Page:Electricity (1912) Kapp.djvu/152

148 magnetic mass is acted upon with the force of one dyne, then the induction at that point is unity, or, as we also may express it, each square centimetre of a surface laid at right angles to the direction of the force at that point is traversed by one line of force. Thus we may define the horizontal component of the earth's magnetism by saying that the induction is 0·18, meaning thereby that each unit of magnetism accumulated on the north end of a compass needle is drawn northwards with a force of 0·18 dynes, the other end of the needle being drawn southward with an equal force. These forces make the needle point north-south. In a dynamo machine there is also magnetic induction, but of vastly greater intensity. In such machines e.m.f. is produced by the motion of wires placed on an armature, which revolves within a system of magnet poles. The clearance space between the face of the poles and the surface of the armature is traversed by magnetic lines of force, and the stronger the induction in this so-called "air space," the higher is the e.m.f. of the machine. The object of the designer is therefore to produce as strong an induction as possible. In modern machines the induction in the air space is of the order of 5000 to