ELEMENTARY LAWS OF CONTINUOUS CURRENTS 9 



For many approximate solutions it is sufficient to re- 

 member the constants for No. 10 wire only, as 1 ohm 

 per 1000 feet and a cross-section of 10,000 circular mils. 

 Other wires may be determined by remembering that the 

 size of the cross-section of a wire doubles for every three 

 numbers on the gauge. For example, No. 7 has an area of 

 20,000 circular mils and a resistance of 0.5 ohm per 1000 ft.; 

 No. 16 has 2500 circular mils cross-section and a resistance 

 of 4 ohms per 1000 ft. 



4. Units. In previous paragraphs we have discussed 

 current, e.m.f., and resistance but have not given the units 

 in which these various quantities are ordinarily measured. 

 Various systems of units have been employed in the past, 

 but we shall work with the so-called practical system. 

 Most of the units in this system are named after famous 

 scientists. The units which we shall use mostly in the suc- 

 ceeding chapters are those of current, e.m.f., resistance 

 and quantity of electricity. In addition to these there are 

 the units of work and power and the units of the magnetic 

 field, etc., which will be taken up later. 



Unit of Current. The unit of current is the ampere. 

 It is defined as that current which when flowing through a 

 standard voltameter (in which a silver plate is immersed in 

 a solution of silver nitrate) will deposit silver at the rate 

 of 1.118 milligrams per second. 



Unit of e.m.f. The unit of electromotive force is the 

 volt. It is defined by fixing the value of the e.m.f. of a 

 standard Weston cadmium cell. By international agree- 

 ment the value of the e.m.f. of such a cell has been taken 



1 



as 1.0183 volts so that we may say the volt is - - of 



1.0183 



the o.m.f. of a standard Weston cell. 



The units of current, e.m.f., and resistance are fixed 

 in their relation to one another by a simple equation called 

 Ohm's law, which we shall take up in the next paragraph. 

 Having fixed two of the units, the volt and ampere, it is 



