and Spark-length for Small Values of the latter. 621 



necessary adjustments. The box was connected to an air- 

 pump through the stopcock (P), and after sealing the lower 

 edges with bees-wax any desired pressure could be obtained. 

 A plan of the battery and galvanometer circuits,- together 

 with the arrangement of the interferometer mirrors, is shown 

 in fig, 3. The potential was furnished by a battery of about 

 five hundred storage-cells, and the sparking-potential was 

 observed by a direct-reading voltmeter properly standardized. 



Fig:. 3. 



The practice during an experiment was as follows : — The 

 surfaces were carefully polished with very fine emery and 

 rouge, set in position and the fringes adjusted in sodium 

 light. A small quantity of drying substance was placed 

 inside and the box sealed on. The air was pumped out and 

 slowly let in again through a series of drying bottles, and the 

 apparatus allowed to stand for a period of 6 to 24 hours. 

 On beginning the observations, the surfaces were brought 

 into contact by means of the interferometer-screw, the point 

 of contact being shown by the deflexion of a sensitive 

 galvanometer (Gr, fig. 3) when the spring-keys K x and K 2 

 were pressed. To avoid any possibility of the potential in 

 this circuit effecting a discharge which would tarnish the 

 surfaces, an ordinary dry cell (B^ was connected to a 

 german-silver resistance-coil (E x ) and a bit of copper wire 

 (c) in series, and the fall of potential across this copper 

 wire w r as the P J), applied to the- points between which the 



