CHAP, iv.] ELECTRICITY AND MAGNETISM. 209 



second (or about 32-2 feet per second). The value differs 

 slightly in different latitudes. At Bristol the value of 

 the acceleration of gravity is g= 981'! ; at the Equator 

 g= 978' I at the North Pole g = 983-1. 

 Force. The unit of force is that force which, acting for one 

 second on a mass of one gramme, gives to it a velocity 

 of one centimetre per second. It is called one Dyne. 

 The force with which the earth attracts any mass is 

 usually called the "weight" of that mass, and its value 

 obviously differs at different points of the earth's surface. 

 The force with which a body gravitates, i.e. its weight 

 (in dynes) is found by multiplying its mass (in grammes) 

 by the value of g at the particular place where the force 

 is exerted. 



Work. The unit of work is the work done in overcoming 

 unit force through unit distance, i.e. in pushing a body 

 through a distance of one centimetre against a force of 

 one dyne. It is called one Erg. Since the "weight" 

 of one gramme is I x 981 or 981 dynes, the work of 

 raising one gramme through the height of one centimetre 

 against the force of gravity is 981 ergs, 

 Energy. The unit of energy is also the erg; for the energy 



of a body is measured by the work it can do. 

 Heat. The unit of heat (sometimes called a calorie] is the 

 amount of heat required to warm one gramme mass of 

 water from o to i (C) ; and the dynamical equivalent 

 of this amount of heat is 42 million ergs, which is the 

 value of Joule's equivalent, as expressed in absolute 

 (C.G.S.) measure. 



These units are sometimes called " absolute" units; the term absolute t 

 introduced by Gauss, meaning that they are independent of the size of any 

 particular instrument, or of the value of gravity at any particular place, or of 

 any other arbitrary quantities other than the three standards of length, mass, 

 and time. It is, however, preferable to refer to them by the more appropriate 

 name of "C.G.S. units," as being derived from the centimetre, the gramme, 

 and the second. 



256. Electrical Units. There are two systems of electrical 

 units derived from the fundamental "C.G.S." units, one set 

 being based upon the force exerted between two quantities of 

 electricity, and the other upon the force exerted between two 

 magnet poles. The former set are termed electrostatic units, the 

 latter electromagnetic units. The important relation between the 

 two sets is explained in the note at the end of Lesson XXX. 

 P 



