710 MAGNETIC ATTRACTION AND REPULSION. 



the covering, are required. The end of the tube where the spiral 

 is to commence should be heated until a little sealing-wax melts 

 upon it, and the coiling of the wire should be begun before the 

 wax cools, so tKat the first turns can be pressed into the sealing- 

 wax, and may thus be secured. After that the turns are wound on 

 very closely and uniformly, until a little more than half the tube is 

 covered ; about forty-five or fifty turns will be requisite for the 

 first layer. A stick of sealing-wax is then held in the flame, and 

 the last turns are just touched lightly with the soft wax to cement 

 them together. When the wax is cold the wire is wound round the 

 tube in the same direction, but the coil is now made upon the top 

 of the other, and placing again each turn close to the preceding 

 one, the last turn will be on the top of the first, and must be 

 secured either with a little sealing-wax or fastened with thread, 

 taking in either case care that the wire cannot unwind itself. At 

 the beginning and end of the coil a free end, 10 cm in length, must 

 'be left. 



The end at the uncovered side of the tube is closed by a small 

 cork of suitable size. The small bar is a piece of iron wire, 4 or 4 cm- 5 

 long and l m> 5 thick. To attract this bar vertically upwards, the 

 tube being held in a vertical position, it is sufficient to employ the 

 current of two strong cells. 



When the tube is held horizontal, and the current is interrupted 

 at the moment when the bar moves with greatest velocity, in con- 

 sequence of the attraction exerted upon it, the bar may be made 

 to shoot out of the tube altogether. The bar is drawn inside the 

 coil by the current, and if the current suddenly stops while the bar 

 is moving rapidly onwards, the motion is continued solely in con- 

 sequence of the inertia of the bar; but if the current is constant 

 the attraction overcomes the inertia, and the bar is soon brought to 

 rest within the coil. For this experiment one end of the spiral 

 is clamped permanently to one of the terminals of the battery, while 

 the other terminal is drawn by the hand along the second end of the 

 spiral, so that the circuit is only closed for a short time. 



For the experiment on the mutual attraction of several pieces of 

 iron it is best to take thicker iron wire than for the first experiment; 

 it should be almost as thick as the bore of the tube is wide, so as 

 just to go in. The pieces to be attracted are of the same thick- 

 ness, but only 15 or 20 mm long. 



In experiments in which it is not intended to show the attraction 

 between the spiral and the bar of iron, but when iron is simply to 

 be made magnetic, as in the movable electro-magnet, fig. 355, the 

 insulated copper wire may be coiled round the bar itself. For this 



