CONSTRUCTION OF BINDING SCREWS. 659 



other side of it, one large enough to receive a square nut of brass 

 3 mm thick, by means of which the binding screw may be drawn 

 tight to the wood. 



To make a binding screw like F, two holes must be drilled through 

 the brass cylinder, one along the axis, the other at right angles to 

 it. The former of these is tapped into a nut, and the screw s is put 

 in from below and fixed with a little soft solder. The screw s should 

 just reach to the bottom of the hole made across the cylinder, so that 

 the wire to be clamped in the hole may rest upon the top of s ; if 

 the screw s did not reach quite up to the level of the horizontal aper- 

 ture, any wire clamped by the screw p would be pressed down into 

 the space left vacant above the end of s, and might be easily broken. 

 A similar precaution is necessary as regards the binding screws 

 C and D in the figure ; the holes for the pressure screws p must 

 only just reach those for inserting the wire which is to_be clamped, 

 and consequently the hollow screw should be made with the * middle 

 tap,' that is, a tap with only just a few threads at the end (see 

 page 94). Such a tap is, however, very easily broken off, and a way 

 out of the difficulty is to tap the nuts right through, and then to 

 insert from the other side small bits of brass wire with a thread cut 

 on them, marked with e in the figure, in which the part of the nut 

 to be made at first, but then to be filled up in this way, is indicated by 

 dots. The best way to obtain these little bits of screw is to cut a screw 

 of the necessary width upon a longer piece of brass, screw it into 

 the hole as far as required, cut off the part left outside with the saw, 

 and smooth the outside afterwards with the file. If the piece fits 

 well into the hole it will remain firm by itself, otherwise a little 

 solder should be used. Only a very small quantity of solder must 

 in any of these cases be used, so that none of it may run into the 

 hole in which the pressure screw p is to move. 



A form of binding screw which can only be made upon a lathe 

 is represented at G in the figure. It is one of the kinds usually sold 

 by the dealers, and differs from the others only by its more elegant 

 shape, and in having instead of a ring a flat milled head for turning 

 the pressure screw. 



Conducting wires for galvanic apparatus require no insulation ; 

 they may rest on the table and be touched with the hand provided 

 the hand is dry. A special insulation is however required where 

 the wires themselves are in contact, side by side, and the passage 

 of electricity from one wire to the next is to be avoided. To insulate 

 such wires they are covered with cotton or silk. Cotton insulates 

 sufficiently well, but silk has the advantage of being more durable, 

 and it is also thinner than a covering of cotton. The latter ad van- 



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