88 Mr. J. J. Waterston on some Electrical 



ving it observe the divergence of the leaves to be 65°. This is 

 termed angle of first contact. We then repeat the process, and 

 again touching the cap, find the angle rises to 75°, a third time 

 to 80°, and so on, converging to a maximum of say 85°, which is 

 termed angle of last contact , and is the angle that truly represents 

 the intensity of the electricity of the induced charge, and there- 

 fore of the inducing power existing at the centre of the sulphur- 

 excited surface. But from the angle of first contact we may 

 infer the angle of last contact. 



Now suppose the limiting angle of last contact due to three 

 contacts of the twin B. D.s, or ^th of the constant charge, is 100° ; 

 and of four contacts of the twins, or -^th of the same constant 

 charge, is 85° ; and of five contacts, or ^nd of the same, is 60°. 

 We can thus determine the relation between the angle of diver- 

 gence and the quantity of electricity on the electroscope. 



The maximum divergence depends on the proximity of the 

 leaves to the sides of the cage, so it varies with the diameter of 

 the cage and length of the leaves. With cage 6 inches in dia- 

 meter and upwards the maximum is 70° ; and 3 inches in dia- 

 meter, 105°, the leaves being |^ths of an inch long; but if fths, 

 the angle extends to about 115°; and if 1 inch, to 130°: the 

 weight of the gold-leaves seems too small to influence the angle, 

 which seems to be fixed by the repulsion between the leaves 

 themselves and the vertical wire that supports them, and by the 

 attraction of the extremity of the leaves to the sides of the cage. 

 This suggests an arrangement with the wire placed horizontal, as 

 perhaps affording a more equable scale. 



Having thus determined the scale of the particular electro- 

 scope, it is easy to find its capacity with reference to the B. D» 

 unit. 



It must be noted that the electroscope cannot be used as an 

 electrometer when the angle of divergence is near the maximum, 

 because the increment of divergence for the same constant incre- 

 ment of charge becomes very minute towards the maximum. 

 By applying the twin B. D.s to divide and subdivide the charge, 

 we can always keep the angle at a safe distance from the maxi- 

 mum. Beyond a certain intensity, which seems to be a fixed 

 quantity, the gold-leaves discharge towards the air, as it is usu- 

 ally termed (see I. and 3.). If we hold a small disk of sealing- 

 wax close to the leaves while we bring an excited electric towards 

 the cap of the electroscope, it will not obtain a charge until the 

 maximum is attained. 



13. The cap of electroscope in contact with a B. D. took as 

 much off its charge as two of the same B. D.s in contact at the 

 same time with each other and with the first ; so its capacity 

 may be valued at 2, that of a B. D. being 1. The following is 



