ELECTRICITY 



19S1 



ELECTRICITY 



of mankind is to a remarkable extent a devel- 

 opment within the memory of men yet living. 



About 700 B.C. a very wise Greek named 

 Thales discovered that a piece of amber rubbed 

 with cloth possessed the power of attracting 

 feathers, pieces of paper and other light bodies 

 to it. Probably other men of that early day 

 noted the same phenomenon, but we only 

 know that Pliny, who died A. D. 114, left the 

 next recorded experiments in the same direc- 

 tion. He knew of the loadstone and likened it 

 to the amber, but did not guess there could 

 be a connection between the two. An English- 

 man named Gilbert, who died in 1603, gave the 

 world the name electricity. Very properly he 

 gave credit to the early Greek who left re- 

 corded experiments with amber. The Greek 

 name for amber is electron, so Dr. Gilbert gave 

 the name electricity to the phenomena which 

 the crude experiments of the day produced. 

 The student who reads in this work something 

 of the lives of Isaac Newton, Volta, Faraday 

 and Davy will learn of the various discoveries 

 and inventions, each of which brought the 

 world one step nearer to present-day electrical 

 knowledge. It would be desirable to learn also 

 that it was Benjamin Franklin who proved to 

 the world that lightning and electricity are 

 identical in meaning. 



Through the later discoveries of Lord Kelvin 

 (Sir William Thomson), Morse, Gray, Bell, 

 Edison, Marconi and others, we begin to be- 

 lieve that the world has reached really the 

 electrical age, although beyond any doubt we 

 are yet on the threshold of electrical discoveries 

 and their applications. 



To-day, with electricity we light our houses; 

 illumine our streets; cook food; run trolley 

 cars at sixty miles per hour and heavy passen- 

 ger trains at nearly as great a speed; lift im- 

 mense weights ; tunnel through solid rock ; heal 

 the sick; talk. over distances of 3,000 miles and 

 send messages even farther through the air 

 without wires. To secure these great benefits 

 electricity is harnessed and applied to its task 

 through proper machinery by which it is 

 brought to the place where work is to be per- 

 formed. 



To understand something of its wonderful 

 properties we must know that electricity will 

 travel through or along certain substances, but 

 will not traverse other substances. The various 

 electrical effects were formerly classified under 

 the headings of static, or {national, electricity, 

 and current electricity. A better classification, 

 in view of the present-day theory is: 



(1) Electrical effects upon nonconductors or 

 very poor conductors of electricity ; 



( 2 ) Electrical effects upon conductors ; 



(3) Electrical effects in space. 



Conductors and Nonconductors. Any sub- 

 stance, such as copper, through which elec- 

 tricity flows freely is called a good conductor 

 of electricity. Any substance through which 

 electricity under ordinary conditions does not 

 flow at all is called a nonconductor, or an insu- 

 lator. Glass is practically a nonconductor. 

 A substance through which electricity flows 

 with great difficulty is a poor conductor of 

 electricity. A wet trolley pole is an example; 

 if the wire were to touch the wet pole a weak 

 current of electricity Would flow along the pole 

 to the ground. On the other hand, if the pole 

 were dry it would not conduct the electric 

 current. 



Electrical Effects upon Nonconductors. Such 

 effects asrare noted below are familiar to us in 

 e very-day experience. Stroking the fur of a 

 cat causes a crackling sound and the hairs to 

 be attracted by the hand. When a person 

 walks across a carpet on a cold day his body 

 becomes electrically charged and a spark passes 

 if the hand is brought near a metal object 

 such as a gas jet. By rubbing briskly with a 

 flannel cloth a rubber comb or penholder may 

 be easily electrified, so that it will attract light 

 objects such as bits of paper. 



When any two objects of different material 

 are rubbed together or merely placed in con- 

 tact each becomes electrically charged and the 

 charges remain on the objects when they are 

 separated. The two charges thus produced act 

 in opposite ways. This may be shown in the 

 following manner: A small piece of pith such 

 as may be found in a corn stalk is rolled into a 

 ball and suspended by a thread. A rubber pen- 

 holder or a rod of hard rubber is then electri- 

 fied by rubbing it with a woolen cloth. If the 

 penholder is then held near the pith ball the 

 ball is first attracted to the penholder, then 

 touches it and takes up part of the electric 

 charge. It then flies away from the penholder. 

 The pith is now repelled by the rubber and 

 attracted by the cloth. The cloth and the 

 rubber each has an electric charge. One charge 

 attracts the charge on the pith ball, and the 

 other repels it. Any electrical charge that is 

 repelled by the charge on the penholder is at- 

 tracted by the charge on the woolen cloth. An 

 electrical charge of the same kind as the one 

 on the rubber penholder is called a negative 

 charge. Any charge of the same kind as the 



