350 Sir William Thomson [April 10, 



iron, wholly bounded by a surface of revolution, with a straight 

 conductor conveying electricity along its axis. 



A screen of imperfectly conducting material is as thorough in 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. 



Fig. 7. 



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

 screening etfect 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 o23posed metal plate, 

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

 metric conditions of our climate. But if the instrument is applied 

 to measure a raj)idly 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 hygro metric conditions, a frequency of 

 alternation of from 50 to 100 per second will more than suffice to 

 render the screening influence of the paper insensible. And a much 



