ELECTROMOTIVE FORCE 



2003 



ELECTROSCOPE 



a pipe. Greater water pressure causes a greater 

 number of gallons per minute to flow through 

 a pipe, and in a similar way a greater elec- 

 tromotive force causes a greater quantity of 

 electricity per second to flow through an elec- 

 tric circuit. 



Electromotive force in a circuit is measured 

 by finding the difference of pressure between 

 two points in the circuit just as one might 

 find the difference of pressure between two 

 points in a water pipe. The difference of 

 pressure between two points in an electric 

 circuit is sometimes called potential differ- 

 ence; this term and electromotive force mean 

 essentially the same thing. The term electro- 

 motive force is usually abbreviated to its 

 three initial letters, e. m. f. The practical unit 

 of electromotive force is the volt (which see). 



The e. m. f . of a battery is the difference in 

 the electric potential between the terminals of 

 the battery. The e. m. f . of single cells of 

 electric batteries is from one to two volts, de- 

 pending on the kind of cell. The e. m. f. of 

 a battery may be compared to the pressure 

 produced by a pump. The battery is the 

 source of electric pressure, as the pump is the 

 source of water pressure. 



When an electric circuit is open, if a bat- 

 tery of a dynamo or other source of electro- 

 motive force is part of the circuit, then the 

 electromotive force tends to cause electricity 

 to jump across the gap. If the gap is not too 

 great there is an electric discharge, or a spark, 

 across the gap; if a discharge does not occur 

 it is because the electromotive force is not 

 great enough to overcome the resistance of 

 the air. -Whenever there is a flow of elec- 

 tricity along a conductor or a discharge of 

 electricity through a non-conducting substance 

 the cause is an electromotive force. 



Very high electromotive force is used in 

 transmitting electric currents over long dis- 

 tances. Such currents are called high tension 

 currents. The e. m. f. of the ordinary electric 

 lighting circuit for houses is from 110 to 120 

 volts. 



In every-day electrical parlance electromo- 

 tive force is often called voltage drop, partic- 

 ularly in referring to the difference of poten- 

 tial between two points on a single wire. Thus 

 it may be said that the voltage drop of a 

 certain trolley line is ten volts, meaning that 

 the difference of electric pressure between the 

 end of the line at the power house and the 

 farther end of the line is ten volts. In this 

 case the e.m. f. between the trolley line and 



the earth might be, for example, 550 volts at 

 the power house and 540 volts at the farther 

 end of the line, the difference being ten volts. 

 When a car starts from the end- of the line 

 nearest the power house, in the example we 

 are considering, the e. m. f., or the voltage 

 drop, from the trolley to the track is 550 volts. 

 When the car has reached the farther end of 

 the line this e. m. f. is 540 volts. It is the 

 e. m. f. between the trolley wire and the track 

 that causes a current to flow from the trolley 

 to the ground through the motor which runs 

 the car. E.E.B. 



ELECTRON, elek'tron. See ELECTRICITY. 



ELECTROPLATING, e lek'tro play ting, the 

 process of depositing a coat of gold, silver, 

 nickel or other metal on a cheaper metal by 

 means of an electric current. This method has 

 almost completely superseded the old Sheffield 

 method, in which the metals were welded to- 

 gether at a high temperature. In electroplat- 

 ing, the article to be plated is immersed in a 

 solution of the desired metal, through which 

 an electric current is then passed. The solu- 

 tion is decomposed by the action of the cur- 

 rent (see ELECTROLYSIS), and fine particles of 

 metal are deposited on the article to be plated. 

 The longer the article is a part of the electric 

 circuit the heavier will be the deposit. If 

 several layers of metal are deposited, the ar- 

 ticle is known as triple-plated or quadruple- 

 plated. 



Electroplating is also used to make repro- 

 ductions of medals and other metal pieces. 

 First a mold or cast of the original must be 

 made; this cast may be of some cheap metal, 

 or it may be of wax or gutta-percha. This 

 mold is carefully rinsed in an acid or caustic 

 alkali solution, and then rinsed again in cold 

 water. The mold may then be immersed in 

 a solution of gold or other metal, as explained 

 above. 



The solutions in commercial use are numer- 

 ous, but for gold, a solution of gold cyanide 

 and potassium cyanide is commonly used, and 

 for silver, a solution of silver cyanide and po- 

 tassium cyanide. Platinum, zinc, copper and 

 brass are among the other metals commonly 

 used in electroplating. A special application 

 of the principles of electroplating has been 

 made to printing; by this method, type, en- 

 gravings and etchings may be reproduced in- 

 definitely. See ELECTROTYPING. 



ELECTROSCOPE, e lek ' tro skope, an instru- 

 ment used for detecting the presence and na- 

 ture of an electric charge. A simple electro- 



