CURRENT-CURVES AND TENSION-CURVES. 179 



current-paths. We naturally think of these paths 

 within the cylinder as planes, so that we obtain current- 

 planes, which enclose each other like the scales of an 

 onion, and which in the section which we figure form 

 closed curves all of which pass through the point A. 

 They are represented on the figure by unbroken lines. 

 On each of these paths a definite fall prevails, as we 

 know that is, in each of these the point immediately 

 on the right nearest to A is the most positive, the ten- 

 sion gradually decreasing toward and up to the middle, 

 where it = 0, then becomes negative, the greatest 

 negative tension being immediately next to A on the 

 left. This is true of all paths or lines of conduction. 

 In each there is a point at which the tension = ; on 

 the right of this the tension = -f 1 ; yet further to the 

 right it = +2, and so on up to the greatest tension at 

 A ; and similarly in each curve, to the left of the zero 

 point there are points at which the tension = 1, 

 2, and so on. If all the points of equal tension are 

 united, the result is a second system of curves, which 

 are at right angles to the current curves, and which are 

 represented in our figure by dotted lines. There is a 

 curve which unites all points at which the tension = 0, 

 another which unites those points at which the tension 

 = + 1, and so on. These may be called tension-curves 

 or iso-electric curves. In the cylinder the section of 

 which is here drawn, these curves evidently represent 

 planes which cut the planes of the currents already 

 mentioned, and which may be called tension-planes or 

 iso-electric surfaces. On the outside of the cylinder 

 these iso-electric surfaces are exposed, and meet the 

 surface in bent lines, which in the simple figure 

 which lies before us are all parallel, that i?, surfaces 



