CONSTRUCTION OF THE ELECTROSCOPE. 573 



in order to remove the greater portion of the vapour of water 

 formed by the heating. When after repeatedly heating the flask 

 and expelling the vapour, all visible moisture is removed, the flask 

 is once more heated and the air sucked out of it for some time by 

 means of the tube, in order to replace by dry air the moist breath 

 blown in previously. As common water after drying off usually 

 leaves a little solid residue on the sides of the flask, it is better to 

 use either distilled water or pure alcohol for rinsing, if the flask is 

 to be perfectly clean, as either of these liquids evaporates without a 

 solid residue. If alcohol is used, the flask should be rinsed with it 

 after having first used water and allowing it to run off ; the alcohol 

 will then be available for burning, as it is not rendered too dilute. 

 The flask should be kept mouth downwards for a day after rinsing 

 it with alcohol; otherwise too much alcohol remains behind, and its 

 inflammable vapour may cause accidents while the flask is being 

 heated. 



The cork may be fixed with sealing-wax, which is placed in suffi- 

 cient quantity along the edge of the flask and melted by the spirit- 

 flame, using the blowpipe as in fig. 121, but directing the flame 

 carefully upon the edge of the flask alone, not upon the middle, as 

 otherwise the cylinder round the wire becomes softened, in which 

 case the wire sinks to the bottom of the flask. 



The instrument should only be charged by induction, for if 

 charged by touching it with a rubbed rod of glass or sealing-wax, 

 the charge may be too great and produce not only disturbances but 

 also too wide a divergence of the leaves, which might tear them. 

 Even when charged by induction care must be taken to bring the 

 inducing body only so near that before applying the finger to the 

 wire the divergence may not be greater than that in fig. 308. 



When once charged the electroscope may be used for an hour or 

 two, if the air is dry ; but after that time the instrument must 

 be charged again if further required. The body to be tested must 

 be brought near the instrument very slowly, and the behaviour of 

 the leaves must be observed from the moment when the body is 

 brought near. If the body and the instrument are charged with 

 the same electricity, the leaves diverge immediately ; if the electri- 

 cities are opposite the leaves will under all circumstances converge 

 at first. When the body to be tested is strongly electrified and 

 pretty near to the instrument, the leaves may possibly diverge after 

 having converged on the first approach of the body, and uncertainty 

 may arise, especially if the body is brought near rather rapidly, and 

 the convergence of the leaves at first was not sufficiently close. 

 The cause of the convergence is easily understood. In charging 



