. The Street railway journal . ously unfair to the producersof small switches which can easily be built of 75 amps,and even 100 amps, per square inch, with a very low tem-perature rise. But in very large switches, even 50 amps,per square inch is too large a figure. Temperature rise, or 676 STREET RAILWAY JOURNAL. [Vol. XVIIL No. 18. a sliding scale of amperes per square inch of contact, wouldbe more fair. The size and number of the main and equalizer cablesfor each generator, as dictated by experience, is an im-portant detail, and is given in the table below:Generator kw Amps. Main circ. milI—
. The Street railway journal . ously unfair to the producersof small switches which can easily be built of 75 amps,and even 100 amps, per square inch, with a very low tem-perature rise. But in very large switches, even 50 amps,per square inch is too large a figure. Temperature rise, or 676 STREET RAILWAY JOURNAL. [Vol. XVIIL No. 18. a sliding scale of amperes per square inch of contact, wouldbe more fair. The size and number of the main and equalizer cablesfor each generator, as dictated by experience, is an im-portant detail, and is given in the table below:Generator kw Amps. Main circ. milI—300,000I—400,000I—750,000 1— 1,500,000 2— 1,500,000 3— 1,000,000 4— 1,500,0006—2,000,000 Cast copper has a conductivity from 30 per cent to 60 percent of that of rolled or drawn copper, altliough there are volts = 5751002003005008001,0002,0003,200 Amps. 1742615228701,39-21, Equalizercirc. milI—300,000I—400,000I—750,000 1— 1,500,000 2— 1,500,000 3— 1,000,000 3— 1,500,000 4— 2,000,000. FIG. I.—STANDARD G. E. RAILWAY BOARD mixtures, very high in copper, which can readily be cast,possessing a conductivity of 80 per cent of drawn copperand presenting a good appearance. Ordinary cast copperis apt to be spongy and have blow holes. Round shapesof copper can be readily drawn or rolled, but thin flat barsare not easily drawn to gage, coming out of the die withsurface blemishes, and untrue to gage. Experience dictatesthat perfect copper bars may only be obtained by rolling,but the drawn stock can be obtained slightly cheaper inprice. Striking distances of 500-volt current-carrying metalsvary with the current strength. By striking distance ismeant the distance between metals maintaining a differenceof potential at which an arc might form. Tests have shownthat with 25 amps, at 625 volts, the safe striking distanceis 2^ ins. with 50 amps., 2V1 ins. 100 4S/s 200 sK 300 400 sVs 500 5 11-16 700. 5?4 1,000 sVa it will be noticed that the necessary d
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