Equipotential Curves and Surfaces, fyc. 9 



equal parts, there is no deflection; but at (151, 50), i.e. at a distance of 

 1 millim. from this plane, there is a deflection of 10 divisions of the 

 scale. Hence the plane which is equidistant from the electrodes is 

 shown to be an equipotential surface. This is the surface at which 

 the potential is zero. 



Case 7. Plate 2. fig. 9 represents three sections of three equipotential 

 surfaces, one through the point (50, 10), another through the point (80, 

 10), and a third through the point (100, 10), the battery-electrodes being 

 placed at distances of 10 milliins. from the ends of the box and 284 

 millims. apart. 



When the battery-electrodes are so near to the ends of the box, the 

 distribution of the electric currents, and therefore the forms of the equi- 

 potential surfaces, will not be the same as in a conductor which is un- 

 limited in every direction ; hence in comparing these experimental results 

 with theory it is necessary to take into account the influence of the ends 

 and sides of the box. 



If we measure the distances from each point of a curve to the battery- 

 electrodes and to the positions of their electric images due to the sides 

 and ends of the box, and if for each point we subtract the sum of the 

 reciprocals of the distances of the negative electrode and its images from 

 the sum of the reciprocals of the distances of the positive electrode and 

 its images, we get a series of numbers which differ very little from one 

 another. 



If r, r', r" . . . . represent the distances from the positive electrode and 

 its images, and r v r x ', r" .... represent the distances from tho negative 



electrode and its images, then 2 ( - — — J is very nearly constant for each 



curve. 



In Plate 2. fig. 9 the negative electrode is so far away from every 

 point of the curve, that all except one of the electrical images at the 

 distant end of the box have been neglected. 



In the case of the curve passing through the point (50, 10), the values 



of 2 f — — — ) for the several points are given in the following table : — ■ 



•0444 

 •0443 

 •0439 

 •0435 

 •0432 

 •0433 



The curves are drawn only on one side of the axis ; but they are 

 symmetrical about the axis, and also, with regard to the plane of zero, 

 potential. 



Treating the next curve in the same way, the coordinates of the first 



