PRELIMINARY NOTIONS. 11 



PRACTICAL UNITS. The practical units, as decided upon by 

 the Congress of 1881, and the Conference of 1884, are as follows : 

 the ohm, the volt, the ampere, the farad, and the coulomb. 



OHM. The ohm is the practical unit of resistance; its 

 legal value is represented by a column of mercury of a section 

 of one square millimetre, and 1 *06 metre long, at the tempera- 

 ture of melting ice. This value corresponds, as near as it is 

 possible to estimate it, to 1 milliard of theoretical units, that 

 is to say, 1 milliard of centimetres divided by one second 



/"the quarter of the terrestrial meridian\ 

 one second 



AMPEKE. The ampere is the practical unit of intensity ; 

 its value is equal to T L of the theoretical unit C.G.S. For 

 industrial purposes the ampere is generally taken as being 

 representative of the quantity of silver deposited per second. 

 According to the recent calculations of Messrs. F. and W. 

 Kohlrausch, this quantity is equal to 0*00111888 gramme. 



VOLT. The volt is the practical unit of electromotive 

 force ; it is the electromotive force which sustains a current of 

 one ampere through a resistance equal to the legal ohm. 



The material value of one volt is approximately equal to 

 the electromotive force of one zinc-copper cell with dilute sul- 

 phuric acid and sulphate of copper, known as the Daniell celL 



COULOMB. The coulomb is the unit of electric quantity. 

 It is the quantity of electricity passing through a circuit during 

 one second when the current is equal to one ampere. 



FAEAD. The farad is the unit of capacity. It is defined 

 by the condition that one coulomb in one farad gives one volt. 



The British Association has adopted the dyne as a unit of 

 force, and the erg as a unit of work. We will describe them, 

 although they are not used in practice. 



DYNE. The dyne is the force which, acting upon the unit 

 of mass (1 gramme), impels it at a speed of one centimetre at the 



these units, which are not used for industrial purposes. The formula of resistance, 



R = L T- l , maybe written - = 5 tlie formula of force, F = M L T- 2 , 



may be written - , and be expressed as the mass of 1 gramme multiplied by 

 1 centimetre and divided by the square of a second. 



