88 PRACTICAL LESSONS IN SCIENCE. 



long as it is surrounded by poor conductors, as glass or air, 

 which are sometimes called insulators. 



It is often desirable to detect the presence of electricity, which 

 is done by the use of the electroscope. This, in the simplest 

 form, consists of a ball of pith from a cornstalk or elder, hung 

 by a silken thread from a glass support. Now, if a glass rod 

 which has been electrified by friction is brought near the ball of 

 pith it will attract the ball, but on coming into contact with 

 the rod the ball receiving electricity from the rod is instantly 

 repelled. Now, if an electrified stick of sealing wax and an elec- 

 trified rod of glass are alternately brought near the pith ball the 

 glass rod will repel and the wax attract the ball, which seems to 

 show that the glass and wax have different kinds of electricity, 

 and that the same kinds of electricity repel each other while un- 

 like kinds attract each other. The electricity produced on the 

 glass is called vitreous or positive electricity ; that developed in 

 the wax is called resinous or negative. They are developed to- 

 gether, positive in the glass and negative in the rubber, the nega- 

 tive in the wax and positive in the rubber. The supposition is 

 that each body is endowed with neutral electricity, and by fric- 

 tion this is decomposed, one body retaining the positive and the 

 other the negative. Another curious phenomenon which compli- 

 cates matters somewhat occurs when glass is rubbed with cat- 

 skin, in which case negative electricity is developed in the glass 

 instead of positive, as when rubbed with the flannel. 



In all electrical experiments the air and the apparatus should 

 be dry, many delicate experiments fail by loss of electricity 

 through the conduction of moisture in the air. 



Frictional electricity seems to rest only on the surface of bod- 

 ies. An insulated conductor charged with either kind of elec- 

 tricity decomposes the neutral electricity of bodies near it, 

 attracting the opposite and repelling the like kinds. The action 

 thus excited is called induction. 



Machines are made for developing electricity by friction, and 

 with them are condensers of some kind for the accumulation of 

 the electricity developed ; sometimes the condenser is part of the 



