Fig. 965—Method of making a "transposition." This is usually done by means of transposition insulators, which are either double insulators, one being screwed to the pin above the other, or else such caps as are shown in [fig. 967]. Such insulators are intended to act as circuit breakers, the particular wire to be transposed being cut and "dead ended," or tied around, on both the upper and lower grooves of the cap. The free end of each length is then passed back and around the insulator and twisted, or sleeve jointed to the other limb of its own circuit.
The telephonic current changes the pressure of its own charging surface as frequently as it alternates, and this fact in itself is amply sufficient to account for a vast weakening of the current before it reaches its destination. The only practicable method of overcoming this annoyance in pole lines is by the arrangement known as "transposition," which is, briefly, the practice of regularly shifting the relative position of the two limbs of each circuit as regards other wires in the same pole system, as shown in [fig. 965].
For short lines and pole systems with only a few wires it is not necessary to transpose very frequently. On longer lines it has been found amply sufficient to transpose once every quarter mile; that is to say to change the relative position of the wires of the different circuits at posts situated about that distance apart. This does not mean, however, that each pair of wires is transposed so often, but that on ordinary sized systems, the transposition of some one circuit is amply sufficient to secure balanced relations and effectually counteract the effects of cross induction. It is a matter which must be carefully calculated and planned in each particular instance in order to secure the best advantages.
Fig. 966.—Telegraph and telephone line glass insulator.
Fig. 967.—Type of insulator used in making a transposition.
Insulators.—Glass and porcelain are employed almost universally for supporting overhead wires. Insulators made of these materials are superior to those made of other material such as hard rubber, or various compounds of vegetable or mineral matter, with the exception perhaps of mica insulators used on the feeders of electric railway lines.