420 Sir W. Thomson. [Apr. 9, 



thickness "* is 0'71 of a cm. ; and the range of current intensity at 

 depth n x 071 cm. from the surface of the screen next the exciting 

 magnet is c~" of its value at the surface. 



Thus (as e 3 = 20'09) the range of current-intensity at depth 2'13 cm. 

 is -fa of its surface value. Hence we may expect that a sufficiently 

 large plate of copper of 2 cm. thick will be a little less than perfect 

 in its screening action against an alternating magnetic force of 

 frequency 80 per second. 



4. Lord Bayleigh, in his "Acoustical Observations" (' Phil. Mag.,' 

 1882, first half-year), after referring to Maxwell's statement, that a 

 perfectly conducting sheet acts as a barrier to magnetic force (' Elec- 

 tricity and Magnetism,' 665), describes an experiment in which the 

 interposition of a large and stout plate of copper between two coils 

 renders inaudible a sound which, without the copper screen, is heard 

 by a telephone in circuit with one of the coils excited by electro- 

 magnetic induction from the other coil, in which an intermittent 

 current, with sudden, sharp variations of strength, is produced by 

 a " microphone clock " and a voltaic battery. Larmor, in his pa 

 on " Electromagnetic Induction in Conducting Sheets and 

 Bodies" ('Phil. Mag.,' 1884, first half-year), makes the followi 

 very interesting statement : " If we have a sheet of conducting 

 matter in the neighbourhood of a magnetic system, the effect of 

 disturbance of that system will be to induce currents in the sheet 

 such kind as will tend to prevent any change in the conformation 

 the tubes [lines] of force cutting through the sheet. This follows 

 from Lenz's law, which itself has been shown by Helmholtz and 

 Thomson to be a direct consequence of the conservation of energy. 

 But if the arrangement of the tubes [lines of force] in the conductor 

 is unaltered, the field on the other side of the conductor into which 

 they pass (supposed isolated from the outside spaces by the conductor) 

 will be unaltered. Hence, if the disturbance is of an alternd^l 

 character, with a period small enough to make it go through a cyok 

 of changes before the currents decay sensibly, we shall have the con- 

 ductor acting as a screen. 



" Further, we shall also find, on the same principle, that a rapi 





rotating conducting sheet screens the space inside it from all magnetic 

 action which is not symmetrical round the axis of rotation." H 



Mr. Willoughby Smith's experiments on Volta-electric induc- 

 tion," which he described in his inaugural address to the Society of 

 Telegraph Engineers of November, 1883, afforded good illustrations 

 of this kind of action with copper, zinc, tin, and lead, screens, and 

 with different degrees of frequency of alternation. His result.- 

 iron are also very interesting: they showed, as might be expected, 

 paratively little augmentation of screening effect with augmen 

 * Collected Papers,' vol. 3, Art cii, 35. 



