the movable member to clutch the copper and prevent breaking as might be the case if the soft wire was gripped too tightly at one point. The device consists of a main body carrying a movable clamping member that is actuated by a lever, joined to the portion or hook to which the pulling 1 up rope is attached with a link. As the strain on the rope becomes greater, the clamping piece grips the copper wire more firmly between the clamp

FIG. 97e.-Typical Wire Clamps.

body and the contact piece it carries. The clamp shown at B is used for iron and is practically the same in general construction as the device previously described except that a cam member is used to clamp the iron wire against the lug on the lower portion of the clamp body on which it rests. The leverage is arranged so the more the pulling

up rope is tightened the greater the grip of the

cam on the wire. Subscriber's

Pole

Connections. ·

The con

mon method of taking a subscriber's connection from a pole line is shown at Fig. 97f. A doublegrooved insulator receives the ends of the line wire which is cut at that point, one end going into the upper, the other end in the lower groove. From these ends a branch circuit is taken, as shown in illustration, two single-grooved insulators being provided, these taking the strain off of the main insulator and forming a secure anchorage for the wires comprising the subscriber's loop.

MAIN LINE

SUBSCRIBER'S LOOP

SUBSCRIBERIS LOOP

MAIN LINE

FIG. 97f.-Subscriber's Pole Connection.

Modern Line Wire Practice.

In the early days iron wire was used exclusively for telegraph and telephone purposes. Steel was found to be far superior to iron in tensile strength, but with the increase in hardness there was also an increase in resistance, which was a serious disadvantage; the purer and softer the iron, the less its strength, but the greater its

conductivity. But even the best of iron wire had. two drawbacks: its resistance, when used between distant points, and its tendency to corrode, even though galvanized, when subjected to the smoky atmosphere of the cities. Copper wire did not have either of these objections, but besides being very costly, it was altogether too weak for use on pole lines. For very long telegraph circuits a socalled compound wire consisting of a steel wire, which gave the necessary tensile strength, with a strip of soft copper folded around it so as to form a concentric cylinder, was developed and used to some extent. But a little later the growth of the telephone industry was responsible for the development of hard-drawn copper wire, which, though inferior to steel and but slightly better than soft iron in tensile strength, is yet very satisfactory, and soon came into general use for both telephone and telegraph lines.

For service wires, both in rear wall and fence distribution and in aerial spans, so-called rubberinsulated twisted-pair wire has now been developed to a state which almost seems to approach perfection in its durability and reliability. A compound containing about 30 per cent. of pure rubber was long ago found to possess the necessary insulating and weather-resisting qualities, and at the same time was within reason in the matter of cost, but mechanically it was not sufficiently serviceable. A cotton braiding woven over the insulation gave it the needed protection,

but the cotton was subject to rapid deterioration when exposed to the weather. The question then resolved itself into one of preserving the braiding from deterioration, and very satisfactory solutions of this problem have been reached in the present methods of filling or impregnating these braids.

The latest innovation in the way of telephone line wire is the new copper-steel wire which is coming into extensive use, both bare and in rubber-insulated twisted pairs. It is made by a patented process, by which a thin sheath of copper envelops and is thoroughly united to a steel core. It has some transmission advantage over steel or iron wire of the same gauge, and it combines the tensile strength of steel with the corrosion-resisting qualities of copper. Although the cost of copper and of copper-steel wire is about the same for equal lengths of a given gauge, the superior tensile strength of the latter makes it possible in many cases to use a much smaller gauge than with the former, with a consequent saving in cost.

Properties of Aluminum Wire.-Some of the physical constants of aluminum as it is now manufactured commercially for electrical purposes are as follows: Melting point, 1,157 degrees F.; elastic limit, 14,000 pounds per square inch; ultimate strength, 26,000 pounds per square inch; modulus of elasticity, 9,000,000; electrical conductivity, 62 per cent.; specific gravity, 2.68; coefficient of linear expansion, .000,012.8. At pres

ent aluminum is used only to a limited extent for large conductors. Its use is thus restricted because of the practical impossibility of employing the ordinary methods of soldering, due probably to the fact that its surface seems to have a coating of oxide on it at all times.

At the present relative cost of the two metals, aluminum is 10 to 15 per cent. cheaper than copper of the same resistance. The weight of a unit length of aluminum wire is only 47 per cent. of a copper wire of the same length and resistance. Consequently, aluminum can cost 2.13 times as much as copper per pound and still cost the same as copper per unit length from the standpoint of electrical resistance. As a matter of fact, however, the price of aluminum at present is less than 2.13 times that of copper per pound, so that it is actually cheaper to use aluminum as an electrical conductor than copper, where other considerations do not enter. At present, however, the aluminum market is so centralized under one common control that there is no market for aluminum junk, and this feature makes it impossible for operating companies to figure that aluminum has any junk value.

Expansion and contraction with changes in temperature is greater than in copper. A solid wire of small size gives a great deal of trouble through breakage due to crystallization of the wire from swaying in the winter. The greater expansion and contraction make it necessary to