41* Sir W. Thomson. [Apr. 



, 



30 (with sin iO and cos t'0 instead of spherical harmonics). But it 

 deserves more complete synthetic investigation, not limited by the 

 approximntionftl conditions of 21, 22, if for no other reason, 

 because of Hertz's mouse-mill. This must, however, be reserved for a 

 future communication. Meantime, it is worth saying that sudden 

 variations of electric current, or alternating electric currents, distri- 

 bute themselves between different straight parallel conductors in the 

 same proportion as static electriBcation is distributed in corre- 

 sponding electrostatic arrangements, whenever the suddenness, or 

 the frequency, is sufficient to cause the impedance by mutual induc- 

 tion of the separate parallel conductors (and therefore, a fortiori, tl 

 impedance by self-induction of each) to be very large in compai 

 with ohmic resistance. Hence Hertz's mouse-mill screening follow 

 (though by utterly different physical action), simply the electrostatic 

 law, except in any case in which his wave-length is less than a 

 siderable multiple of the diameter of his mouse-mill. 



II. "On Variational Electric and Magnetic Screening." 

 Sir W. THOMSON, P.R.S. Received April 1, 1891. 



1. A screen of imperfectly conducting material is as thorough ii 

 its action, when time enough is allowed it, as is a similar screen of 

 metal. But if it be tried against rapidly varying electrostatic force, 

 its action lags. On account of this lagging, it is easily seen that the 

 screening effect against periodic variations of electrostatic force will 

 be less and less, the greater the frequency of the variation. This is 

 readily illustrated by means of various forms of idiostatic electro- 

 meters. Thus, for example, a piece of paper supported on metal in 

 metallic communication with the movable disc of an attracted disc 

 electrometer annuls the attraction (or renders it quite insensible) a 

 few seconds of time after a difference of potential is established and 

 kept constant between the attracted disc and the opposed metal plate, 

 if the paper and the air surrounding it are in the ordinary hygro- 

 metric condition of our climates. But if the instrument is applied to 

 measure a rapidly alternating difference of potential, with equal 

 differences on the two sides of zero, it gives very little less than the 

 same average force as that found when the paper is removed and all 

 other circumstances kept the same. Probably, with ordinary clean 

 white paper in ordinary hygrometric conditions, a frequency of alter- 

 nation of from 50 to 100 per second will more than suffice to render the 

 screening influence of the paper insensible. And a much less frequency 

 will suffice if the atmosphere surrounding the paper is arti6cially 

 dried. Up to a frequency of millions per second, we may safely say 

 that, the greater the frequency, the more perfect is the annulment of 



