The fibre strip which supports the German silver resistance wire is shown in Figure 80. It is eight inches long, three-eighths of an inch wide and one-eighth of an inch thick. If the experimenter has access to a lathe it is possible to wind the wire on the fibre strip under tension with the aid of the screw feed so that the wire will go on very tightly and with a small air space between the turns. When a lathe is not available it will be necessary to groove the top and bottom of the fibre strip with the aid of a three-cornered file. The grooves should be evenly spaced and as close together as possible. They serve to prevent the wire from slipping and the adjacent turns becoming short circuited. The wire used to wind the strip should be No. 24 B. & S. Gauge German Silver wire and should be bare. Wind the wire tightly into the grooves in the strip and fasten both ends by looping through a small hole bored in the ends of the fibre for that purpose. The fibre strip can be bent so that it will fill in the groove in the wooden base by heating it in the flame of a bunsen burner. It should not, of course, be heated too hot or the fibre will burn. Warm it thoroughly and bend into a circle of the same diameter as the groove in the base. Then force the strip into the groove with the two ends opposite the two binding post holes. The strip should fit into the groove tightly so that there is no danger of it shifting or slipping out. [Illustration: FIG. 80.—The German-silver Resistance Wire is wound around a Fibre Strip.] The details of the switch lever, knob and binding posts, are shown in Figure 81. The lever is made of sheet brass cut into the shape shown and is one and three-quarter inches long. The knob is similar to those used on typewriters for turning the paper roller. It is fitted with a threaded stem moulded into the head. These knobs may be obtained at almost any supply house carrying goods for experimenters. The lever is slipped over the stem and clamped tightly against the knob by a nut on the under side. The end of the stem or shaft fits into the hole in the centre of the wooden base. Slip a washer over the end and then thread on a brass washer. Tighten the nut up until the end of the lever makes firm contact with the resistance wire but is not so tight that it binds and cannot be easily moved by turning the knob. When the right position for the nut is found, solder it to the stem so that it cannot change its adjustment. [Illustration: FIG. 81.—The Lever, Knob, Binding Posts, etc.] The completed rheostat is shown in Figure 82. The right hand terminal of the German Silver wire is connected to the right hand binding post. The other binding post is connected to the washer around the lever shaft on the under side of the base. The wires should be laid in grooves cut in the bottom of the base for that purpose.