1891.] On Variational Electric and Magnetic Screening. 41 



screening by the paper; and this statement holds also if the paper be 

 thoroughly blackened on both sides -with ink, although possibly in 

 this condition a greater frequency than 50 to 100 per second might 

 he required for practical annulment of the screening. 



2. Now, suppose, instead of attractive force between two bodies 

 separated by the screen, as our test of electrification, that we have 

 as test a faint spark, after the manner of Hertz. Let two well 

 insulated metal balls, A, B, be placed very nearly in contact, and 

 two much larger balls, E, F, placed beside them, with the shortest 

 distance between E, F sufficient to prevent sparking, and with the 

 lines joining the centres of the two pairs parallel. Let a rapidly 

 alternating difference of potential be produced between E and F, 

 varying, not abruptly, but according, we may suppose, to the simple 

 harmonic law. Two sparks in every period will be observed be- 

 tween A and B. The interposition of a large paper screen between 

 E, F, on one side, and A, B, on the other, in ordinary hygrometric 

 conditions, will absolutely stop these sparks, if the frequency 

 be less than, perhaps, 4 or 5 per second. With a frequency of 

 50 or more, a clean white paper screen will make no perceptible 

 difference. If the paper be thoroughly blackened with ink on both 

 sides, a frequency of something more than 50 per second may be 

 necessary; but some moderate frequency of a few hundreds per second 

 will, no doubt, suffice to practically annul the effect of the interposi- 

 tion of the screen. "With frequencies up to 1000 million per second, 

 as in some of Hertz's experiments, screens such as our blackened 

 paper are still perfectly transparent, but if we raise the frequency to 

 500 million million, the influence to be transmitted is light, and the 

 kened paper becomes an almost perfect screen. 



Screening against a varying magnetic force follows an opposite 

 aw to screening against varying electrostatic force. For the present, 

 I pass over the cage of iron and other bodies possessing magnetic 

 susceptibility, and consider only materials devoid of magnetic sus- 

 ceptibility, but possessing more or less of electric conductivity. 

 However perfect the electric conductivity of the screen may be, it 

 has no screening efficiency against a steady magnetic force. But if 

 the magnetic force varies, currents are induced in the material of the 

 screen which tend to diminish the magnetic force in the air on the 

 remote side from the varying magnet. For simplicity, we shall 

 suppose the variations to follow the simple harmonic law. The 

 greater the electric conductivity of the material, the greater is the 

 screening effect for the same frequency of alternation ; and, the 

 greater the frequency, the greater is the screening effect for the same 

 material. If the screen be of copper, of specific resistance 1640 



. cm. per second (or electric diffusivity 130 sq. cm. per second), and 

 h frequency 80 per second, what I have called the " mhoic effective 



