The Moving Picture World (July 1907)

294 THE ROVING PICTURE WORLD. ample, in working out wires by this method we will take, for instance, a wire whose diameter measures a quarter of an inch; now, % hich equals .250 inch, or 250 mils, then the area in circular mils is 250 multiplied by 250, equals 62,500 circular mils. Now, again, all electrical energy is not conveyed through round wires, but often through square bars, such as in panel and switchboard work, and in a case of that kind you proceed the same way, except you multiply the width of the bar by its thickness, which gives you the area of the same in square mils; of course, you must remember to measure the bar in mils, as, for instance a bar of copper one inch wide by % inch in thickness, will be as follows: One inch equals 1,000 mils and V4 inch equals 250 mils, so, there- fore, 1,000 multiplied by 250 equals 250,000 square mils in area. Sometimes the metal aluminum is used for wires and electrical conductors, because this metal has been so much improved in the reduction from its oxide that it can now be obtained at a price that can compete with copper, but its conductivity is only about 60 per cent that of copper, so that to use a wire of aluminum instead of copper, it must be of a larger cross-sectional area if the same resistance is to be maintained; but as aluminum is so much lighter than copper a larger cross section can be used and still compete with copper, although the cost of aluminum may be considerably higher. For line- construction work it is more difficult to handle than the other metal, because joints are hard to solder and make, also on account of the lower tensile strength of aluminum over that of copper there is a greater liability of the spans breaking down, although some of the alloys are very strong and tough; but a comparison of some of the properties of the two metals will be interesting and a guide for future work in this line. Comparison of the Properties of Aluminum and Copper. Aluminum. Copper. Conductivity (for equal sizes) 5410.63 I. Weight (for equal sizes) .33 I. Weight (for equal length and resistance) 48 1. Price, aluminum at 29 cents, copper at 16 cents (bare line wire) 1.81 .1. Price (equal resistance and length bare line wire) • 868 I. Tensile strength (pounds per square inch, . hard drawn) 40,000 60,000 Of the other metals used for wires, iron is used largely for telegraph and telephone lines, and is seldom employed for electric light and power on account of its high resist- ance. Iron wire is very often used for resistance coils, but only on account of its cheapness, because it is unre- liable as a permanent resistance on account of its ready affinity for the oxygen of the atmosphere through rust- ing and therefore changing its resistance. For those who want to use it for this purpose or any other I here- with give a table of its properties. 14-53 18.06 22.04 2748 33-30 . 438s 5744 76.33 9366 i a 12040 1 > 16480 tgi The various grades of iron wire on the market termed "E. B. B.," meaning "Extra Best Best"; "B for "Best Best," and "Best." Steel wire is often because it is cheaper and of a higher resistance iron, as will be seen from the table given above, and] has the advantage of greater tensile strength. In most-resistances of the better class German is used where a high resistance is required together 1 reliability, and I therefore append a table of its propertk] The resistance of German silver wire varies as you will perceive from the above table, according! the materials and methods of manufacture used. Gern silver is an alloy of copper, zinc and nickel and has! resistance from 18 to 28 times that of copper, and" resistance changes very slightly with the changes of t perature; this feature makes it very suitable for ~ ance coils and rheostats. Table No. 4 gives the properties of German silver 1 containing 18 per cent and 30 per cent, of nickel inj composition. There are other alloys used, but the tables of properties can be obtained from their manufacturer^ Having gone into the subject of wires, we W" e pass on to joints, splices, methods of making the e and materials used. (To be continued.)