Table of Resistance and lengths per lbs. & 100 yards of cotton covered copper wires.
| Birmingham Wire Gauge. | Diameter in 1000th of an inch. | Yards per lb. | Ohms. per lb. | Ohms. per 100 yards. |
|---|---|---|---|---|
| No. 12 | 100 | 9 | 0·0342 | 0·0038 |
| 14 | 80 | 15 | 0·0850 | 0·0094 |
| 16 | 62 | 24 | 0·2239 | 0·0249 |
| 18 | 48 | 41 | 0·6900 | 0·0766 |
| 20 | 41 | 59 | 1·2100 | 0·1333 |
| 22 | 32 | 109 | 3·1000 | 0·3444 |
§ 67. Whatever gauge wire be selected, it must be carefully insulated, to avoid all chance contact with nails, staples, metal pipes or other wires. The best insulation for wires employed indoors is gutta-percha, surrounded with a coating of cotton wound over it, except only in cases when the atmosphere is excessively dry. In these,
as the gutta-percha is apt to crack, india-rubber as the inner coating is preferable. If No. 18 wire be used, the thickness of the entire insulating coating should be thick enough to bring it up to No. 10 gauge, say a little over 1/10th inch in diameter. There is one point that will be found very important in practice, and that is to have the cotton covering on the wires leading to the bells of a different colour from that on the return wires; in other words, the wires starting from the zinc poles of the battery to the bells, indicators, relays, etc., should be of a different colour from that leading from the carbon poles to the bells, etc. Attention to this apparently trifling matter, will save an infinite amount of trouble in connecting up, repairing, or adding on fresh branch circuits. For outdoor work, wire of the same gauge (No. 18) may generally be used, but it must be covered to the thickness of 1/10" with pure gutta-percha, and over this must be wound tape served with Stockholm tar. Wires of this description, either with or without the tarred tape covering, may be obtained from all the leading electricians' sundriesmen. Many firms use copper wire tinned previous to being insulated. This tinning serves two good purposes, 1st, the copper wire does not verdigris so easily; 2ndly, it is more easily soldered. On the other hand, a tinned wire is always a little harder, and presents a little higher resistance. Whenever wires are to be joined together, the ends to be joined must be carefully divested of their covering for a length of about three inches, the copper carefully cleaned by scraping and sand-papering, twisted tightly
and evenly together, as shown in [Fig. 73 A], and soldered with ordinary soft solder (without spirits), and a little resin or composite candle as a flux. A heavy plumber's soldering iron, or even a tinman's bit, is not well adapted for this purpose, and the blowpipe is even worse, as the great heat melts and spoils the gutta-percha covering. The best form of bit, is one made out of a stout piece of round copper wire ¼" thick with a nick filed in its upper surface for the wire to lie in (see [Fig. 73 B]). This may be fastened into a wooden handle, and when required heated over the flame of a spirit lamp. When the soldering has been neatly effected, the waste ends a and b of the wire should be cut off flush. The wire must then be carefully covered with warm Prout's elastic or softened gutta-percha, heated and kneaded round the wire with the fingers (moistened so as not to stick) until the joint is of the same size as the rest of the covered wire. As a further precaution, the joints should be wrapped with a layer of tarred tape. Let me strongly dissuade
the fitter from ever being contented with a simply twisted joint. Although this may and does act while the surfaces are still clean, yet the copper soon oxidises, and a poor non-conducting joint is the final result.
"That'll do" will not do for electric bell-fitting.
Fig. 73.