INSULATION. 561 



with the electric wire it affects the pendulum as little 

 as the dry stick of sealing-wax. 



A conductor can be electrified only when contact 

 between it and other conductors is avoided, so that the 

 escape of its electricity is prevented. The conductor 

 must therefore be supported exclusively by non-con 

 due tors, in which case it is said to be insulated. 



A. body may be insulated by suspending it on silk threads or 

 placing it on supports of glass or sealing-wax. Supports of sealing- 

 wax are easily made, but also easily broken ; glass supports are 

 rarely good, unless they are coated with shellac ; this is a resinous 

 substance, usually brown, but there exists also a bleached sort which 

 is nearly white. A solution of it in spirits of wine is sold as a 

 furniture polish under the name ' shellac-varnish.' This is usually 

 rather opaque ; in order to obtain a nice transparent varnish for glass 

 used in electrical experiments, let it stand until the whole has separ- 

 ated into two layers, of which the lower one is light brown, opaque 

 and thick, while the upper layer is dark brown, transparent and thin; 

 carefully pour off the upper layer and use it for varnishing glass ; 

 the lower layer may be used for wood. The clear solution must be 

 applied with a camel-hair brush to the warmed glass, if the coating is 

 to be bright and transparent ; if the glass is cold, the coating is 

 dull, whitish, and turbid. The glass should be warmed over a lamp 

 sufficiently that the varnish may dry immediately on it when applied 

 by the brush, care being taken that the brush is not too much 

 wetted. There should be no hissing sound when the brush is 

 applied, otherwise the coating will contain bubbles and the glass 

 may even crack. The varnishing should first be practised upon a 

 few broken pieces of glass, so as to obtain a correct estimate of the 

 proper temperature required. Avoid drawing the brush over places 

 already varnished, or they will get spoiled. If the coating is a 

 failure, remove it by rubbing the whole over with a cloth dipped in 

 spirit of wine. A solution of sealing-wax in spirit of wine is often 

 used for the same purpose as the shellac- varnish, but such a coating 

 does not look so well, being opaque and dull. If used, the solution 

 must be stirred before being applied, since the heavier substances of 

 which sealing-wax is made, sink to the bottom when left to stand. 

 The shellac -varnish should be purchased, not made ; it dissolves far 

 too slowly and forms lumps in the liquid. 



Shellac is a first-rate insulator, and it condenses much less vapour 







