The instrument is now practically complete. [Illustration: Fig. 117.—Knife-Contact.] In order to use the Wheatstone bridge, it is necessary to have a set of resistances of known value. The resistance of any unknown circuit or piece of apparatus is found by comparing it with one of the known coils. It is just like going to a store and buying a pound of sugar. The grocer weighs out the sugar by balancing it on the scales with an iron weight of known value, and taking it for granted that the weight is correct, we would say that we have one, five, or ten pounds of sugar, as the case may be. The Wheatstone bridge might be called a pair of "electrical scales" for weighing resistance by comparing an unknown coil with one which we know has a certain value. The next step is to make up some standard resistance coils. Secure some No. 32 B. & S. gauge single-cotton-covered wire from an electrical dealer and cut into the following lengths, laying it straight on the floor but using care not to pull or stretch it. 1/2 ohm coil—3 feet 1/2 inch 1 ohm coil—6 feet 1 1/4 inches 2 ohm coil—12 feet 2 1/2 inches 5 ohm coil—30 feet 6 1/4 inches 10 ohm coil—61 feet 20 ohm coil—122 feet 30 ohm coil—183 feet 50 ohm coil—305 feet These lengths of wire are then wrapped on the spools in the following manner. [Illustration: Fig. 118.—Resistance-Coil. _A_ shows how the Wire is doubled and wound on the Spool. _B_ is the completed Coil.] This method of winding is known as the non-inductive method, because the windings do not generate a magnetic field, which might affect the galvanometer needle used in connection with the Wheatstone bridge as described later on. Each length of wire should be doubled exactly in the middle, then wrapped on the spools like a single wire, the two ends being left free for soldering to the terminals as shown in Figure 118, B. The spools may be the ordinary reels upon which cotton and sewing-silk are wrapped. The terminals of the spools are pieces of stout copper wire, No. 12 or No. 14 B. & S. gauge. Two pieces of wire about three inches long are driven into holes bored in the ends of each spool. A small drop of solder is used permanently to secure the ends of the coil to each of the heavy wire terminals. The spools are then dipped into a pan of molten paraffin and boiled until the air bubbles cease to rise. The spools should be marked 1, 2, 10, 20, 30, and 50, according to the amount of wire each one contains as indicated in the table above. How to Use a Wheatstone Bridge for Measuring Resistance The instrument is connected as in Figure 116. The unknown resistance or device to be measured is connected across the gap at _B_. One of the standard known coils is connected across the gap at _A_. A sensitive galvanometer or a telephone receiver and two cells of battery are also connected as shown.