GALVANIZED SUPPORTING STRANDS. What weight per foot will a half-inch ordinary strand support if the strain is one-half the breaking weight, the span 120 feet, and the deflection .01 of the span or 1.2 feet? One-half the breaking weight of a half-inch ordinary galvanized strand is 4 160 pounds. The value of S for above span and deflection, table page 50, is 1 500.2. Dividing 4 160 by 1 500.2 we find the total weight per foot to be 2.773 pounds. Deducting from this the weight per foot of the half-inch galvanized strand we have 2.263 pounds as the weight per foot of cable that this strand will support. While it is true that a factor of safety of two in this work is too small, yet the cables help in a great measure to carry their own weight. It is believed that galvanized strands will easily carry the loads indicated on page 39. This strand is composed of seven wires, twisted together into a single strand. CURRENTS. FUSING EFFECTS OF CURRENTS. Table giving the diameters of wires of various materials which will be fused by a current of given strength. W. H. PREECE, F.R.S. FUSING EFFECTS OF CURRENTS.-(Continued.) Table showing the amperes required to fuse wires of various sizes and materials. NOTE.—The size of “cut-outs,” or fuses for electric-lighting circuits, can be taken at once from the first table. Pure copper wire makes the best and most reliable cut-out or fuse, and should never be less than one inch in length between the terminals to which it is fixed so as to prevent the cooling effect of the terminals. HEATING EFFECTS OF CURRENTS. A REPORT read before the Edison Convention, at Niagara Falls, August, 1889, by A. E. Kennelly, gives complete formulæ and tables based on experimental data, showing the heating effects of electric currents. This report was published in the Electrical World, beginning with the edition of November 23, 1889. The tables in this book are taken from curves constructed from data given in the above report. The table page 43 gives the rules of the various insurance companies, together with one column giving the current whose double would cause a rise of 40° C. This is the safe carrying capacity recommended in Kennelly's report. The table page 44 gives the diameters of various wires and the current they will carry with a specified rise in temperature. The wires are insulated, and the conditions are similar to those met with in house wiring in mouldings or conduits. The table page 45 is computed for bare wires suspended indoors, and gives the current carried with the corresponding rise in temperature. The table page 46 is computed for outdoor wires, not insulated. In these tables all wires are solid. Insulation increases the current a wire will carry with a given rise in temperature, because the radiating surface is increased, and for the same reason a strand will carry a larger current than a solid wire. One square inch of bright copper radiates .003 9 watts per degree Centigrade rise in temperature, and one square inch of blackened copper, .009 watts, under the same conditions. Convection seems to be dependent only on length, and may be taken at .053 watts per foot per degree Centigrade rise. |