98 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



wires are pushed sidewise by the magnetic field causing the 

 cylinder A to rotate in the direction of the curved arrow. 



52. Strength of current magnetically defined. Consider a 

 straight electric wire stretched across a uniform magnetic field 

 of which the intensity is one gauss, the wire being at right angles 

 to the field as described in the foregoing article. The force in 

 dynes with which the field pushes sidewise on one centimeter of 

 this wire has been adopted as the fundamental measure of the 

 strength of the current in the wire. This force-per-unit-length-of- 

 wire-per-unit-field-intensity is called simply the strength of the 

 current in the wire ; let it be represented by 7. The force push- 

 ing sidewise on / centimeters of the wire is //, and, if the field 

 intensity is H gausses instead of one gauss, the force is H 

 times as great, or IIH\ that is, 



(28) 



in which F is the force in dynes pushing sidewise upon / centi- 

 meters of wire at right angles to a uniform magnetic field of which 

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

 in the wire. 



Definition of the ampere. The ampere 

 is 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. That is, 

 F in equation (28) is expressed in dynes 

 when / is expressed in centimeters, H in 

 gausses, and / in abamperes. 



In the early days of the development of the theory of electricity and magnetism, 

 a great variety of arbitrary units was used. Thus, 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 particular galvanometer. The introduc- 

 tion of a uniform system of units was due chiefly to Weber and Gauss in Germany and 

 to Maxwell and Kelvin in England. This uniform system of units was based on the 

 units already in use in mechanics, the centimeter, the gram, and the second, and the 

 units of this c.g.s. system were called absolute units. 



