234 OUR PHYSICAL WORLD 



copper wire some 20 inches long and bend it so as to make a 

 rectangle 2 inches long and i^ inches wide of several parallel 

 turns of the wire. Bend out one end of the wire at the middle 

 of one end of the rectangle so that it extends out about ^ inch in 

 the plane of the rectangle and at right angles to its end. Simi- 

 larly bend out about i| inches of wire at the other end of the 

 rectangle and strip off its insulation. We will call this last end 

 of the rectangle its top. Now bend this wire at the top of 

 the rectangle so as to make in it a square open on one side, the 

 plane of the square to lie at right angles to the plane of the 

 rectangle. File off the ends of the wire until they are smooth 

 and rounded. Cut a small piece of sheet copper about i X^ inch. 

 At one end of this make a dent, and near the other end punch 

 a hole with a nail point. Fasten one end of a 2 -foot length of 

 copper wire to this copper strip through the hole and the other 

 end to a free pole of three dry batteries connected in series. 

 Connect one end of another 2-foot length of wire to the other 

 free pole of the batteries and bend the other end, which has been 

 bared, to make a small semi-circle. Lay the bit of copper sheet 

 on the end of the middle finger of your left hand, set the tip of 

 the wire that projects from the bottom of the rectangle in the 

 dent in this, and hold the other end of the wire of the rectangle 

 against the ball of your thumb. Hold the rectangle thus, verti- 

 cally, between the ends of the bar magnets. Now hold the second 

 wire in your right hand and bring its free curved end into contact 

 with the wire of the open square. The wire rectangle should 

 now rotate rapidly on its axis. You may have to start the rota- 

 tion with a light push of the ringer. Suppose the bit of sheet 

 copper is connected with the carbon (positive) pole of the battery. 

 The current enters the wire through it, and in our diagram passes 

 up the wire at the right side of the rectangle. If the adjacent pole 

 of the magnet is the north pole, the wires tend to attract it, or 

 since the wires are free to move and the heavy magnet does 

 not move readily, the wire will be attracted by the magnet and 

 so turns to the right. (A more exact explanation is given later, 



