1902.] 



Theory of the Capillary Electrometer. 



225 



mechanical stresses are in some way distributed evenly over the whole 

 area of contact ; so that local action, to which perhaps we should 

 ascribe the permanent current through the electrolyte with solid 

 electrodes observed by Bouty, is prevented. I have already published 

 my opinion that there is no electrolysis, properly so called, in a well- 

 made electrometer. 



A point of practical interest arises in connection with the rapid 

 leakage of the charge in damp weather. How far does this defective 

 insulation affect the records obtained with the apparatus 1 Taking 

 the worst conditions, namely, when the time of half -discharge was 

 65 seconds, and the severest test, namely, photographing the discharge 

 of a small Leyden jar charged to half a volt into the electrometer, we 

 have the following data : The passage of the sensitive plate occupies 

 O'l second, i.e., ^-jj of the time of half -discharge. A simple calcula- 

 tion will show that the total loss of charge in O'l second would be 

 about one-tenth per cent., and as such a record would be completed in 

 less than 0*01 second, the difference would be quite inappreciable. 



It may be of interest to give some data as to the quantity of 

 electricity which an electrometer will detect. 



I took an ordinary gold-leaf electroscope with a brass knob 1 inch 

 in diameter, and charged it so that the leaves diverged at an angle of 

 about 20°. I then touched one of the terminals of the electrometer 

 with the knob of the electroscope — the mercury instantly shot right 

 out of the field. I then tried smaller charges, and found that a 

 permanent excursion of fully 1 cm. was caused by a charge that pro- 

 duced a barely visible divergence of the gold leaves. 



In order to measure the minimum quantity of electricity required 

 to cause a visible movement of the meniscus, I earthed one pole of a 

 four-cell accumulator, and touched the other with an insulated brass 

 ball 3*3 cm. diameter, freshly polished, but not lacquered. On 

 touching one of the terminals of the electrometer with the ball thus 

 charged, there was a sudden upward jerk of the meniscus, not followed 

 by any return. After six such charges, the level of the image of the 

 meniscus had risen fully 1 mm. On reversing the charges on the sphere, 

 the direction of the movement of the meniscus was reversed. With 4 

 volts the excursions were smaller, but well marked, and with 2 volts 

 they were just visible. The total quantity of the charge therefore was 



o . o o 



x = 0-011 electrostatic unit. But as the screen on which 



2 300 



the excursions were observed was placed 81 cm. from the lens, whereas 

 the photographs are taken at a distance of 126 cm., and as details 

 are discernible on the photographs with a lens that are quite invisible 

 to the naked eye, it may be safely said that a quantity equal to 

 electrostatic unit will produce a measurable excursion. 



The capacity of this electrometer, measured by the method of mix- 



VOL. LXX. R 



