Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 Excerpt: ...in phase with one another, or, in other words, their phase-difference is zero. During each period they attain their maxima values, positive and negative, together, and pass through zero values together, &c. The graph for power marked W represents the product of volts and amperes. It will be seen that it is always positive since the product of the two negative values in the second half of Fig. 64.--Graph for Power when the E.M.F. and Current are in Phase. the period must give a positive result. The graph, moreover, drops to zero twice during each alternation. It should be understood that it is the mean value of the power over a complete cycle that is meant when we speak of alternating current power. If instantaneous power is referred to this is specified. The instantaneous power or rate of working, that is, the actual value of the power at any instant, is obtained by multiplying the values of the E.M.F. and current at that instant. The mean power is the total work done during one cycle or half a cycle, divided by the corresponding time. In the case just considered the power can be at once determined by multiplying the readings of ammeter and voltmeter, but in all other cases this cannot be done. 51. Value of the Power when the Phase-difference is (a) 90, (b) greater than 90. Suppose next, the case where the current lags by a quarter of a period, that is to say, the angle of phasedifference is 90". As an analogy we might take a rotating crank connected to a heavy piston which is being driven up and down without friction. In this case we have F = ma, and the acceleration is ahead of the velocity by an angle of 90 (see Fig. 65). Hence the force is in advance of v by an angle of 90, or, which means the same thing, v lags behind F by an angle of 90. It i...
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