562 ELECTRICAL INDUCTION. 



on its surface than most kinds of glass ; the varnish coating is 

 specially intended for preventing the condensation of vapour. 



Electricity is also produced when conductors are 

 rubbed, but as the conductor is usually held in the 

 hand while being rubbed, the electricity produced 

 escapes as fast as it is generated. A conductor must 

 hence be insulated before the electricity produced by 

 rubbing it can be observed. 



Upon a stick of ordinary sealing-wax a smooth 

 copper coin is fastened ; holding the end of the sealing- 

 wax between the thumb and forefinger of the right 

 hand, the coin is rapidly yet softly moved across a 

 piece of fur spread out in the palm of the left hand, the 

 motion resembling that used for cleaning a painter's 

 brush; a single stroke is enough to render the coin 

 sufficiently electrical to attract the cork ball sus- 

 pended by the linen thread at a distance of 1 or 2 cm . 



45. Induction. The Electroscope. The Electrophorus. 

 A special class of phenomena is observed, when an 

 electric body is brought near to an unelectric insulated 

 conductor, but not so near as to cause a spark to pass 

 between the two bodies or electricity to be transmitted 

 in any other way. These phenomena may be best 

 studied by means of a conductor consisting of two 

 halves, which can be removed from one another, for 

 example, two coins insulated by being attached to sticks 

 of sealing-wax. One of the sticks is fixed below to a 

 small board, so as to stand vertically, the coin being in 

 a horizontal position at the top. The other stick is 

 held in the left hand, so that the edges of the two coins 

 are just touching, while a rubbed glass rod is brought 

 near to them with the right hand, as in fig. 304. Since 



