196 



ELECTROMETRY. 



between two concentric cylinders (80), the inner cylinder being in 

 connection with the source and the outer one with the earth. 

 Using a gold leaf electrometer as a measuring instrument, Gaugain 

 established that the striking density is constant if the diameter of the 

 outer cylinder is varied ; but that its value changes with the diameter 

 of the inner cylinder, and increases as this diameter diminishes. 



Mr. Macfarlane* used the same method for striking distances 

 between o'i cm. and i - o cm. in diameter. If V is the difference 

 of potential and x the striking distance between the two plates, 

 he finds that the results are exactly expressed by the formula 



V = 66*94\/^ :2 + o. 205^, 

 that is to say, by the branch of a hyperbola the real axis of which 



Fig. 135- / JT7 



corresponds to the distances and the imaginary axis to differences 

 of potential. In the case of a ball and a disc, or of two balls, the 

 curve closely approaches a parabola. 



In order to investigate the law of the striking distances between 

 two balls, M. Mascartf used with advantage an electrical valve. The 

 two knobs of a spark discharger, put separately in connection with a 

 conductor B and a point in front of it, are connected with the two 

 poles of a Holtz's machine, and for each striking distance the 



* MACFARLANE. Trans. Roy. Soc. Edin., Vol. xxvin., p. 633. 1878. 

 t MASCART. Traitt d' Electricite Statique, Vol. n., pp. 87, 93. 1876. 



