610 Dr. J. W. Nicholson on the Accelerated 



The following simple experiment illustrates this effect. 

 Portions of a piece of thin manganin wire are insulated with 

 glass, the rest being left bare. When placed in a current of 

 air and heated electrically the bare pieces of wire glow 

 brilliantly, but the portions covered by the glass are quite 

 dark and are therefore at a much lower temperature. 



In very high tension systems for the electric transmission 

 of power the overhead wires are sometimes surrounded with 

 coronas which appreciably increase the transmission losses. 

 The author has previously suggested that the losses would 

 be diminished by insulating the overhead wires with a suitable 

 material of high electric strength. The above analysis indi- 

 cates that this procedure instead of diminishing the pei missible 

 current in the wires would actually, in many cases, allow an 

 appreciably greater current to be transmitted for the same 

 rise of temperature of the wire. 



In conclusion, I have to thank Professor Charles Lees, 

 F.R.S., for his kind help in giving me a long list of references 

 to papers on this subject. 



LXIII. The Accelerated Motion of an Electrified Sphere. 

 By J. W. Nicholson, M.A., D.Sc* 



WHEN a sphere carrying a surface charge is placed in 

 a uniform field of electric force at any instant of 

 time, it is set into motion under the mechanical action on its 

 electrification during the adjustment necessary for the satis- 

 faction of the new conditions at its surface. A direct solution 

 of the appropriate electromagnetic relations, with a determi- 

 nation of the motion of the sphere, has been given by 

 Mr. Gr. W. Walker + for the general case in which the 

 sphere is assumed to possess a Newtonian mass in addition 

 to its inertia of! electrical origin, and in which the applied 

 field of force is small. 



The application of the quasi-stationary principle to accele- 

 rated motions has never received formal justification, and in 

 addition to certain general considerations tending to throw 

 doubt upon its validity for such prjblems, Walker has 

 obtained, in a later paper J, a formula for the transverse 

 inertia of a moving sphere which is not in accord with that 

 derived by Abraham with the aid of this principle. The 



* Communicated "by the Author. 



t Proc. Kov. Soc. 1906, p. 260. 



t Phil. Trans. 1910, vol. 210. p. 145. 



