150 Mr. R. F. Ear hart on the Sparking Distances 



bank of 2000 storage-cells so arranged that they might be 

 used in units of 25 cells, or by special connexions of single 

 cells. The cells were intended primarily for supplying small 

 currents at high voltages, and had small capacity. It is 

 highly desirable that small capacity be used ; otherwise, owing 

 to the disruptive discharge, the surfaces would be greatly 

 marred upon the passage of a spark. 



Surfaces. — The surfaces as described above consist of one 

 spherical surface and a plane surface. These were made 

 optically perfect, free from scratches, and were polished with 

 dry rouge. It is necessary, owing to pitting of the surfaces 

 due to discharge, that a fresh surface be presented each time 

 a reading is taken. This may be readily accomplished by 

 rotating the bodies about an axis not in the line of symmetry. 

 The means of accomplishing this will be apparent upon con- 

 sulting figure 1. 



Method of Procedure. — The surfaces were brought into 

 contact and separated by a distance considerably greater than 

 a given difference of potential required for discharge. The 

 number of fringes crossing the field during the displacement 

 was noted. Call this number a. The potential was then 

 established and the motion of the carriage reversed. The 

 number of fringes of retrograde movement until discbarge 



occurred was noted. Call this number b. Then — —- = 



It 



distance in wave-lengths between plates, since the passage of 



a fringe corresponds to one-half wave-length displacement 



of the carriage. In fig. 2, the plan of the electrical portion 



of the apparatus is indicated. As in fig. 1, D and W 



represent the surfaces. These are connected to binding-posts 



B and B'. Circuit 1 contains the E.M.F., which may be 



varied from 2 volts to 5000 volts. Circuit 2 includes the 



voltmeter. Circuit 3 includes the galvanometer and small 



E.M.F. (about 1/100 volt) for determining the point of 



contact. All circuits are provided with suitable double-pole 



switches. 



After the plates are separated, circuit 3 is opened, and a 

 difference in potential between D and D' established hy 

 closing circuit 1. The voltmeter will register this potential- 

 difference. Upon the passage of a spark the resistance of 

 the dielectric breaks down, causing a sudden drop in the 

 potential existing between D and D 7 . Thus the voltmeter 

 serves not only to measure differences in potential, but also' 

 to detect the point where discharge occurs. 



Before the readings the surfaces were polished wi th dry 

 rouge, and freed from dust-particles by blowing them with a 

 jet of dust-free air. 



