TRANSACTIONS OF SECTION A. 565 



5. Beport of ilie Committee on the Phenomena accompanying the 

 Discharge of Electricity from Points. — See Reports, p. 189. 



6. On the Electrification of Needle Points in Air} 

 By A. P. Chattock. 



The author measures the streugth of the electrostatic field at the surface of a 

 needle point by the mechanical force exerted by the field upon the needle parallel 

 to its axis ; and justifies experimentally the formula 



r 



where /is the field strength at a point of radius of curvature r, and P the mecha- 

 nical pull upon it. 



Values of/ at the instant of discharge in air are given for air pressures, varying 

 from 10 cm. to 76 cm. of mercury ; the measurements havino- been made on needle 

 points, for which the values of r lie between 7 x 10"^ cm. and 6 x IQ-^ cm. It is 

 shown that for radii less than about 10"'^ cm. the product /x ;-»'8 is fairly constant ; 

 its value at 76 cm. mercury pressure being 16'5. 



In the light of these results the possible ways are discussed in which resistance 

 to discharge may arise at a point. The conclusion is arrived at that the resistance 

 at a clean point is due to the formation of Grotthuss chains of the air molecules 

 surrounding the point ; and it is shown that, on this view, the charges carried by 

 the gas atoms are probably of the same order of magnitude as those carried by the 

 same atoms in electrolytes. 



The variations of/ with air pressure are then referred to, and are shown to be 

 in accordance with the Grotthuss chain hypothesis so far as they go. 



7. On the Measurement of Liquid Resistances.'^ Py J. Svvinbubne. 



To avoid errors due to variations of resistance or polarisation at the electrodes, 

 the fall of potential over a portion of the electrolyte is measured. Siphon tubes 

 are arranged to connect the feeling points with vessels containing non-polarisable 

 electrodes in a suitable electrolyte. Various ways of arranging the apparatus are 

 described. 



8. The Surface-Tension of Ether and Alcohol at Different Temperatures. 

 By Professor William Ramsay, Ph.D., F.B.8. 



Measurements of the ascent of these liquids in a calibrated capillary tube were 

 made at temperatures varying from that of the atmosphere to within a short 

 distance of the critical point. These measurements, combined with determinations 

 of the angle of contact of the meniscus of the liquid with the walls of a containing 

 narrow-bore tube, and also with a knowledge of the densities of the liquid and the 

 vapour, give data for calculating the surface-tension. The results go to prove 

 that surface-tension is not a linear function of temperature. It is apparently 

 related to the heat of the vaporisation of the liquid in a somewhat simple manner. 



The angle of contact of the liquid with the tube walls varies in a remarkable 

 manner with the temperature. While, at temperatm-es for ether up to 160°, the angle 

 of contact is a small and a gradually decreasing quantity, at that temperature it is 

 zero : with rise of temperature above 160°, the angle of contact increases slowly at 

 first, rapidly as the temperature approaches the critical, until at the critical point it 

 is a right angle. It is remarkable that above 160° no bubble will stick in the tube, 

 but ascends to the top ; whereas below that temperature a babble will remain in 



• Printed in extenso in the Phil. Mag. September 1891. 



* Published in full in Electrical Revieiv, August 28. 18^' . 



