Buy, also, twelve small porcelain knobs that are the right size to fit inside the large tubes. These should have holes bored through them to admit screws. Construct a frame of hard-wood to accommodate the tubes, as shown in the drawing, and leave one end loose. With slim screws make the porcelain knobs fast to the top and bottom strips of the frame, as shown in Fig. 14. The porcelain rods will fit over these and will thus be held securely in the frame, one small knob entering the tube at each end, as indicated by the dotted lines in Fig. 14. The first porcelain tube to the left is wound with No. 22 German-silver wire, the next with No. 20, the third with No. 18, then Nos. 16, 14, and 12; so that in this field a broad range can be had for a current of 110 volts. The coils are connected in series, as explained for the other rheostats, and the leading wires brought down to the back of a switchboard of which Fig. 13 A is the front and Fig. 13 B the rear view. The switchboard is made of thin slate or soapstone; or a fibre-board may be employed. Fibre-board is especially made for electrical work, and can be had from a large supply-house in pieces of various thickness, three-eighths of an inch being about right for this board. Brass bolts and nuts and copper washers are used for the contact-poles, and when the ends of the leading wires are looped around the bolts the nuts are to be screwed down tightly so as to make good contacts. This rheostat may be used when lying on a table, or it can be hung up by means of two screw-eyes driven in the top of the frame, as shown in Fig. 13 A. A convenient form of rheostat for fine wire and high resistance is shown in Fig. 15. This is on the plan of the well-known Wheatstone rheostat and does not require a switchboard nor a series of coils. Two rollers, one of wood the other of metal or brass-covered wood, are set in a frame, and by means of a handle and projecting ends with square shoulders, one or the other of the rollers may be turned so that the wire on one winds up while on the other it unwinds.