10 ELECTRICAL MACHINERY 



really needless to define the unit of resistance except in 

 terms of the ampere and volt. However, the definition is 

 often given in terms of a column of mercury so we add 

 it here. 



Unit of Resistance. The unit of resistance is called the 

 ohm and is defined as that amount of resistance offered 

 by a column of pure mercury 106.3 cm. long, having a uni- 

 form cross-section and weighing 14.4521 gms. at C. 



Unit of Quantity. The unit of quantity is the coulomb. 

 It is defined as that quantity of electricity which is con- 

 veyed past any point in a circuit in one second of time 

 by a current of one ampere. 



6. Ohm's Law. It was experimentally determined by 

 the celebrated physicist, Ohm, that there existed a direct 

 proportionality between the e.m.f. in a given circuit and 

 the current which flowed through the circuit. For a given 

 circuit, if the e.m.f. was increased to twice its value, the 

 current also increased to twice its value. If the e.m.f. 

 was reduced to one-tenth of its value the current was 

 decreased in the same ratio. When a direct proportionality 

 exists between two variables, it is always possible to express 

 the relation in the form of an equation by the use of a proper 

 constant. 



If 7 = current in the circuit, 



E = e.m.f. in the circuit; 



then Ohm discovered that I<*E and so we may put E = H, 

 where k is some constant which will generally be different 

 for every circuit. Now this constant k is really what we 

 call the resistance of the circuit (designated by R) and so 

 we have the well known Ohm's law, 



E=IR, . . (3) 



which may also be written 



/= or R = 



