THE MAGNETIC MEASUREMENT OF CURRENT. 19 



which the intensity is H gausses, and / is the strength of the 

 current in the wire. 



Definition of the ampere. 

 The ampere is defined as one 

 tenth of an abampere. 



Definition of the abampere. 

 A wire is said to carry a current 

 of one abampere when one 

 centimeter of the wire is pushed 

 sidewise with a force of one 

 dyne, when the wire is stretched 

 across a magnetic field of which 

 the intensity is one gauss, the 

 wire being at right angles to the 

 field. The current I in equa- 

 tion (i) is expressed in ab- 

 amperes when F is expressed 

 in dynes, / in centimeters and 

 H in gausses. 



The c.g.s. system of electrical units. In earlier days, the 

 resistance of a particular piece of wire would be used as a unit 

 of resistance, the electromotive force of a particular voltaic cell 

 would be used as a unit of electromotive force, and current 

 values were often specified in terms of the deflections of a par- 

 ticular galvanometer. The introduction of a uniform system 

 of units was a great improvement on this old procedure, and it 

 was brought about chiefly by Weber and Gauss in Germany and 

 by Maxwell and Kelvin in England. This uniform system of 

 units was based on the units already in use in mechanics, namely, 

 the centimeter, the gram and the second; and the units of this 

 c.g.s. system were called absolute units to distinguish them from 

 the units formerly used. 



The electrical units now almost universally employed, namely, 

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

 forth, are not the c.g.s. units but convenient multiples or sub- 

 multiples of them. The c.g.s. units as a rule have no names, 

 therefore it is convenient to call the c.g.s. unit of current the 



