Dr. Oliver Lodge on Opacity* 393 



their slipping along the boundary of a conductor, but it 

 retards their passing across the boundary and entering a 

 conductor. As regards waves entering a conductor, the 

 effect of conductivity is a screening effect, not a trans- 

 mitting effect, and it is the bad conductor which alone has 

 a chance of being a transparent medium. 



It may be convenient to telegraphists, accustomed to 

 think in terms of the " KR-law " and comparing equa- 

 tions (2) and (10), to note that the quantity 4c7t/jl/<t — that 

 is, practically, the specific conductivity in electromagnetic 

 measure (multiplied by a meaningless 47T because of an 

 unfortunate initial convention) — takes the place of KR (i. e. of 

 RS), but that otherwise the damping-out of the waves as 

 they enter a good conductor is exactly like the damping-out of 

 the signals as they progress through a cable ; or again as elec- 

 trification travels along a cotton thread, or as a temperature 

 pulse makes its way through a slab ; and yet another case, 

 though it is different in many respects, yet has some simi- 

 larities, viz. the ultimate distance the melting-point of wax 

 travels along a bar in Ingenhousz's conductivity apparatus, — - 

 the same law of inverse square of distance for effective reach 

 of signal holding in each case. 



Now it is pointed out by Mr. Heaviside in several places 

 in his writings that, whereas the transmission of high- 

 frequency waves by a nearly transparent substance corre- 

 sponds by analogy to the conveyance of Hertz waves along 

 aerial wires (or along cables for that matter, if sufficiently con- 

 ducting) , and whereas the absorption of low-frequency waves 

 by a conducting substance corresponds, also by analogy, to the 

 diffusion of pulses along a telegraph-cable whose self-induction 

 is neglected — its resistance and capacity being prominent, — 

 the intermediate case of waves of moderate frequency in a 

 conductor of intermediate opacity corresponds to the more 

 general cable case where self-induction becomes important and 

 where leakage also must be taken into account ; because it is 

 leakage conductance that is the conductance of the dielectric 

 concerned in plane waves. This last is therefore a real, and 

 not only an analogic, correspondence. 



Writing R : Si L : Q 1 for the resistance, the capacity (" per- 

 mittance"), the inductance, and the leakage-conductance 

 (" leakance ") respectively, per unit length, the general 

 equations to cable-signalling are given in Mr. Heaviside's 

 Electromagnetic Theory ' thus : — 



at ax at dec 



