[Illustration: FIG. 110.--Magnetic action: _Unlike poles of magnets attract each other_.] [Illustration: FIG. 111.--Magnetic action: _Like poles of magnets repel each other_.] =Magnetic Force.=--This is the force with which a magnet attracts or repels another magnet or any piece of iron or steel. The force varies with the distance, being greater when the magnet is nearer and less when the magnet is farther off. The following are the laws relating to magnetic force: 1. _Like magnetic poles repel one another; unlike magnetic poles attract one another._ 2. _The force exerted between two magnetic poles varies inversely as the square of the distance between them._ =Magnetic Circuit.=--The path taken by magnetic lines of force is called a magnetic circuit; the greater part of such a circuit is usually in magnetic material, but there are often one or more air gaps included. The total number of lines of force in the circuit is known as the _magnetic flux_. =Ques. How is magnetic flux measured?= Ans. By a unit called the maxwell. Named after James Clerk Maxwell the Scottish physicist. =Ques. What is the maxwell?= Ans. _The amount of magnetism passing through every square centimetre of a field of unit density._ =Ques. What is the unit of field strength?= Ans. The gauss. =Ques. What is a gauss?= Ans. _The intensity of field which acts on a unit pole with a force of one dyne. It is equal to one line of force per square centimetre_. Named after Karl Friedrich Gauss, the German mathematician. =The Magnetic Effect of the Current.=--Hans Christian Oerstead, the Danish scientist, discovered in 1819 that a magnet tends to set itself at right angles to a wire carrying an electric current. He also found that the way in which the needle turns, whether to the right or left of its usual position, depends: 1, upon the position of the wire that carries the current, whether it be above or below the needle, and 2, on the direction in which the current flows through the wire. [Illustration: FIG. 112.--Illustrating Maxwell’s “corkscrew rule” for relative directions of current and lines of force. According to the rule: _the direction of the current and that of the resulting magnetic force are in the same relation to each other as is the forward travel and rotation of an ordinary corkscrew_, Thus, in the figure, if a current flow through the wire _ab_ in the direction from _a_ to _b_, the magnetic lines will encircle the wire in the direction of the curved arrow _ro_ which shows the direction in which the corkscrew must be turned to advance in the direction of the arrow _n_.] To keep these movements in mind numerous rules have been suggested, of which the following will be found convenient: =Corkscrew Rule=.--_If the direction of travel of a right handed corkscrew represent the direction of the current in a straight conductor, the direction of rotation of the corkscrew will represent the direction of the magnetic lines of force._