Effective Resistance and Inductance of a Helical Coil. 11 



of the rods and cones on the retina (only the main essentials 

 o£ which are referred to in this paper), seem to be still the 

 subject of much discussion among physiologists to whom we 

 must look for the exact interpretation of their bearing on 

 problems in photometry. Dr. Edridge Green, for instance, 

 in a recent lecture before the Optical Society*, expressed his 

 dissent from the theory of vision based on the supposed be- 

 haviour of the rods and cones, and expressed the view that 

 visual impulses are only received through the latter organs. 

 But the general phenomena on which the theory mainly 

 rests, and which have been utilized in this paper, seem to be 

 well authenticated. 



VI. The Effective Resistance and Inductance of a Helical 

 Coil. By J. W. Nicholson M.A., D.ScA 



ri^HE problem of the propagation of alternating currents 

 JL in wires has been solved in but few cases. For a 

 single wire isolated in space, a solution has been given by 

 Sommerfeld J. If the return current be conducted along a 

 concentric sheath, instead of by the dielectric, the problem is 

 that treated earlier by Sir J. J. Thomson §. The case in 

 which the return current flows along a parallel wire has 

 been discussed by Mie ||, and to a first approximation, in a 

 simpler manner, by Morton *[, who has given, in other 

 papers **, a detailed examination of the distribution of forces, 

 and applied his method also to the case of a greater number 

 of wires. 



The influence of electrostatic capacity in wires renders 

 the mathematical investigations very complicated, but from 

 a practical point of view, the main object to be attained is a 

 knowledge of the effective inductance and resistance of the 

 wires when the capacity and leakage are not so great as to 

 produce a sensible attenuation of the wave in a short 

 distance. 



The concentration of the current in the outer regions of 

 the wires, when the frequency is high, completely changes 

 the character of the inductance and resistance, and a know- 

 ledge of their values for steady currents is frequently of 



* Optical Society (London) Oct. 21, 1900. 



t Communicated bv the Physical Society : read November 20, 1909. 



% Wied. Ann. lxvii. p. 233 (1899). 



§ Recent Researches, p. 2(52. 



|| Ann. der Phys. ii. p. 202 (1900). 



11 Phil. Mag. Dec. 1900. 



** Phil. Mag. May 1901, Sept. 1902, June 1903. 



