Inductive Capacity of Solids under Varying Fields, 79 



features : — Three heavy brass plates, A, B, and C (fig. 1) , 

 each 12 x 14 inches, are fastened parallel to one another, and 

 are capable of being separated each from the other through a 

 range of several inches. Each plate is held in position by 

 four insulating strips of ebonite which extend out from its 

 edge ; four iron rods, with a screw-thread cut upon their 

 entire length, pass through the ends of the ebonite strips, 

 which may be secured in any position upon the rods by bolts 

 one above and one below each strip (figs. 1, 2, and 3). At 

 the centre of each of the two outside plates, and perpendicular 

 to them, is fastened a heavy brass tube. A rod, half of which 

 is brass and half ebonite, slides in this tube, the ebonite 

 portion extending between the plates ; and the whole is capable 

 of being moved in the tube by means of a rack and pinion. 

 A scale, divided into twentieths of a centim., is marked upon 

 the brass portion of the rod, and a vernier is attached to the 

 side of a section cut in the tube. Each of the ebonite rods 

 carries at its end between the large plates a thin plane plate 

 of glass ; its surface facing the centre plate being covered 

 with a thin metal foil. These two small plates are as 

 nearly as possible parallel to the parallel large plates, and 

 their position between them is given by the vernier scale. 

 The two outside plates, B and C, are connected by a brass 

 rod, as shown in fig. 1. At the centre of this rod is a brass 

 ball kept well polished, which is capable of adjustment 

 towards or away from the centre plate, by having the ends of 

 the brass rod made to slide in tubes fitted to the two outside 

 plates. The ball may also be rotated about the rod as an axis, 

 or moved up and down. Another ball, nearly opposite the 

 first, is fastened to the centre plate by a short metal pin, 

 which permits it also to be rotated. This arrangement of the 

 two balls allows them to have a wide range of adjustment in 

 relation to each other, and permits different portions of their 

 untarnished surfaces to be brought opposite. 



This apparatus is arranged as follows : — The two outside 

 plates are connected to the earth. One terminal of a large 

 induction-coil is also connected to the earth, while the 

 other is attached to the ball, n, which is fastened to the centre 

 plate. When the balls are separated about 6 or 8 millim., 

 sparks pass between them when the coil is put in operation. 



We have here an oscillating electric system. The lines of 

 force divide themselves between the two halves of the appa- 

 ratus, reaching from the centre plate to the two outside plates. 

 In the region of the small plates, the field may be considered 

 as practically uniform. Hence, if these two small plates are 

 at equal distances from the centre plate, which we consider for 



