616 Prof. Skinner on the Drop of Potential 



in the position of the supports should appreciably affect the- 

 length. Thus in the case of a standard yard the ends of 

 the bar seem a much less advisable position for the supports 

 than the points answering to a/l= n/1/3. This, of course, 

 has been recognized since Airy's time. Again, the type B 

 has, at least theoretically, the conspicuous advantage that 

 even large variations in the position of the supports are of 

 secondary importance. It is not, of course, impossible that 

 there may be some compensating disadvantages, apart from 

 mere expense or difficulty of construction. In a bar of rect- 

 angular section there would seem less grounds for fearing 

 difference in elastic property or density between the material 

 above and below the neutral plane than in a bar of unsym- 

 irietrical section like B or C. In the event of differences of 

 elasticity between the material in the flange and in the upper 

 and lower limbs, the plane of unstretched fibres might not 

 coincide with the horizontal plane through the line of centres. 

 If heterogeneousness of this kind existed to any appreciable 

 extent, it would militate against the advantages claimed for 

 the type B. 



LIX. On the Conditions controlling the Drop of Potential at 

 the Electrodes in Vacuum- Tube Discharge. By Clarence 

 A. Skinner, Adjunct Professor of Physics in the University 

 of Nebraska*. 



IT is a fact with which those investigating the passage of 

 electricity through rarefied gases are well acquainted,, 

 that it requires a very high potential to force a discharge 

 directly across the space between the electrodes, when these 

 are brought within a certain small distance of each other. 

 This phenomenon has been studied by several investigators f. 

 The writer, conceiving that the relatively high potential 

 required to produce the discharge under these conditions 

 should be of the nature of a " drop " at the electrodes, sought 

 to decide the point by experiment. The experimental results 

 supported this view, and, bringing some new facts to light, 

 led to a more complete investigation of the conditions con- 

 trolling the drop of potential at the electrodes. The results 

 of this investigation form the subject-matter of this paper. 



* Read "before the American Association for the Advancement of 

 Science, August 1901. Communicated by the Author. 



t See J. J. Thomson, Recent Researches in Electricity and Magnetism, 

 p. 160:; E. Wiedemann, Wied. Ann. xx. p. 767 (1883); lxiii. p. 242 

 (1897) ; A. Wehnelt, Wied. Ann. lxv. p. 511 (1898). 



