. A text-book of electrical engineering;. i = I-2l . .{a).. Fig, 141 In Fig. 141 the time t is plotted along the horizontal axis OG, while thecorresponding values of the short-circuit current i are set up as equation (a) we see that, when t = o, i = I, *~ 2t=T, 1 = 0, 156 Electrical Engineering We thus get the straight diagonal line shown in the figure. At any time tfrom the commencement of short-circuit, represented by OC, we havei = BC, ii = I-i = AB, i^=^I + i = BD. The ordinates measured between the horizontal hne EF and the dia-gonal represent the current i^ in the traihng
. A text-book of electrical engineering;. i = I-2l . .{a).. Fig, 141 In Fig. 141 the time t is plotted along the horizontal axis OG, while thecorresponding values of the short-circuit current i are set up as equation (a) we see that, when t = o, i = I, *~ 2t=T, 1 = 0, 156 Electrical Engineering We thus get the straight diagonal line shown in the figure. At any time tfrom the commencement of short-circuit, represented by OC, we havei = BC, ii = I-i = AB, i^=^I + i = BD. The ordinates measured between the horizontal hne EF and the dia-gonal represent the current i^ in the traihng tip of the brush. Now, the areaof contact of the trailing tip is proportional to T — t. The current density under this brush tip is therefore proportional to „^ which we see from 2/ the figure is a constant ratio equal to -~:. We see, then, that the current density under the brush is constant at every stage of the is therefore no sparking since the current in the traiUng tip decreasesproportionately with the decreasing area, until as
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