552 ELECTRICITY PRODUCED BY FRICTION. 



force which they do riot exhibit under ordinary circum- 

 stances. This attractive force was first discovered in 

 rubbed amber, and from the Greek word for amber 

 (rjXeKTpov, electron), bodies which manifest this attrac- 

 tion have been called c electric/ while the cause of the 

 attraction, and of other phenomena related to it, has 

 been termed ' Electricity.' As long as bodies are in 

 their usual state they are said to be 'neutral ' or 'non- 

 electric ; ' to render them electric, we must ' electrify ' 

 them by operations which have been partly already 

 alluded to or will be described hereafter, and the bodies 

 are then often called 'electrified 'or c electrically excited.' 



Substances used for electrical experiments must be perfectly dry. 

 Sealing-wax, ebonite, and glass may be dried by gently heating 

 them over a lamp ; sulphur is apt to break to pieces if not heated 

 very cautiously. The pieces of flannel or silk used for rubbing 

 must also be dried thoroughly by holding them for a short time 

 before a fire, without scorching them, or, in the summer, by placing 

 them in the sun. A stick of glass, about l cm thick, and 40 or 50 cm 

 long, is much used for electrical experiments. Some kinds of glass, 

 however, are apt to condense the vapour of the atmosphere on the 

 Surface, so that an invisible layer of moisture is formed ; such glass 

 rods will, even after being heated, refuse to show signs of electricity. 

 The student should therefore endeavour to procure from the dealers 

 at least two glass rods which will become at once electrified when 

 rubbed, provided they are not visibly moist, in which case they need 

 only be wiped with a cloth to render them again available for the 

 experiments. 



Amalgam for electrical experiments consists mostly of a mixture 

 of mercury, zinc, and tin, mixed in proportions which are variously 

 stated by different writers. Its preparation should not be attempted 

 by an unpractised hand ; the student should rather buy a small 

 quantity ready made, and if it is not to be had, he should scrape the 

 silvering off a piece of old looking-glass, although this consists of 

 mercury and tin only. A useful amalgam is obtained by rubbing 

 15 grammes of mercury with an equal weight of crumpled tinfoil in 

 a china or stone mortar, until a crumbly mass is obtained. A piece 

 of flannel or leather, upon the surface of which powdered amalgam 



