50 ohm coil—305 feet
These lengths of wire are then wrapped on the spools in the following manner.
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.