696 EXPERIMENTS ON AMPERE'S LAWS. 



a I and c d of the square, one projecting from the middle of the side 

 a &, the other end being parallel to the first, at a distance of 6""" 

 from it. To the middle of the side b c a small ring and hook of 

 covered wire is attached, the construction of which will be rendered 

 sufficiently clear from fig. 352 A, which shows the square when com- 

 pleted. 



A second square is also made in the same way, having five turns 

 of wire, but having each side only ll cm long. In making it, the 

 tack e is driven into the board a little more to the left, and the tack 

 / a little more to the right, so that the free ends may be somewhat 

 longer. The distance between the two parallel ends should in this 

 square be 12 mm ; the binding of the wires with thread should there- 

 fore be commenced not in the middle, but G 1 31 from the middle, and 

 end 6 mm from it. These two ends must be bent so as to be 

 horizontal when the square is held vertical. The square itself is 

 fixed with sealing-wax to one end of a strip of wood 10 cm long, 

 l cm wide, and 5 mm thick. Two holes are made through the wood 

 with a red-hot wire, and each of the wire ends is drawn through 

 one of them as shown in fig. 352. 



The larger square is suspended from the frame represented in fig. 

 35 (page 36) by means of a thread, to the end of which the 

 S-shaped hook is tied, which passes through the ring attached to the 

 square. The other end of the thread is passed first through a small 

 hook screwed into the middle of the cross-bar of the frame, and then 

 tied several times round a small tack driven into one of the uprights. 

 The ends of the wire are immersed in mercury, which allows the cur- 

 rent to enter at one end and to pass out again at the other. 



In the middle of a small block of wood, fig. 351 JB, about 10 cm 

 square, and 2 or 2 cm> 5 thick, a hole is bored with the centrebit, 8 

 or 10 cm deep and 18 mm wide. In the middle of this hole a smaller 

 one is bored right through with a stout gimlet. A piece of glass 

 tubing, 5 or 6 mm wide, is heated at one end in the spirit or gas 

 flame until by the softening of the edges the bore has contracted so 

 far that the wire used in the preparation of the squares will just 

 pass through without friction, but not loosely. When the tube has 

 cooled, a cut is made with the three-square file 2 from the end, 

 and this length broken off. A piece of copper wire, 5 cm long and 

 }mm thick, is surrounded near one end with a small collar of sealing- 

 wax, and cemented into the wider end of the little glass tube in the 

 manner explained on page 637. The little tube is then inserted 

 from below into the hole bored in the wood, as shown in fig. 351 

 B ; if necessary the hole must be widened with the rat-tail. If the 

 tube fits tight in the hole no further cementing is required ; other- 



