ELECTRIC CHARGE. THE CONDENSER. iSl 



charge at a distance of one centimeter, so that, if the two charges q 1 and q ff are 

 expressed in "electrostatic" units, and F in dynes, equation (7 2 ) becomes 



'=* o> 



It is convenient to call the electrostatic system of units the Faraday units in order 

 to distinguish them from the c.g.s. units which are employed throughout this text. 

 Thus, the Faraday unit of electric charge is a charge which will exert a force of one 

 dyne upon an equal charge at a distance of one centimeter in air ; the Faraday unit 

 of electric field intensity is an electric field of such strength that it will exert a force 

 of one dyne upon a body which carries one Faraday unit of charge ; the Faraday unit 

 of electric current is the flow of one Faraday unit of charge per second through a wire ; 

 the Faraday unit of magnetic field intensity is a field which will push sidewise with a 

 force of one dyne upon each centimeter of a wire carrying one Faraday unit of electric 

 current ; the Faraday unit of magnetic pole is a pole of such strength that it will be 

 acted upon by a force of one dyne in a magnetic field of one Faraday unit intensity ; 

 and so on. The electrostatic system of units (Faraday units) are extensively used in 

 advanced treatises on Electricity and Magnetism. In this text, however, the electro- 

 magnetic system of units will be used, that is to say, either the c.g.s. units, such as 

 the abampere, the abohm, the abvolt, the abcoulomb, the ab farad, etc., or the so- 

 called practical units such as the ampere, the ohm, the volt, the coulomb, the farad, 

 etc. 



Number of Faraday units of charge in one abcoulomb. A given pair of charges 

 attract ( or repel ) each other with a definite force at a given distance apart according 

 to equation (72), in which q and q' are expressed in coulombs and F is expressed 

 in joule-units of force. If F is expressed in dynes, the number which expresses it 

 must be ten million times as large, so that the right-hand member of equation (72) 

 must be multiplied by lo 7 to give F in dynes. The force with which two concen- 

 trated charges, each equal to one abcoulomb (10 coulombs), would repel each other 

 at a distance of one centimeter apart is ioo.5/47r joule-units of force, or !O 9 .#/47r 

 dynes, according to equation (72) ; by substituting this value offeree in equation (i), 

 above, placing q' = q' f , and solving for q 1 we find the number of Faraday units of 

 charge in one abcoulomb, namely, 3 X Iol - This result is equal to the velocity of 

 light in air in centimeters per second. See Art. 146. 



101. Electrostatic attraction of parallel plates. Consider two 

 parallel metal plates connected to a battery as shown in Fig. 107. 

 Let a be the area of each plate, x their distance apart, and E 

 the electromotive force between them. The two plates constitute 

 a condenser of which the capacity is 



i ka 



C =- B -^ 



according to equation (63), where k is the inductivity of the 



