ELECTRICITY 



1988 



ELECTRICITY 



one on the woolen cloth is called a positive 

 charge. Whenever any object is electrified it 

 has either a positive or a negative charge. 



When two unlike charges, a negative and a 

 positive, are separated by an air space, they 

 attract each other. The air resists this action 

 and tries to hold the charges apart. If the 

 attraction becomes strong enough to overcome 

 the resistance of the air a spark passes, which 

 is a discharge through the air. A stroke of 

 lightning is a series of such discharges in the 

 clouds or between a cloud and the earth. A 

 discharge through the air is always accom- 

 panied by heat and light. In such a discharge 

 the electricity is for an instant in motion. 



Two charges of the same kind repel each 

 other. Two pith balls charged from a rubber 

 rod and suspended by threads repel each other, 

 because they have charges of the same kind. 

 The human body may be electrified if one 

 stands on hard rubber or other insulating ma- 

 terial and touches one knob of an electrical 

 machine in operation. The hair then bristles 

 up, because all the hairs have the same kind 

 of charge and these charges repel each other. 



To sum up: Electrical effects upon non- 

 conductors consist of: 



(1) Electrical charges at rest the charges 

 being of two kinds, positive and negative, two 

 unlike charges attracting each other so that a 

 strain is produced in the space between them ; 



(2) Electrical charges in violent motion that 

 is, a discharge through air or some other poorly 

 conducting substance, the discharge being ac- 

 companied by heat and light. 



Electrical Effects upon Conductors. If an 

 object which is a good conductor of electricity 

 receives an electric charge, the charge distrib- 

 utes itself over the entire surface of the object. 

 If a copper wire receives an electric charge at 

 one end the charge flows along the wire until 

 it is distributed along its entire length. This 

 happens so quickly that it is practically in- 

 stantaneous. On the other hand, if a glass rod 

 receives a charge at one end the charge re- 

 mains at that end. An electric charge cannot 

 flow along a glass rod, but remains on that 

 part of the rod which first receives it. One 

 end of a glass rod may be charged and the 

 other end uncharged, but it is impossible to 

 charge one end of a metal rod without the en- 

 tire rod becoming charged. This is the differ- 

 ence between conductors and nonconductors. 



If a metallic conductqr such as a copper wire 



receives a positive charge at one end and a 



negative charge at the other end, the two 



charges flowing along the wire in opposite 



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directions tend to neutralize each other. Thus, 

 if the two knobs of an electric machine are con- 

 nected by a copper wire there is a flow of 

 electricity along the wire which is practically 

 instantaneous, and the machine is duscharged. 

 During the discharge there is a current of 

 electricity lasting only a small fraction of a 

 second. If the positive and negative charges 

 at the ends of the wire could be renewed as 

 rapidly as the discharge takes place, there 

 would be a continuous flow of electricity along 

 the wire a real electric current. This could 

 be done by continuing to turn the handle of 

 the electric machine, but it would not be a 

 practical method ; the current produced by the 

 electric machine is too small. There are two 

 practical devices that can be used; they are 

 the electric battery and the dynamo. Either 

 of these when properly connected to the ends 

 of the wire furnishes a continual supply of 

 positive electrification at one end of the wire 

 and negative at the other end. These two 

 kinds of electrification flow along the wire in 

 opposite directions, neutralizing each other. 



Definition. An electric current, then, con- 

 sists of a continuous flow of positive electrifica- 

 tion in one direction and negative electrifica- 

 tion through the same conductor in the oppo- 

 site direction. The direction in which the 

 positive charge flows is always taken as the 

 direction of the current. 



What Is Electricity? The best answer that 

 can be given to this question is to state clearly 

 what scientists generally believe electricity to 

 be in other words, to state the modern theory 

 of electricity. It must be remembered that 

 scientists are every year discovering important 

 new facts regarding electricity and that new 

 discoveries may change the theory. With this 

 understanding, we shall proceed to state the 

 modern theory, known as the electron theory, 

 as if it were proven so completely that it could 

 never be questioned. 



Before the discovery of electrons an atom 

 was supposed to be the smallest particle of 

 matter. Atoms are so small that no micro- 

 scope makes them visible; they combine to 

 form molecules, and a molecule is therefore 

 larger than an atom. Yet molecules are so 

 small that it takes millions of molecules of air 

 to fill an ordinary thimble. It would tako 

 about two and one-half million atoms of mer- 

 cury to make a row one inch long. If we write 

 the figure 4 followed by twenty-six leros, we 

 would have approximately the number of mole- 

 cules of hydrogen that would weigh one gram; 



