. A text-book of electrical engineering;. robably different, so that we cannot 172 Electrical Engineering draw simply the geometrical tangent. The difference between the ordinatesof the curves E and V is equal to the ohmic pressure drop for a certain valueof the current, while the ordinates of the curve V give us the correspondingterminal pressure. Experimentally the process is of course reversed, sincewe observe the terminal pressure and add to it the calculated armaturedrop to find the electromotive force. The results can be plotted in another manner, namely, by setting out the external resi
. A text-book of electrical engineering;. robably different, so that we cannot 172 Electrical Engineering draw simply the geometrical tangent. The difference between the ordinatesof the curves E and V is equal to the ohmic pressure drop for a certain valueof the current, while the ordinates of the curve V give us the correspondingterminal pressure. Experimentally the process is of course reversed, sincewe observe the terminal pressure and add to it the calculated armaturedrop to find the electromotive force. The results can be plotted in another manner, namely, by setting out the external resistance R, the quotient y, as abscissae and plotting both V and I as ordinates (Fig. 155). Since the product I (R + R^) is equal tothe constant electromotive force, the curve representing I must be a rect-angular hyperbola. Its asymptotes are the horizontal axis and a verticalline through a point representing — /?„. The curve I cuts the vertical axisin the point A. This corresponds to no external resistance, or, in other words, B. Fig- 154 Fig- 155 the dynamo is short-circuited, and OA = ^- is the short-circuit current. Ra This point lies, naturally, quite outside the working limits, and could onlybe found by plotting the curve for a very small value of the excitation. The limit of the curve V in Fig. 155 can be found from the considerationthat the terminal pressure is equal both to the product IR and to the dif-ference E — On short-circuit it must be zero, since the terminals areelectrically equivalent to a single point and can have no difference of po-tential. The whole electromotive force is then used up in driving the enor-mous short-circuit current through the armature resistance. On the otherhand, the terminal pressure attains its maximum value and becomes equalto the electromotive force E when the external resistance becomes infinitelygreat, on open-circuit. As we have already indicated, the curves would be modified in an actualcase by the demagnetis
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