40 



ELECTRICITY. 



(52) ELECTROSTATIC UNITS. 



The C.G.S. electrostatic unit of quantity or charge is that 



at 

 By 



quantity of electricity (q) which would repel an equal quantity at 

 the distance of one centimetre in air with the force of one dyne. 



Coulomb's law F = 



q x q 



The unit of current (i) is the current in which the unit of quantity 

 passes in a second. 



Unit difference of potential (v) exists between two points when 

 the expenditure of an erg of work is required to bring a unit of +.. 

 electricity from one point to the other against the electric forces. 



A conductor has unit capacity (c) when unit charge raises it to 

 unit potential (e.g. an isolated sphere of 1 cm. radius, has unit 

 capacity). 



The surface density of a conductor at any point is measured by 

 the number of units of electricity, supposed to be uniformly dis- 

 tributed, per square centimetre of its surface. 



The resistance of a conductor (r) is measured by the difference of 

 potential at its extremities divided by the current produced in it 

 thereby. The resistance of a conductor is also measured by the time 

 required for the passage of a unit of electricity through it, when 

 unit difference of potential is maintained between its ends. 



The specific inductive capacity (k) of a dielectric is measured by 

 the ratio of the capacity of a condenser made of it to that of an 

 air condenser of equal size. 



(53) ELECTROMAGNETIC UNITS. 



The C.G.S. unit of current (/) is that current which when 

 passed through a circuit a centimetre long bent into the arc of a 

 circle one centimetre in radius (subtending a radian at the -centre) 

 produces a magnetic field of unit-intensity at the centre. 



The C.G.S. unit of quantity (Q) is the quantity of electricity 

 which when passed through a circuit in a second produces a unit- 

 current. 



The C.G.S. unit of electromotive force (E) or potential exists 

 between two points when one erg of work is expended in bringing a 



