Experiments and Inductions. . 87 



The cage consists of a brass cylinder, on the top of which is 

 stuck a disk of lac with oval hole in centre, in which was fitted, 

 when soft, an oval plug of shell lac that had been melted upon 

 the wire when heated. The lower end of wire was filed to a 

 knife-edge about i-th of an inch long. This edge, when heated, 

 was brought down on a surface of soft sealing-wax (resin and 

 beeswax), the sides then cleaned, and the knife-edge rubbed on 

 paper so as to leave only the slightest coating of wax. It was 

 then brought down transversely upon the centre of a strip of goldr 

 leaf 1 \ inch long and -j^th of an inch broad (being less in breadth 

 than knife-edge of wire), lying loosely on gold-beaters'' paper; 

 After being trimmed at edges, the angle of divergence is very 

 clearly brought out. It is most convenient to use thick gold- 

 leaf which can be handled. I used the thinnest (what is sold in 

 books and used by gilders) in most of the experiments; but 

 latterly I mounted the electroscope with a leaf ten times as thick, 

 obtained from the gold-beater, and found that there was no 

 sensible difference in the divergence for the same intensity of 

 charge. 



Two square openings were cut on the opposite sides of the 

 brass cylinder and covered with thin plates of talc, and on the 

 off side an angular scale was applied, which enabled the diver- 

 gence of the leaves to be measured roughly, the maximum being 

 about 110°. The insulation was almost perfect; even in rainy 

 weather the leaves would stand at their extreme angle for 10 

 minutes without sensible diminution. 



To obtain some idea of the relation between the intensity of 

 the charge and the angle of divergence, twin brass disks (termed 

 13. D.) were provided (fig. 8), and an insulating handle provided 

 to each, composed of a slender stick of fine hard sealing-wax. If 

 we charge one B. D. inductively from a constant source, such as 

 centre of an excited cake of sulphur (see 35.), then touch it 

 with the second B.D., we divide the charge into two equal 

 •parts; then discharge B. D. 2, and touch B.D.I again with 

 it, we divide the charge into four parts, and so on ; three touch- 

 ings giving ^, and four - 1 1 ^. The cake of sulphur which served 

 as electrophorus was stuck on the top of a cylinder of sealing- 

 wax, as shown in fig. 20. Another cake of the same with shell- 

 lac handle rested upon it, the two surfaces being in close 

 contact. 



As an example of graduation, suppose, after exciting the cake 

 and putting on the cover, we leave it in contact for half an hour ; 

 then taking off the cover we apply a B. D. to the centre of excited 

 cake, and touching it with a wire take off an induced charge ; and 

 reducing this to -^th by four contacts with the twin B. D., we 

 touch the cap of the electroscope with its edge, and after remo- 



