1891.] on Electric and Magnetic Screening. 351 



less frequency will suffice if the atmosphere surrounding the paper is 

 artificially 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 screening by the paper ; and this statement holds also 

 if the paper be thorouohly blackened on both sides with ink, although 

 possibly in this condition a greater frequency than 50 to 100 per 

 second might be required for practical annulment of the screening. 



Kow, suppose, instead of attractive force between the 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 difierence 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 between 

 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 con- 

 ditions, 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 interposition 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 blackened 

 paper becomes an almost perfect screen. 



Screening against a varying magnetic force follows an opposite 

 law to screening against varying electrostatic force. For the present 

 I pass over the case 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 efi'ect 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 

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

 with frequency 80 per second, what I have called the " mhoic effectivt^ 



