﻿On the Resistance of a Fluid to a Plane. 409 



The strip began to break down under the action of the 

 alternating current at 900 volts ; at 3000 volts it was emitting 

 small local arcs, some crimson, others vio!et. After the 

 application for some time of this high voltage the number 

 and brilliancy of the arcs diminished, — the insulation had 

 apparently improved. At 5500 volts there were bright dis- 

 charges similar to the first. These were not of the nature of 

 long sparks, but of small local arcs. 



Rod No. 6 gave way at 6500 volts and exhibited the same 

 apparent improvement in insulation. The arcs shot out in 

 miniature flames, like very small blowpipe blasts; locally, as 

 in the case of the strip. 



With rod No. 9 sparking commenced at 1500 volts, having 

 the appearance of small beads of crimson and violet light- 

 burning themselves out at fixed points. When these speci- 

 mens had become cool their insulation was again tested. 

 Dielectric. Resistance. 



Strip = oo . 



No. 6 = 70,750 megohms. 



No. 9 = 2020 megohms. 



The strip thus appears to have recovered entirely. No. 6 

 has fallen to a quarter of its first value, and No. 9 has greatly 

 improved, under this trying ordeal. 



Rod No. 15 was compounded of iron filings, Sr, &c, with 

 a view to obtaining a brilliant discharge. The salts brought 

 the insulation down rather low ; there was a great deal of 

 heat generated, and little else. Such a rod should probably 

 be made simply of metallic powders intermixed with the 

 dielectric. 



XL VII. On the Resistance of a Fluid to a Plane kept moving 

 uniformly in a direction inclined to it at a small angle. By 

 Lord Kelvin*. 



§ 1. "J" ET q be the velocity ; i its inclination to the plane ; 

 I A and w, v its components in and perpendicular to the 

 plane. We have 



u = q cos i, v=q sin i. 



§ 2. Suppose now the moving body to be not an ideal 

 infinitely thin plane, but a disk of finite thickness very small 

 in comparison with its least diameter, and having its edges 

 everywhere smoothly rounded. If the fluid is in viscid and 

 incompressible, and the boundary containing it perfectly 

 * Communicated by the Author. 



