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Jointing of Overhead Wires

(By “G. Ripfast”)

In the early days of telephony, galvanized iron wire was about the only wire available. The price of copper was high and if that metal was erected, the wire quickly sagged, this sag being especially noticeable after a very hot summer. Continual regulation was soon found to be not only expensive but useless with soft metal, and electrical engineers of that day looked around for something more serviceable. Bronze was at length tried, and has now stood the test of more than 40 years' use. Hard drawn copper was devised for trunk lines and is at present fairly cheap, which, with skilled craftsmanship in its erection, should last a century. But bad workmanship on either bronze or copper lines will considerably shorten their lives, adding enormously to the “repair” account. It behoves every man in an erecting gang, therefore, to eschew possible faults in completing the job.

In erecting hard drawn wires (bronze or copper) the most important work is in “Jointing.” When dealing with galvanized iron wire, a joint called the “Britannia” was evolved. it is clumsy; still, and this is the most important part, the heat applied to solder the splice is not sufficient to burn the “temper” out of the wire, and if by any chance it were, the ultimate cooling would partially, if not wholly, restore that quality. But it is a different matter when jointing hard drawn bronze or copper. The heat of a soldering bit is sufficient to rob either of its hard drawn quality wherein lies its great tensile strength; and when it is remembered that both bronze and copper are softened by first being heated and then plunged into cold water, it will be seen how important it is to preserve the “temper” of the wire; for no amount of subsequent cooling can possibly restore this indispensable property. In the jointing of these materials therefore, too much care cannot be used, for their power to resist storms and snowfalls is materially increased thereby. In early work many differently made joints were tried and finally abandoned in older countries, and this article is prompted by finding that a joint long ago found deficient is still being used in New Zealand.

In the absence of a testing machine it is yet quite easy to ascertain which is the superior joint; and this applies equally to bronze or copper. Take a length of wire on which a sensible joint has been made (as shown in figure 1), but not soldered. Attach one end firmly to a beam overhead (one having “round corners” is best), fix an empty oil drum securely to the other end so as to be two or three inches off the floor, and proceed cautiously to fill up the drum with sand until the wire breaks. The break will not occur at the joint, and the joint unsoldered as it is, will not come undone. Weigh the oil drum and its contents and note down.

Take a similar length of the same gauge wire from the same coil and make the ordinary (“asylum”) joint on it without solder. Follow the same procedure as in the first test. Before half the quantity of sand previously used is returned to the oil drum the joint will have come undone. Repeat the test with soldered joints (the soldering of the joint in figure 1 not to exceed half an inch in length; the soldering of the other to be anything that satisfies a foreman ganger).

The shortness of the soldered part (figure 1) is the wire's salvation. The heat has not time to flow far along the joint, and the radiation from its two diameters is rapid. The joint in figure 2, consisting mostly of three diameters, takes a greater heat to effect the soldering and quickly reaches the ends of the splice, softening the central wire instantly. This joint, in daily practice, is usually soldered right up to the extremities, which makes the case worse. The test shows that the strength of joint in figure 1 does not depend on the soldering, but such union is rendered imperative in order to maintain good metallic conductivity, otherwise the oxidisation caused by the atmosphere would very soon render the line useless.

Any member of an erecting staff seeing the above tests carried out will not again make fool joints of figure 2 pattern, or do anything that might soften the wire on which he is working.—Verb Sap.