=23. Note.= An oily sewing-needle may be floated without the cork by carefully lowering it to the surface of the water. All magnets, pieces of iron and steel, knives, etc., should be removed from the table when trying such experiments. Why? =24. Directions.= (A) Place the little bar magnet (the needle) upon the floating cork, turn it in various positions, and note the result. _=25. North-seeking Poles; South-seeking Poles; Pointing Power.=_ It should be noted that the _point_ swings to the north, provided the needle is magnetized as directed in Exp. 9. This is called the north, or north-seeking pole. The N-seeking pole is sometimes called the marked pole. For convenience, we shall hereafter speak of the N-seeking pole as the N pole, and of the S-seeking pole as the S pole. We shall hereafter speak of the tendency which a bar magnet has to point N and S, as its _pointing power_. An unmagnetized needle has no pointing power. _=26. The Magnetic Needle; The Compass.=_ A small bar magnet, supported upon a pivot, or suspended so that it may freely turn, is called a _magnetic needle_. When balanced upon a pivot having under it a graduated circle marked N, E, S, W, etc., it is called a _compass_. These have been used for centuries. (See Apparatus Book for Home-made Magnetic Needles.) =EXPERIMENT 11. To study the action of magnets upon each other.= _Apparatus._ Two magnetized sewing-needles (magnetized as in Exp. 9); the cork, etc., of Exp. 10. =27. Directions.= (A) Float each little bar magnet (needles) separately to locate the N poles. (B) Leave one magnet upon the cork, and with the hand bring the N pole of the other magnet immediately over the N pole of the floating one. Note the result. (C) Try the effect of two S poles upon each other. (D) What is the result when a N pole of one is brought near a S pole of the other? =EXPERIMENT 12. To study the action of a magnet upon soft iron.= _Apparatus._ A magnetized sewing-needle; cork, etc., of Exp. 10; a piece of soft iron wire, 3 in. long; iron filings. =28. Directions.= (A) Test the wire for magnetism with filings. Be sure that it is not magnetized. If it shows any magnetism, twist it thoroughly before using. (Exp. 19.) (B) Float the magnetized needle (Exp. 10), then bring the end of the wire near one pole of the needle and then near the other pole. (C) Place the wire upon the cork, hold the needle in the hand and experiment. _=29. Laws of Attraction and Repulsion.=_ From experiments 11 and 12 are derived these laws: (_=1=_) _=Like poles repel each other=_; (_=2=_) _=Unlike poles attract each other=_; (_=3=_) _=Either pole attracts and is attracted by unmagnetized iron or steel.=_