CHAPTER II. 



ELEMENTARY THEORY OF ELECTROMAGNETISM. 



THE MAGNETIC MEASUREMENT OF 



CURRENT. 



13. Strength of electric current magnetically defined. Con- 

 sider a straight electric wire stretched across a uniform magnetic 

 field, the wire being at right angles to the field as shown in Fig. 

 15. Let us suppose, for a moment, that the field is of unit 



N 



wire 



irection of 

 current 



Fig. IS. 



intensity. The force in dynes with which this unit field pushes 

 sidewise on one centimeter of the electric 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-inten- 

 sity is called simply the strength of the current in the wire, and it is 

 represented by the letter /. The force pushing sidewise on / 

 centimeters of the wire is // dynes; and if the field intensity is 

 H gausses instead of one gauss, then the force is H times as 

 great, or IIH dynes. That is 



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

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



18 



