2 Chant — Experimental Investigation into the 



trie. He compares the transmission of the effect into the 

 metal to the transmission of motion into the inner portions of 

 liquid in a cylindrical vessel when the vessel is given a rota- 

 tory vibration about its axis. 



Especial attention was drawn to the subject by Hughes,* 

 who treated the question experimentally. 



In 1886 Rayleighf published his well-known paper in which 

 he obtained expressions for the resistance and self-induction of 

 a straight conductor carrying a periodic current. For very 

 rapid oscillations the resistance 



R' = ^pl^R) (1) 



in which I is the length of the conductor, //. its magnetic per- 

 meability, R its resistance to steady currents and p = 2irn, 

 where n is the frequency. 



In 1890 Stefan, J in a paper on electric oscillations in straight 

 conductors, also obtained formulas for the resistance and self- 

 induction. With very high frequency his expression for the 

 resistance is 



R' = Rtt££ 



/? w 



where a is the radius of the conductor and a its specific resist- 

 ance. This formula is equivalent to that given by Rayleigh. 

 He remarks that for very great frequencies metallic conductors 

 act much as though without resistance, but electrolytes behave 

 very differently on account of their very high resistance. He 

 finds that for a cylindrical copper conductor l cra in diameter, 

 with a frequency of 50 millions, the current density at a depth 

 of 0-004 cm is only 1/100 of that at the surface ; while for a 

 tube of equal size of carbon disulphide the current density at 

 the center is but 0*8 per cent lower than at the surface, — in 

 other words, the current is practically uniform. 



If, now, the action enters the conductor from the surround- 

 ing dielectric and is prevented from penetrating very far by 

 the rapidity of the oscillations, it is evident that very thin 

 layers of metal should be sufficient to ward off electrical undu- 

 lations, either by absorption or reflection. 



In a paper published in 1889 Hertz§ described experiments 

 made to find out how thick a metallic film was needed to 

 screen from his rapid oscillations. Tinfoil, Dutch metal and 

 gilt paper acted perfectly. The thickness of the metal on the 

 latter he estimated at l/20 mm though it was probably much 



* Jour. Soc. Tel. Engineers, Jan. 28, 1886. 



f il On the Resistance and Self-induction of Straight Conductors," Phil. Mag., 

 May, 1886, p. 382; Scientific Papers, vol. ii, p. 486. 

 % Wied. Ann., xli, p. 400, 1890. 

 § Electric Waves, p. 160. 



