The switch is an important part of every rheostat, and should be carefully and accurately made. One of the simplest and most practical switches is constructed from a short, flat bar of brass or copper having a knob attached at one end and a hole provided at the other through which a screw may pass (see Switches, chapter iii.). The contact-points are made from one or two copper washers, with the holes countersunk so that a machine screw of brass, with a flat head, will fit the hole snugly. The top of the head will then be flush with the top of the washer, as shown at Fig. 7 A. The bolt is passed down through a piece of board, then slate or soapstone, and caught with a washer and nut, as shown at Fig. 7 B. A loop of wire is passed about the bolt end, then another nut is screwed tightly over it to hold it in place, as well as to lock the first nut. The binding-posts that hold the inlet and outlet wires may be made of bolts and nuts also, as shown at Fig. 7 B; but the bolt must be passed through the switchboard so that the head is at the rear and the ends project out to receive the nuts. A very compact and simple rheostat and switch is shown in Fig. 8. It is composed of a base-board, eight blocks of hard-wood, and a top strip used as a binder to lock the upper ends of the blocks together. The hard-wood blocks are three-quarters of an inch thick, one inch and a half wide, and four inches long. A small hole is made near each end of the block and through one of them an end of the wire is passed. The wire is then wound round the block, taking care to lay it on evenly, and with about one-eighth of an inch of space between each strand. When the opposite hole is reached, pass the end of the wire through it and clip it. The block will then resemble Fig. 7 C. There should be three or four inches of wire at each end for convenience in connection, and when the eight blocks are wound they are to be mounted on end at the rear side of a base-board measuring ten inches long, three inches wide at the ends, and nine at the middle (or across the face of the switchboard to the rear edge behind the blocks). Use slim steel-wire nails and glue to attach the blocks to the base; or slender screws may be employed. Across the top lay a piece of wood a quarter of an inch in thickness, and drive small nails or screws down through it and into the blocks. [Illustration: _=Fig. 8=_ _=Fig. 10=_ _=Fig. 9=_ COMPACT FORMS OF RHEOSTATS] Connect the ends of the coils together in series, as already described, and carry the wires under the base-plate in grooves cut with a [V]-shaped chisel. If the sunken wires are bothersome, the work may be avoided by running the wires direct to the foot of the contact-points and elevating the rheostat on four small blocks that may be screwed, or nailed and glued, under the corners, as shown in Fig. 8. These will raise the base half an inch or more above the table on which the rheostat will rest so as to allow room for the under wires.