202 



Mr. G-. J. Burcll On the Interpretation of [Feb. 11, 



Inasmuch as this expression does not contain p, it is evident that the 

 duration of the P.D. between two leads both of which are situated on 

 the same side of P is independent of the position of the point at which 

 the electrical change originates, and is conditioned solely, so far as the 

 wave-front is concerned, by the distance between the two leads. On 

 the other hand, the period of delay between the moment at which an 

 E.M.F. is first established at P, and the commencement of the P.D. 

 between the leads, varies directly with the distance of P from B. 



The case is otherwise when leads on both sides of P are taken, e.g., 

 B and D. The wave-front reaches B when t ± = (p - b)/% and it 

 reaches D when to — (p - d)j — v. 



Suppose D is nearer to P than B is, this will make a P.D. in the 

 same direction as in the previous example. Then t 2 < t v and the 

 duration of the P.D. is, 



t = p-b_d-p = 2p-(b + d) 

 1 2 V V V 



Since by the diagram d >p > h, we may write b + h = p and 

 p + 1 = d. Then 



h-U = (k-lj/v. 



This expression is clearly a maximum when either h = or I = 0. 

 It will be observed that the sign of the resulting P.D. varies according 

 as k or I is greater. Hence the duration of the P.D, will be greatest 

 when one of the leads coincides with P, zero if P is midway between 

 them, and it will vary in sign according as D or B is nearer to the 

 point at which the E.M.F. oiiginates. The delay will be zero if either 

 electrode coincides with P, and cannot exceed the maximum of 

 (d - b)l'2v, at which point the duration becomes zero, and the value of 

 the P.D. consequently vanishes. 



The next step is to trace the time relations of the P.D. between the 

 terminals which may result from the removal of the source of E.M.F. 

 If the condition of electrical activity after having invaded the Avhole 

 conductor begins at any moment to subside simultaneously at all 

 points, and if the rate of subsidence is the same at all points of it, no 

 P.D. will be produced between any two points, however situated. If 

 the electrical activity begins to subside before it has invaded the entire 

 conductor, and while the wave-front is still between the two electro- 

 meter leads, then if it subsides simultaneously over the whole part 

 affected, the P.D. between the leads will simply fall to zero. 



If the electrical activity, after having invaded the whole conductor, 

 subsides first at the two ends of it, persisting longest at P, where it 

 originated, the period of the subsidence will be marked by a second 

 P.D. between the leads, of the same sign as that evoked by the wave- 

 front of electrical change. 



