Equipotential Cu?*ves and Surfaces, fyc. 3 



of this small tube a needle was held by a spring ; and when the required 

 point was found, by pressing down the spring a hole could be made in 

 the tinfoil, thus marking the position of the tracing-electrode. 



The best form of contact is probably by means of needle-points, on 

 which shoulders of metal 2 or 3 millims. in diameter are soldered, and 

 which are pressed tight on the tinfoil. By placing a sheet of paper 

 underneath the tinfoil disk the forms of equipotential curves are at once 

 pricked out, and may afterwards be drawn. For illustration in lectures 

 the sheet of tinfoil may be placed in front of a lamp, and the forms of 

 equipotential curves or lines of flow may be thrown on a screen. If the 

 curves be traced on a circular disk of the size of or smaller than the 

 condensing-lens, the whole series of equipotential curves on it may be 

 thrown upon the screen at the same time. 



Case 1. Plate 1. fig. 1 represents a sheet of tinfoil 310 millims. 

 square, in which A and B, the battery-poles, are 126 millims. apart ; 

 and the line AB is nearly parallel to a side, and passes through the 

 centre O of the square ; the point O is equidistant from the two poles. 



Not far from the centre of the sheet, and in the smaller curves through 

 an angle varying from 60° to 90° about the electrodes, these curves 

 coincide with circles ; and in other parts of the curves, when the influence 

 of the edge is taken into account, by supposing the distribution due to 

 charges in the positions of the electric images, as will be afterwards 

 explained, the agreement with the curves as given by theory is remarkably 

 exact. 



If a large sheet of tinfoil be taken, and the battery-electrodes be 

 placed far away from the edge of the sheet, then at all points not near 

 the edge of the sheet the forms of the equipotential curves will be very 

 nearly the same as in a sheet of infinite extent. 



In all such cases (as will be afterwards shown) the equipotential curves, 

 when there are only two battery-electrodes in connexion with the sheet, 

 are circles having their centres on the straight line passing through 

 the two electrodes ; and the lines of flow are also arcs of circles which 

 pass through the two poles. 



Case 2. Plate 2. fig. 2 represents a circular sheet of tinfoil, 210 millims. 

 in diameter, with the electrodes on the circumference, and at a distance 

 from one another equal to the radius. The electrodes were small 

 binding-screws placed as closely as possible to the edge of the disk. 

 The differences of potential between two successive equipotential curves 

 have been measured by the deflections of the needle of the galvanometer. 



The deflections were as follows : — 



From 7c to a 150 



„ a to b 50 



„ h toe 50 



„ c to e 50 



„ e to./ 50 



b2 



