42 ELECTRICITY. 



q = it which gives i if the unit of time be given ; 

 energy q X v which gives v if the unit of energy be given ; 



V T~' T~ 



^ = which gives r ; 



q = vc which gives c. 



For Electromagnetics : 



/ = H X ^ which gives I if the intensity of the mag- 

 netic field (H) and length of the radius (a) 

 of the circular current and its angle (a) at 

 tho centre be given ; 



Q It which gives Q, if the unit of time be given ; 

 energy = Q X E which gives E if the unit of energy be given ; 



F 



I - which gives R ; 

 J\> 



Q - EG which gives C. 



(56) PRACTICAL UNITS. 



Since the C.G.S. electromagnetic units are found to be incon- 

 veniently large or small, multiples and submultiples of them are 

 used in practical work. 



The practical unit of current, called the AMPERE, is T V f the 

 C.G.S. unit, and is the current produced by the electromotive force 

 of a volt acting through an ohm. 



The COULOMB is the quantity of electricity which flows per second 

 in a current of one ampere ; it is T V of the C.G.S. unit of 

 quantity. 



The FARAD is the capacity of a condenser which when charged 

 with one coulomb has a potential of one volt. It is 10 - 9 C.G.S. 

 unit of capacity. In practice the microfarad (10- 15 C.G.S. unit of 

 capacity) is generally used. 



The VOLT is the electromotive force required to produce a current 

 of one ampere in a circuit the resistance of which is one ohm. It 

 is 10 8 C.G.S. units of potential. A Daniell cell has an electromotive 

 force of rather more than one volt. 



The practical unit of resistance, the OHM, is 10 9 C.G.S. units of 

 resistance. It is nearly represented by a standard platinum-silver 

 wire prepared by a Committee of the British Association and known 

 as the B. A. unit of resistance. 



Current 

 Quantity 

 Capacity 



Electromotive force 

 Resistance 



ampere 



cou 



ilomb 



farad 



volt 



ohm 



10 - 1 C.G.S. units. 

 10- 1 



io- 9 



10 8 

 10* 



