Demonstration. -Through the middle of a thick card with a smooth surface, like bristol board, thrust a piece of No. 12 copper wire. Connect the ends of this wire with a battery arranged to give its maximum current, and while the card is supported in a horizontal position, sift iron filings over it, and strike it lightly with a pencil. If the current is great enough, the filings will show that the wire is surrounded by circular lines of magnetic force having the axis of the wire for their center. A small magnetic needle will Set itself tangent to these circles at any point.
If the current passes up the wire, as in Fig. 1, the direction of the lines of force is counterclockwise. By the direction of the magnetic lines is meant, …, the direction in which the + end of the magnetic needle will point. If the current is sent down the wire, then the direction of the lines of force will be clockwise.
While there is no difficulty in showing the magnetic effect of the current in a single wire by the use of a small magnetic needle or compass, it is necessary to use a current of from 20 to 30 amperes in order to show the circular character of the lines of force by the filings. If a current of this amount cannot be obtained, the same effect can be produced by sending a current of one ampere through a vertical coil (of about 25 turns) which pierces the card. Fig. 2 indicates how this may be done.
The relation between the direction of the current flowing in a conductor and the direction of the resulting lines of magnetic force
around it, may be stated as follows: Grasp the conductor in the right hand, with the thumb pointing in the direction in which the current is flowing; then the fingers will point in the direction of the lines of magnetic force.
