572 CONSTRUCTION OF THE ELECTROSCOPE. 



A flask of hard glass with a short neck of the kind used for 

 some chemical operations, will serve for constructing an electro- 

 scope. A piece of brass wire, from 8 to 12 cm long and 2 mm thick, 

 is straightened, and softened at both ends in the flame ; one end is 

 hammered flat and shaped with a smooth file into the form shown 

 at C in fig. 307. The other end is either bent into a small ring, D 

 in fig. 307, introduced into a bullet mould and lead poured round 

 it ; or it is bent as shown at E in fig. 307, and a small round disc 

 of metal soldered upon it. The bullet or the disc must have a per- 

 fectly smooth surface the bullet should be carefully cut smooth 

 with a sharp knife ; a worn small copper coin will serve very well 

 for the disc. The brass wire must be well insulated by surrounding 

 the portion which is in the cork with sealing-wax or still better 

 with shellac. The wire is heated along that part until the wax or 

 shellac melts upon it ; a sufficient quantity is then placed upon the 

 wire, and while it cools the whole is rolled between the fingers so 

 as to form a cylinder about as thick as a common pencil. A hole is 

 then bored in the cork into which the cylinder should fit tightly, 

 and so that it projects a little beyond the cork on both sides. 



Genuine gold leaf must be used ; imitation gold leaf, which is 

 made of brass, is too stifi for the purpose. It requires some skill to 

 cut the required strips from a sheet of gold leaf; an unskilled hand 

 wastes a great deal of material in the attempt, and it is therefore 

 much better to have the strips cut and fixed to the flat end of the 

 wire by a skilled mechanician. They are fixed by solution of gum 

 arabic, or white of egg, of which a trace is spread upon both flat 

 sides of the wire. They should be about 3 mm wide, and from 3 to 

 5 cm long, so that in no position they can reach the glass. In intro- 

 ducing the wire with the leaves every current of air must be 

 avoided, or they will be blown aside and adhere to the glass, in 

 which case it is impossible to separate them again without tearing 

 them. It is advisable, if the student attempts the operation him- 

 self, to cover nose and mouth by a cloth. 



The flask must be thoroughly dried before introducing the cork 

 with the wire and leaves. Flasks with narrow necks may be dried 

 in the following manner. After being thoroughly washed and rinsed 

 with clean water, the flask is clamped mouth downwards in the 

 retort-stand or placed with the mouth upon a piece of folded blot- 

 ting-paper. After being kept in this position for an hour or two 

 the greater portion of the adhering water will be removed ; the 

 flask is then held over the lamp, and constantly turned until it is 

 as hot as the hand can just bear. A glass tube is now introduced 

 which reaches to the bottom of the flask and air is blown into it, 



