260 Messrs. Porter and Morris on the Measurement 

 A 



by 



and add the result to the existing ordinate at that 



t r + 9 

 point. 



Dr. Sumpner has pointed out (Phil. Mag., 1888, vol. xxv. 

 p. 470) a simple construction by means of which equations of 

 the above form (3) can be graphically solved . The following 

 is a convenient method of applying this construction : — 



A A 7 (see fig. 2) is the given function, which in the present 



e 

 case is - plotted in amperes upon a time base, and BC is any 



ordinate ; if then a length CD, equal to the actual value of 



Fig. 2. 



B' 



Time in seconds 



A 



r + g 



in seconds, be measured off from C towards the left, 



and a line DE be drawn parallel to the direction of the curve 

 AA' at C, we have at once 



EB=EC+CB 

 = CD^+CB 



A 



d 



© 



r + g dr r 



