SCIENCE OF COMMON THINGS. 



277 



Conductors and non-conductors of electricity. 



Electrical machines. 



facilitate its passage are called conductors ; those that 

 retard or almost prevent it, are called non-conductors. 



1803 What substances are good conductors of electricity ? 



The metals, charcoal, the earth, water, and most fluids, 

 except oils, the human body, etc., are good conductors. 



1804 What substances obstruct the passage of electricity, or are "non- 

 conductors ?" 



Glass, resin, oil, silk, sulphur, dry air, etc., etc., are 

 non-conductors. 



1805 What is an electrical machine f 



An electrical machine is an arrangement by which 

 quantities of electricity can bexollected and discharged. 



The electrical machine most usu- 

 ally employed consists of a large 

 circular plate of glass, see Fig. 82, 

 mounted upon a metallic axis, and 

 supported upon pillars fixed to a 

 secure base, so that the plate can, 

 by means of a handle, W, be turned 

 with ease. Upon the supports 

 of the glass, and fixed so as to 

 press easily but uniformly on the 

 plate, are four rubbers, marked r r 

 r r in the figure ; and flaps of silk, 

 s 5, oiled on one side, are attached 

 to these, and secured to fixed sup- 

 ports by several silk cords. "When 

 the machine is put in motion, these 

 flaps of silk are drawn tightly 

 against the glass, and thus the 

 friction is increased, and electricity 

 excited. The points p p collect 

 the electricity from the glass, and 

 convey it to the conductor, c, which is sup- 

 ported by the glass rod g. 



Fig. 83 represents another form of an 

 electrical machine, constructed on similar 

 principles. S being a glass cylinder turn- 

 ing on an axis, Y the conductor, F the 

 rubber, A A supports. 



1806 What is the theory of electricity 

 most generally adopted ? 



The theory proposed by Dr. 



ine tneory proposed by Ur. 

 Franklin: this supposes the ex- 

 istence of a single, imponderable 



Fig, 83. 



