Experiment on Pressure due to Waves

Abstract

I HAVE seen both in the Physikalische Zeitschrift (January) and in the Physical Review (February) an account of an experiment by Prof. R. W. Wood to demonstrate the pressure due to waves, and which he suggests as a lecture demonstration of the effect observed by Lebedeff and by Nichols and Hull. The same experiment is quoted by Prof. Poynting in his address on this subject to the Physical Society of London (Phil. Mag., April). I venture to suggest that the experiment, which consists in setting a small windmill in motion by means of Leyden jar discharges maintained by a transformer, will bear a different explanation. It was shown long ago (1793) by Kinnersley, of Philadelphia, in his “Electrical Thermometer,” that a jar discharge produces in air a violent explosive effect, which we should now explain by the repulsion between constituents of the current in opposite phase to one another. The repulsive force may be very great. I think it is this explosive effect that Prof. Wood shows in the experiment, and not the pressure due to reflection of a continuous train of waves. I do not think that the suggestion is new, but it appears to me that the same cause may account for the disruption which occurs when lightning strikes a building, an instance of which is recorded in NATURE of April 13 (p. 565) in the displacement of some of the blocks of the small pyramid.