Free End of a Parallel Wire System. 361 



gate the propagation of electrical disturbance along resisting 

 wires, we shall get exactly equations (3) and (4) with E- in 

 place of 47rF. But on Maxwell's theory, as Heaviside has 

 pointed out, the correspondence is not exact ; for, with the 

 resisting wires, we shall no longer have strictly plane waves, 

 as the electrical force is no longer normal to the wires ; more- 

 over the dissipation of energy does not take place throughout 

 the plane but in the wires alone, and, unless the distance 

 between the wires is small in comparison with the wave length, 

 we must expect to find noticeable differences between the 

 approximate theory and the results of experiment. 



It is in this lack of complete correspondence that the expla- 

 nation is to be found for the apparent discrepancy mentioned 

 at the beginning of this paper. To make the matter some- 

 what clearer, let us consider first the conditions when the ends 

 are bridged. Let the perfectly conducting wires end nor- 

 mally upon a plate of infinite extent with perfect electrical con- 

 ductivity ; then, when any wave plane reaches it, the electric 

 force will be instantly annulled throughout the plane, but the 

 magnetic force will persist, and we must therefore have a 

 reflected sheet with its electrical force in the reversed direc- 

 tion, and its magnetic force in the same direction, as in the 

 incident sheet. This can occur since the two sheets are mov- 

 ing in opposite directions. Exactly at the plate we shall have 

 a node of E and Y, and a loop of H and L If, instead of 

 using the infinite plate, we bridge with a straight, perfectly 

 conducting wire, we shall have nearly, but not quite, the same 

 result ; for now the electrical force is not instantly annulled 

 throughout the whole plane, but only in the central portion ; 

 and the outlying tubes of electric force must travel inward 

 toward the bridge with the speed of light, thus generating 

 lines of magnetic force which surround the bridge and, in 

 general, making the reflection a complicated one. But the 

 most conspicuous effect will be a delay in the vanishing of the 

 electric force and a consequent apparent shift of the potential 

 node (or current loop) beyond the bridge. As, with circular 

 wires of equal section, one-half the whole number of unit 

 tubes in the plane lie within a circle whose diameter is the 

 distance between the wires, we should expect this shift to be 

 not far from one-half the distance between the wires. As a 

 matter of fact, a displacement of the node beyond the termi- 

 nal bridge of about this amount is always observed, but it 

 excites no surprise, since even the most elementary considera- 

 tions make it natural to add half the length of the bridge to 

 the parallel wires, the middle of the bridge being, obviously, a 

 center of symmetry. It is, however, frequently assumed that 

 this correction would disappear if the bridge were a perfect 



