.IHSIR.ICrS OF lir.l.l. SYSII.M IFXIINICAL PAl'EKS 7<)i 



ditions for Schottky's formula for the thermionic current from a fila- 

 ment to a coaxial cylinder. — The formula must fail when, due to space 

 charge, the potential at any distance x(r— x— R) from the axis is less 

 than Vr= (R--x=) x^R'-r-), V being the potential of the filament with 

 respect to the cylinder, and r and R the radii of filament and cylinder 

 respectively. This is more restricti\e than the condition for failure 

 which has been previously assume<l. 



Variation of the Photo-electric Effect with Temperature in the Alkali 

 Metals,* Herbert E. Ives and A. L. Johnsrud. Special cells having 

 a hollow central cathotle were immersed in liquid air for an extended 

 fM.-ri<Kl to condense any gases present on the outer alkali metal coated 

 walls. By a stream of e\aporating liquid air, the temperature of the 

 cathixle was held at temperatures between +20 and — 180°C. In 

 these cells the variation of photo-electric current with temperature 

 in sotlium, potassium and rubidium is continuous. The effect is 

 relati\ely small for sodium, showing hardly at all for blue light or 

 white light, but clearly for yellow light. The behavior of rubidium is 

 similar to that previously reported for potassium. In a second form 

 of cell, potassium was collected in a deep pool. By slowly cooling the 

 metal from the molten conditions, smooth crystalline surfaces were 

 obtained. With these annealed potassium surfaces, the variation of 

 photo-electric current with temperature is represented by curves vary- 

 ing systematically in shape with the color of the light, and the effect 

 is far greater than previously reported, amounting, for \eliow light, to 

 a variation of 10 to 15 times between room and liquid air temperature. 

 When the surface is roughened curves of the previously reported type 

 are obtained. Small pools give erratic effects, showing changes in 

 opposite directions for different portions of the temperature range. 

 It is concluded that the variation of photo-electric effect is intimately 

 connected with the strains produced in the surface by expansion and 

 contraction with temperature. 



Echo Suppressors for Long Telephone Circuits," A. B. Clark and R. C. 

 Mathes. A device has been developed by the Bell System for suppress- 

 ing "echo" effects which may be encountered under certain conditions 

 in telephone circuits which are electrically very long. This device 

 has been given the name "echo suppressor" and consists of relays in 

 combination with vacuum tubes, which are operated by the voice cur- 

 rents so as to block the echoes without disturbing the main trans- 

 mission. 



' Physical Review, Vol. 25, No. 6, page 893, June, 1925. 

 > Jour. .\. I. E. E.. Vol. XLIV, .No. 6, page 618, June, 1925. 



