con- 

 ditions are unfavorable for their observance. Electron images of a 

 Pintsch single crystal filament reveal alternate active and inactive 

 bands parallel to the filament axis. Thorium eruptions occur only on 

 the active bands. With a polycrystalline ribbon the surface migration 

 of thorium from the eruption centers is isotropic; with a single crystal 

 ribbon there is a strongly preferred direction of migration. X-ray 

 analysis shows that the surface is a (211) plane and that the preferred 

 direction of migration agrees with the (111) direction in this plane. 

 During the process of thoriating a filament the relative emissions from 

 different grains change by substantial amounts; in many cases the 

 change is so great that the relative emissions are reversed. Measure- 

 ments of work function differences between grains gave values ranging 

 up to 0.6 volt. 



The Mechanism of Hearing as Revealed through Experiment on the 

 Masking Effect of Thermal Noise.^ Harvey Fletcher. In an electri- 



1 Phys. Rev., September 15, 1938. 

 ''Proc. Nat'l. Acad. Set., July 1938. 



670 



