The wooden roller may be made from a piece of curtain-rod one inch in diameter, and it should have a thread cut on it. This will have to be done on a screw-cutting lathe, and any machinist will do it for a few cents. There should be from twelve to sixteen threads to the inch--no more--although there may be as few as eight. Twelve will be found a good number, as that does not crowd the coils and the risk of their touching is minimized. The ends of the roller should have bearings that will fit in holes made in the end-pieces of the frame, and at one end of each roller a square shoulder is to be cut, as shown at A in Fig. 16. A short handle may be made from two small pieces of wood, as shown at B in Fig. 16. It must be provided with a square hole so that it will fit on the roller ends. The metal roller may be made from a piece of light brass tubing one inch in diameter through which a wooden core is slipped; or it can be a piece of brass-covered curtain-pole with the ends shaped the same as the wooden one. The wood roller should have a collar of thin brass or copper (or other soft metal except lead) attached to the front end; or several turns of wire may be made about the roller so as to form a contact-point. A piece of spring brass, copper, or tin rests on this collar and is held fast under a binding-post, which in turn is screwed to the wooden frame. A similar strip of spring metal is held under another post on the opposite side of the frame and bears on the metal roller. [Illustration: _=Fig. 15=_] [Illustration: _=Fig. 16=_]