352 Sir William Thomson [April 10, 



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

 depth n x • 71 cm. from the surface of the screen next the exciting 

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



Thus (as €^ = 20-09) the range of current intensity at depth 

 2 '13 cm. is ^^Q of its surface value. Hence we may expect that a 

 sufficiently large plate of copper of 2 J cm. thick will be a little less 

 than perfect in its screening action against an alternating magnetic 

 force of frequency 80 per second. 



Lord Kayleigh, in his " Acoustical Observations/'f after referring 

 to Maxwell's statement, that a perfectly conducting sheet acts as a 

 barrier to magnetic force, J describes an experiment in which the 

 interi)osition 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 paper 

 on " Electromagnetic Induction in Conducting Sheets and Solid 

 Bodies "§ makes the following very interesting statement: — "If we 

 have a sheet of conducting matter in the neighbourhood of a 

 magnetic system, the effect of a disturbance of that system will be 

 to induce currents in the sheet of such kind as will tend to prevent 

 any change in the conformation of 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 alternating character, with a period small enough 

 to make it go through a cycle of changes before the currents decay 

 sensibly, we shall have the conductor acting as a screen. 



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

 rotating conducting sheet screens the space inside it from all magnetic 

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



Mr. Willoughby Smith's experiments on " Yolta-electric induc- 

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

 Telegraph Engineers of November 1883, afforded good illustration 

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

 with different degrees of frequency of alternation. His results with 

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

 comparatively little augmentation of screening effect with augmenta- 

 tion of frequency. This is just what is to be expected from the fact 



* ' Collected Papers,' vol. 3, art. oil. § 35. 

 t rhil. Mag. 1882, first half-year. 

 X ' Electricity and Magnetism,' § G6o. 

 § Phil. Mag. 1884, llrst half-year. 



