



[ '-2X ] 



LXXIII. Dust Figures produced by Electric Sparks. By 

 Edwin H. Barton, D.Sc, F.R.S.E., Professor of Experi- 

 mental Physics, and Walter B. Kllby, B.Sc, Research, 

 Scholar, University College, Nottingham *. 



[Plates XXI. & XXII.] 



HE problems of dust figures have recently attracted 



T 



considerable attention, papers on the subject having 

 appeared by T. J. Richmond t> by J. Robinson J, and by 

 Marsh and Nottage§. 



Asa result o£ these investigations, the quantitative relation 

 between the relative pitches of the stria? and the intensities 

 of the air currents producing them may be regarded as 

 established. But there still appeared room for much further 

 work of a varied qualitative character. We accordingly set 

 out to obtain, where possible, dust figures exhibiting in 

 turn the effects of waves which suffered reflexion, refraction, 

 interference, and diffraction. As the experiments progressed 

 other lines of work suggested themselves and were followed 

 up, frequently with interesting results. 



Out of the many experiments tried photographs of twenty 

 of: the figures are here given. But, before passing on to 

 consider these in detail, it may be well to indicate in outline 

 the chief results obtained, which are as follows : — 



1. Reflexion from a concave mirror showed a distinct 



focus. Separate exhibition of the incident "and re- 

 flected beams was not secured. 



2. Refraction was shown where the waves passed under a 



cover plate. 



3. Interference effects were clearly exhibited in the space 



between two spark-gaps. 



4. Diffraction was obtained under a variety of circum- 



stances. 



5. Whispering-gallery effects were shown distinctly, both 



when specially sought and also as subordinate features 

 in many other cases. 

 b\ Variation of Pitch of the striaa with the intensity of 

 the disturbance producing them was clearly exhibited. 



* Communicated by the Authors. 



t Phil. Mag. pp. 771-782, Nov. 1909. 



% Phil. Mag. pp. 180-187, July 1909; pp. 476-485, April 1910: 

 pp. 268-270, February 1911. 



§ Proc. Phvs. Soc. of London, pp. 264-276, vol. xxiii. part 3v. June 15, 

 1911. 



