A rheostat of round blocks standing on end is shown at Fig. 9 A. These are pieces of curtain-pole, four inches long and wound with loops of No. 16 or 18 wire, as shown at Fig. 9 B. The loop and loose ends are caught with staples, and when arranged on a base-board they are to be connected in series as before described. One long, slim screw passed up through the base-board and into the lower end of the block will hold each block securely in place. To keep it from twisting, a little glue may be placed under the blocks so that when the screw draws the block down to the base it will stay there permanently upon the hardening of the glue. The leading wires should be slipped under the washers forming the contact-points of the switch; or they may be carried under the board to the nuts that hold the lower ends of the bolts. Another form of rheostat (Fig. 10 A) is made by sawing a one-inch curtain-pole into four-inch lengths and cross-cutting each piece with eight or ten notches, as shown at Fig. 10 B. These pieces are screwed and glued fast along each side of a base-board eight inches wide and fourteen inches long, so that the notches face the outer edges of the board. The strand of wire passes round these upright blocks and fits into the notches so as to prevent them from falling down. The top end of wire at each pair of blocks is made fast by a turn or two of another piece of wire and a twist to hold it securely; then the loose end is carried down through a hole and along under the board to the foot of a contact-point. Any number of these upright coils may be made, and on a long board the switch may be arranged at one side instead of at the end, as shown in Fig. 10 A. When making ten or more coils it is best to use three or four sizes of wire, beginning with fine and ending with coarse. For instance, in a twelve-coil rheostat make three coils of No. 26, three of No. 22, and three of No. 18; or if coarser wire is required use Nos. 20, 16, and 12. German-silver comes bare and insulated. It is preferable to have the fine wire insulated, but the heavier sizes may be bare, as it is cheaper; moreover, if heated too much the insulation will burn or char off. When cutting out the coils always begin at the end where the finer wire is wound; then as the current is admitted more freely the heavier wires will conduct it without becoming overheated.