206 THE REALITIES OF MODERN SCIENCE 



tance of 1 centimeter. The defining equation for field 

 intensity, H, is then 



# = - (2) 



ra 



where F is the force in dynes which the field exerts 

 on a pole of m units. 



Experiments during the years immediately follow- 

 ing Oersted's discovery developed the fact that the 

 magnetic field due to a current in a conductor is directly 

 proportional to the magnitude of the current, to the 

 length, I, of conductor if it is everywhere the same 

 distance, r, from the point where the field is being 

 measured, and inversely as r. A unit for current 

 was therefore adopted as that current which flowing 

 through 1 cm. of the arc of a circle 1 cm. in radius 

 would establish at the center of the arc a field of unit 

 intensity. Such a current would then exert one dyne 

 of force on a unit pole at the center. This is the 

 electromagnetic unit of current in the C.G.S. system. 



The action between such a current and the unit 

 pole is, however, in the nature of a stress, and the pole 

 reacts on the conductor pushing it sidewise with an 

 equal and opposite force. The reaction on the con- 

 ductor is therefore 1 dyne. The current is pushed 

 sidewise by a magnetic field of unit intensity, for the 

 unit pole at the center would exert 1 dyne of force on 

 a similar pole anywhere on an arc 1 cm. from it. The 

 field in which the conductor is placed is unity and the 

 length of the conductor 1 cm. If field, current, or 

 length is increased, other things being equal, the force 

 exerted on the conductor is proportionately increased. 



