210 



Mr. G. J. Burch. On the Interpretation oj [Feb. 11, 



separately v, the velocity of propagation, p the distribution of the 

 points of origin, the duration of the electromotive effect, tt its rate 

 of development, h . tt its rate of subsidence, and q the influence of 

 conductors which do not reach the second lead. 



As has been already stated, the problem is rendered more difficult 

 by the fact that there must always be an escape of current within the 

 bundle from one conductor to another, the amount of which cannot 

 be determined. 



Velocity of Propagation. 



The function v, i.e., the velocity of propagation, can, as is well 

 known, be easily determined by varying the interval between P and B. 

 This may be done by shifting B with respect to A, or by changing 

 the position of P. 



And the value of v may then be deduced by comparison of the records 

 obtained before and after the change. 



The comparison may be made either between the times at which 

 the curve commences, or between the times at which the direction of 

 the movement of the meniscus changes. 



The commencement of the curve is sometimes difficult to detect, 

 otherwise it would be the best to use, for it depends simply on f(b,p, n), 

 and so long as pi is greater than b (i.e., the points of origin are all 

 outside the leads) the delay of the beginning of the first phase is 

 directly proportional to the time required by the wave to travel from 

 PtoB. 



But if B is brought within the region occupied by the points of 

 origin, there is a discrepancy owing to the fact that the rate of rise 

 of P.D. is altered, and lessened. 



The apex of the curve is frequently very well defined. It is some- 

 times referred to as marking the end of the first phase and the 

 beginning of the second. This is approximately but not strictly 

 true. The change of sign occurs a short but variable time after the 

 change of direction of the movement of the mercury.* 



Taking however the time of the actual zero P.D., which can be 

 easily determined by interpolation on the derived curve, it is neces- 

 sary to inquire whether this varies directly with the distance of B 

 from P. 



Zero P.D. is reached when 



+ 2l/> 3 *> v> <?) - *> p* o> k •*■,?) = o. 



Each term of this expression represents a series of values of P.D. 

 forming a curve which is not symmetrical unless, among other condi- 

 tions, the function / (q) is absent. 



* Hermann does not appear to have noticed that I had drawn attention to this 

 fact in 1892. (' Phil. Trans.,' A, vol. 183, p. 103.) 



