Potential and Lines of Force of a Circular Current. 19 



oscillation as the spark passed. Heating for one minute, 15 

 divisions. 



Spark-gap 12 mm. Discharge was not through the oil, and 

 there was no deflexion except the oscillation described before. 



Spark-gap 15 mm. Discharge was through the oil, but 

 there was no deflexion except a permanent displacement when 

 the spark passed, caused by the jar of the explosion. 



The experiments described confirm the results of my former 

 paper, that the elongation which may occur when a dielectric 

 is charged can be fully accounted for by causes other than 

 electrical stresses. Of these, heating is undoubtedly the most 

 important, although the mechanical attraction of adherent 

 armatures and the slightest non-uniformity in the field also 

 produce appreciable errors. 



The heat developed is probably, at least in my experiments, 

 due more to currents along the surface of the dielectric than 

 those through it, if the substance is a good insulator. As 

 shown by my observations, the elongations, if they occur, lag 

 behind the potential, and this effect is much diminished when 

 a brass tube with rounded edges is used for the charged 

 armature instead of tinfoil or silvered glass. When such 

 an armature, separated from the dielectric, is charged, the 

 heating effect is apparently so much retarded as not to interfere 

 with rapid readings, and as a consequence no elongation 

 occurs. 



Until these extraneous causes can be eliminated or balanced, 

 and an elongation observed which is indisputably due to the 

 electrical field, I am compelled to believe either that the effect 

 of electrostriction on matter is zero or that it is very much 

 smaller than has been supposed. 



University of Cincinnati, 

 February 1903. 



11. On the Potential and Lines of Force of a Circidar 

 Current. By H. Nagaoka, Professor of Physics, Impe7nal 

 University, Tokyo *. 



1. 'T^HE potential of a circular electric current or of a 

 A vortex ring of infinitely small section was investi- 

 gated by Lord Kelvin t, Maxwell J, Hicks §, and Minchin||. 

 They express the solid angle subtended by the circle by means 



* Communicated by the Author. 



t Lord Kelvin, Trans. E. S. E. (1869). 



I Maxwell, ' Treatise on Electricity and Magnetism,' vol. ii. chap. 14. 

 § Hicks, Phil. Trans. 1881, p. m^. 



II Minchin, Phil. Mag. xxxv. (1893). 



C2 



