130 TRANSMISSIVITY IN PASSIVE IRON WIRES 



evidently retarded earlier) ;i this retardation is more readily seen at 

 low temperatures (3°C.), and in general in slowly moving than in 

 rapidly moving waves. 



It is well known that the advance of the wave is due to local cathodic 

 reduction at the boundary region between the active and the passive 

 areas of the metal, the passive area being cathodal.^ The diagram 

 (Fig. 1) illustrates the conditions. The active area is anodal and the 

 adjoining passive area cathodal. The local intensity of the current 

 passing between the solution and the passive metal at any point 

 decreases with increase in the distance from the boundary line X, 

 because of the increase in the electrical resistance of the circuit which 

 includes the point under consideration. This resistance depends 

 almost entirely upon the length and the specific electrical resistance 



:Jl. 



m 



Fig. 1. Indicating the conditions of the local circuit at the boundary between 

 the active and the passive areas; the direction of the current (positive stream) 

 is indicated by the arrows. The active region (shaded) is anodal, the passive 

 cathodal. See text. 



of the column of electrolyte between the point in question and the 

 boundary line. The local reducing action, which is a function of the 

 local intensity of the current, thus decreases as the distance from the 

 boundary increases, i.e. in the order A < B < C, and beyond a cer- 

 tain distance from the boundary {e.g. XY) it will be insufficient to 



^ Adrian (Adrian, E. D., /. Physiol., 1914, xlviii, 53) finds no evidence of change 

 in the velocity of the excitation wave in the nerve as it passes along a region of 

 decrement; but, as he himself points out, his observations do not refer to the rate 

 of conduction immediately before the extinction of the wave. He concludes 

 that "if the rate of conduction is ever affected by the size of the disturbance 

 it can be only when the disturbance is so small as to be on the verge of extinction." 

 In the passive wire also the rate of transmission does not undergo evident retard- 

 ation until immediately before extinction. 



2C/. Bennett, C. W., and Burnham, W. S., J. Phys. Chem., 1917, xxi, 107. 

 For the resemblances to protoplasmic transmission, cj. Lillie, R. S.,Am. J. Physiol., 

 1916, xli, 126; Science, 1918, xlviii, 51; 1919, 1, 259, 416; J. Phys. Chem., 1920. 

 xxiv, 165. 



