[Illustration: FIG. 113.--Experiment showing direction of lines of force in the magnetic field surrounding a conductor carrying an electric current. A piece of copper wire is pierced through the center of a sheet of cardboard, and carried vertically for two or three feet then bent around to the terminals of a battery or other source of current. If iron filings be sprinkled over the card while the current is passing, they will arrange themselves in circles around the wire, thus indicating the form of the magnetic field surrounding the conductor. Compass needles may also be used to show the direction of the lines of force at any point.] [Illustration: FIG. 114.--Right hand rule to determine the direction of magnetic field around a conductor carrying a current. The thumb of the right hand is placed along the conductor, pointing in the direction in which the current is flowing, then, if the fingers be partly closed, as shown in the illustration, the finger tips will point in the direction of the magnetic whirls.] =Ques. What is the effect of a current flowing in a loop of wire?= Ans. If, in figs. 116 and 117, the current flow in the direction indicated by the arrow, the lines for magnetic force are found to surround the loop as shown; all the lines leave on one side of the loop and return on the other; accordingly, a north pole is formed on one side, and a south pole on the other. [Illustration: FIG. 115.--Right hand palm rule to determine the direction of the magnetic field around a conductor carrying a current: Place the palm of the outstretched right hand above and to the right side of the wire with the fingers pointing in the direction of the current, and the thumb extended at right angles, that is, pointing downward. The direction in which the thumb points will indicate the direction of the magnetic whirls.] [Illustration: FIG. 116.--Lines of force of a circular loop. If a current flow through the loop in the direction indicated, the lines of force both inside and outside the loop, will cross the plane of the loop at right angles, and all those which cross the loop on the inside will pass through the plane in one direction (downwards in the figure), while all on the outside will return through the plane in the opposite direction.] =Solenoids.=--A solenoid consists of a spiral of conducting wire wound cylindrically so that, when an electric current passes through it, its turns are nearly equivalent to a succession of parallel circular circuits, and it acquires magnetic properties similar to those of a bar magnet. [Illustration: FIG. 117.--Lines of force of a circular loop. If the loop pass through a piece of cardboard at right angles to its plane, and the current flow as indicated, the dotted lines on the cardboard will represent the direction of the lines of force in the plane of the cardboard. The student should verify the lines of force as here given by applying the corkscrew rule.]