406 Lord Rayleigh. The Influence of [Mar. 13, 



I. " The Influence of Electricity on Colliding Water Drops." 

 By Lord Rayleigh, F.R.S. Received February 27, 1879. 



It lias been known for many years that electricity has an extra- 

 ordinary influence upon the behaviour of fine jets of water ascending 

 in a nearly vertical direction. In its normal state a jet resolves itself 

 into drops, which even before passing the summit, and still more after 

 passing it, are scattered through a considerable width. When a 

 feebly electrified body is brought into its neighbourhood, the jet 

 undergoes a remarkable transformation, and appears to become co- 

 herent ; but under more powerful electrical action the scattering be- 

 comes even greater than at first. The second effect is readily attributed 

 to the mutual repulsion of the electrified drops, but the action of 

 feeble electricity in producing apparent coherence has been a mystery 

 hitherto. 



It has been shown by Beetz that the coherence is apparent only, 

 and that the place where the jet breaks into drops is not perceptibly 

 shifted by the electricity. By screening various parts with metallic 

 plates, Beetz further proved that, contrary to the opinion of earlier 

 observers, the seat of sensitiveness is not at the root of the jet where 

 it leaves the orifice, but at the place of resolution into drops. As in 

 Sir W. Thomson's water- dropping apparatus for atmospheric electri- 

 city, the drops carry away with them an electric charge, which may 

 be collected by receiving the water in an insulated vessel. 



I have lately succeeded in proving that the normal scattering of a 

 nearly vertical jet is due to the rebound of the drops when they come 

 into collision with one another. Such collisions are inevitable in con- 

 sequence of the different velocities acquired by the drops under the 

 action of the capillary force, as they break away irregularly from the 

 continuous portion of the jet. Even when the resolution is regularised 

 by the action of external vibrations of suitable frequency, as in the 

 beautiful experiments of Savart and Plateau, the drops must still 

 come into contact before they reach the summit of their parabolic 

 path. In the case of a continuous jet the " equation of continuity " 

 shows that as the jet loses velocity in ascending, it must increase in 

 section. When the stream consists of drops following the same path 

 in single file, no such increase of section is possible, and then the 

 constancy of the total stream requires a gradual approximation of the 

 drops, which in the case of a nearly vertical direction of motion cannot 

 stop short of actual contact. Regular vibration has, however, the 

 effect of postponing the collisions and consequent scattering of the 

 drops, and in the case of a direction of motion less nearly vertical 

 may prevent them altogether. 



