Equipotential Curves and Surfaces, &c 7 



only for a short time, the polarization may be kept small on the gal- 

 vanometer-electrodes, provided they are not moved far away from the 

 same equipotential surface. 



After a few preliminary experiments to determine how far the method 

 was practicable, I began a definite series of experiments in March 1872. 

 For the experiments in dilute sulphuric acid, in sulphate of copper, and in 

 sulphate of zinc, I have employed a rectangular wooden box, 1 foot long, 

 8 inches broad, and 8 inches deep. On the edges of the box are fastened 

 paper millimetre-scales ; a piece is cut out of the middle of the ends of 

 the box, and a sliding piece fitted in to carry the battery-electrodes. 

 These sliding pieces are capable of motion parallel to the sides of the 

 box, so as to place the battery-electrodes at different distances from one 

 another (see Plate 1. fig. 8). The galvanometer-electrodes were placed 

 firmly in brass tubes, which were accurately placed on T pieces of wood 

 so as to be in a line with the point of intersection of two edges of the 

 T piece, and to be vertical when the T pieces are placed on the edges of 

 the box. By this means the rectangular coordinates of the point could 

 at once be read off on the sides and ends of the box. 



The first experiments were made with the points at a depth of 

 10 centims. below the surface, the box being nearly full of liquid. 



In making the experiments the current was reversed, and the readings 

 of the galvanometer taken on both sides of zero for each position of the 

 electrodes. 



The battery employed was 20 Leclanche cells, the resistance of each 

 cell being nearly 3 ohms, and the electromotive force about 1J of 

 Daniell's cell. The strength of the battery-current was measured by a 

 tangent-galvanometer, of the form of Helmholtz's double galvanometer ; 

 the deflection of the needles during these first experiments wns gene- 

 rally 46°. The galvanometer-electrodes could be brought up to within 

 1 centim. of the centre of the box. 



The object of the first experiments was to determine the changes of 

 potential for equal changes of distance in a direction parallel to the sides 

 of the box. 



The battery-electrodes were placed at 2 millims. distance from the 

 end of the box, i. e. at a distance of 300 millims. from one another. 

 The line joining them was one axis of coordinates, the other axis being 

 the upper edge of one end of the box. One of the galvanometer-elec- 

 trodes was placed at the point (150, 50), and the other was first placed 

 at (30, 50), and moved 1 centim. at a time in a direction parallel to the 

 sides of the box. The deflections were taken at each point, and are 

 recorded in the following Table : — 



