CHAP, v.] ELECTRICITY AND MAGNETISM. 279 



322. Electromagnetic Units. The preceding magnetic 

 units give rise to the following set of electrical units, in which 

 the strength of currents, etc., are expressed in magnetic measure. 

 The relation of this "electromagnetic" set of units to the 

 "electrostatic" set of units of Art. 257 is explained in Art. 

 36S- 



Unit Strength of Current. A current has unit strength when 

 one centimetre length of its circuit loent into an arc of 

 one centimetre radius (so as to be always one centim. 

 away from the magnet-pole) exerts a force of one dyne 

 on a unit magnet-pole placed at the centre (Art. 196). 



Unit of Quantity of Electricity is that quantity which is 

 conveyed by unit current in one second. 



Unit of Difference of Potential (or of Electromotive -force}. 

 Potential is work done on a unit of electricity ; hence 

 unit difference of potential exists between two points 

 when it requires the expenditure of one erg of work to 

 bring a unit of -j- electricity from one point to the other 

 against the electric force. 



Unit of Resistance. A conductor possesses unit resistance 

 when unit difference of potential between its ends causes 

 a current of unit strength (i.e. one unit of quantity per 

 second) to flow through it. 



323. Practical Units. Several of the above "absolute" 

 units would be inconveniently large and others inconveniently 

 small for practical use. The following are therefore chosen 

 instead, as electromagnetic units : 



Electromotive-force. The Volt, = io 8 absolute units (being 

 a little less than the E.M.F. of one DanielPs cell). 



Resistance. The Ohm, = i6 9 absolute units of resistance 

 (and theoretically the resistance represented by the velo- 

 city of one earth-quadrant per second). (See Art. 364.) 



'Current. As a practical unit of current, that furnished by a 

 potential of one volt though one ohm is taken, being 



io ~~ of an absolute (electro-magnetic) unit of current, 

 and is known as "one weber-per-second." .''.. 



Quantity. The Weber, io~ absolute units of quantity 

 of the- electromagnetic system. 



Capacity. The Farad, = io~ (or one one -thousand- 

 millionth) of absolute unit of capacity. 



