736 Dust Futures produced by Electric Sjjarhs. 



beyond the bend moved away from it. But, in the part 

 between the spark and the bend, the striae divided themselves 

 into two sets side by side, one set moving towards the bend 

 while the other moved towards the spark. This appeared to 

 show that a circulatory motion of the air was occurring in the 

 tube. It was then noticed that such a circulation might be 

 attributed to a side glancing into the tube of the chief energy 

 from the spark which was not quite symmetrically placed at 

 the end of the tube. This asymmetrv was then reversed and 

 immediately a reversal of the creeping motions of the stria? 

 was observed. 



Inclination of Strice by Fork. — It seemed of interest to test 

 the effect, if any, on the stria? in a tube, produced by the 

 vibrations of a tuning-fork. A tube £■- 1113I1 bore and about 

 2 feet long was used, the spark being near one end. The 

 other end of: the tube was about level with the centre of the 

 mouth of the resonance-box of a fork of frequency 256 per 

 second. When the fork was bowed alone no effect on the 

 dust was perceived. When the sparks passed alone the 

 usual transverse stria? immediately appeared. When, how- 

 ever, the fork was bowed while the sparks were passing, the 

 striae set themselves at an inclination like the ordinary 

 down-strokes in writing. But when the dust-tube was 

 inserted into the resonance-box so that this open end of the 

 tube was near the closed end of the box, and the experiment 

 repeated, the stria? set themselves at a backward slope. 



Other Tests. — Since the stria? appear to be produced by 

 the air-currents in their vicinity, it seemed desirable to use 

 some other gas in place of the air. Thus, in one tube C0 2 

 was used alternately with air, but the mean of the measure- 

 ments failed to detect any decided change in the pitch of the 

 stria?. 



Magnetic and electric fields were also used on the dust- 

 tubes, but no marked changes in the stria? were thereby 

 produced. 



Conclusion. 



It appears from these experiments that dust figures may 

 be obtained corresponding in their chief features to the 

 ordinary optical phenomena of reflexion, refraction, inter- 

 ference, and diffraction ; but that, in addition, the air 

 currents sometimes acquire circulatory or vortex motions, 

 and thus introduce into the dust figures features not present 

 in the optical cases to which they were expected to be 

 analogous. 



University College, Nottingham. 

 Jury -5th, 1912. 



