. American telephone practice . f No. 34 wire, two inparallel. These two wires are wound side by side throughout their INDUCTION COILS. 75 length, and give the equivalent area of one No. 31 wire. The re-sistance of the primary coil is .38 ohm and that of the secondary75 ohms. The terminals of the secondary coil are shown at a b anda b in Fig. 56. After the coil is wound, the small wires of thesecondary are attached to larger wires inside of the spool-head, sothat the danger of breakage will be diminished. These leading-outwires are coiled in a tight spiral, as shown in Fig. 57, in order toavoi
. American telephone practice . f No. 34 wire, two inparallel. These two wires are wound side by side throughout their INDUCTION COILS. 75 length, and give the equivalent area of one No. 31 wire. The re-sistance of the primary coil is .38 ohm and that of the secondary75 ohms. The terminals of the secondary coil are shown at a b anda b in Fig. 56. After the coil is wound, the small wires of thesecondary are attached to larger wires inside of the spool-head, sothat the danger of breakage will be diminished. These leading-outwires are coiled in a tight spiral, as shown in Fig. 57, in order toavoid breakage and also to give a considerable length of wire inmaking connections where it is needed. More recent experience shows that a smaller diameter of corethan that used in the above-described coil gives better results, thesmaller core losing, perhaps, a little in loudness, but gaining per-ceptibly in clearness and crispness. The use of two fine wires inparallel, as found in this coil, is illustrative of a worn-out FIG. 58.—INDUCTION COIL WITH TERMINALS ON HEADS. This practice was once quite commonly resorted to in various coilwindings for telephone use, on account of some fancied theoreticalgain in efficiency. The gain, however, was not real and the practicewas undesirable, expensive and useless. Undesirable, because twosmall wires are more easily injured mechanically than one large wireof the same carrying capacity; expensive, because the labor in wind-ing is considerably greater than in the case of a single wire, as isalso the first cost of the finer wire. In modern coils a paper tube is generally used in place of the , being thinner, it allows the winding to be placed closer to thecore, an advantage in point of efficiency and economy. A much better method of terminating the wires leading from aninduction coil than that of ending them in pigtails is to solderthem directly to brass or German silver contacts secured to the coil 76 AMERICAN TELEPHONE PRA
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