ELECTRICITY. 



39 



Measures expressed on this system are denoted by C.G.S. The 

 unit of velocity is one centimetre per second. The unit of accelera- 

 tion is that in which unit velocity (one centimetre per second) is 

 added (algebraically) per second. 



The unit of force, called the DYNE, is the force which acting on a 



tram for a second produces in it a velocity of a centimetre per second, 

 ince a body after falling from rest for a second at Greenwich has a 

 velocity of 981 cms. per second, a dyne is ^ T of the weight of a 

 gram at Greenwich, or 1000 dynes are about the weight of 1*019 

 gram. 



The unit of work, called the ERG, is the amount of work done by 

 a dyne in acting through one centimetre. Energy is measured by 

 the work which it can do, and is therefore also expressed in ergs. 

 (For the unit of power called the Watt cf. 58.) 



Since very large and very small quantities have to be expressed 

 by means of the same unit, it is convenient to use the prefix mega- 

 or megal- to express a million times the unit ( x 10 6 ), and the prefix 

 micro- to express a millionth part of the unit ( x 10 - 6 ). 



Thus a megadyne means 1,000,000 dynes (rather more than the 

 weight of a kilogram, and a megalerg means 1,000,000 ergs (rather 

 more than '01 kilogram-metre). 



(51) MAGNETIC UNITS. 



The unit magnetic pole is one of such a strength that it repels an 

 equal pole at the distance of one centimetre with the force of one 

 dyne. 



Unit difference of magnetic potential exists between two points 

 when an erg of work must be expended to bring a unit N- seeking 

 pole from the one point to the other against the magnetic forces. 



A field of unit intensity is one which acts on a unit N- seeking 

 pole with the force of one dyne. 



Magnetic density is measured by the number of unit poles the 

 magnetism per unit of surface is equivalent to. 



The moment of a magnet is nearly the product of the strength of 

 either of its poles by the distance between them. The intensity of 

 magnetisation of a uniformly magnetised body is the quotient of its 

 moment by its volume. 



Dimensions. 



Strength of pole 

 Potential 

 Intensity of field 

 Magnetic moment 

 Intensity of 

 magnetisation 



p (force x distance 2 )* 

 v = work -r- strength of pole 

 i force ~- strength of pole 

 Ip length x strength of pole 



j = moment -f- volume 



