5. Name three different electric circuits that you know exist. Which are open and which are closed circuits?
6. Are voltaic cells used in your home? If so, for what purpose are they used? On open or closed circuits? Have you seen them? what kind are they?
[CHAPTER XII]
THE MAGNETIC EFFECT OF ELECTRIC CURRENTS. ELECTRICAL MEASUREMENTS
(1) The Magnetic Effect of Electric Currents
255. The Magnetic Effect.—Of all the effects of electric currents, it is generally conceded that the magnetic effect is the one of greatest practical importance, and it is also the one most extensively used. An experiment illustrating this effect has been described in Art. 239. This experiment shows that an electric current, if parallel to a magnetic needle, and near it will deflect the north-seeking pole of the needle to the right or left depending upon the direction of the current flow. This deflection of the magnetic needle is due to the fact that surrounding every electric current are magnetic lines of force. It is this magnetic field of the current that causes the needle to turn. The position taken by the needle is the resultant of the forces of two magnetic fields; one, the earth's field, the other, that of the current.
256. Right-hand Rule for a Conductor.—To show the presence of the magnetic field about a current, pass a thick copper wire vertically through a sheet of paper, and connect the ends of the wire to a source of current. While the current (this should be as much as 10 amperes if possible) is flowing, sprinkle iron filings upon the paper and tap gently. The filings will arrange themselves in circles about the wire showing the magnetic field. (See Fig. 229.) The needle of a magnetoscope tends to place itself parallel to the lines of force of this field and from this action or tendency the direction of the magnetic lines about a current may be determined. The following rule is helpful and should be memorized: Grasp the conductor with the right hand with the outstretched thumb in the direction that the current is flowing. The fingers will then encircle the wire in the direction of the lines of force. This rule may be reversed, for, if the fingers of the right hand grasp the wire so as to point with the magnetic field, then the current flows in the direction in which the thumb points. (See Fig. 230.)
