Figs. 2,493 and 2,494.—Steam engine indicator cards, illustrating in mechanical analogy, the misuse of the term effective as applied to the pressure of an alternating current. The card fig. 2,493, represents the performance of a steam engine taking steam at 60 lbs. (gauge) pressure and exhausting into the atmosphere. The exhaust line being above the atmospheric line shows that the friction encountered by the steam in flowing through the exhaust pipe produces a back pressure of two lbs. Hence at the instant represented by the ordinate y, the effective pressure is 60 - 2 = 58 lbs., or using absolute pressures, 74.7 - 16.7 = 58 lbs., the virtual pressure being 60 lbs. gauge, or 74.7 lbs. absolute. Now, the back pressure may be considerably reduced by exhausting into a condenser as represented by the card, fig. 2,494. Here, most of the pressure of the atmosphere is removed from the exhaust, and at the instant y, the back pressure is only 6 lbs., and the effective pressure 74.7 - 6 = 68.7 lbs. Thus, in the two cases for the same virtual pressure of 60 lbs. gauge or 74.7 lbs. absolute, the effective pressures are 58 lbs. and 68.7 lbs. respectively.

In the measurement of alternating current, it is not the average, or maximum value of the current wave that defines the current commercially, but the square root of the mean square value, because this gives the equivalent heating effect referred to direct current. There are several types of instrument for measuring alternating current, and they may be classified as

Electromagnetic or Moving Iron Instruments.—This type of instrument depends for its action upon the pull of flux in endeavoring to reduce the reluctance of its path. This pull is proportional to the product of the flux and the current, and so long as no part of the magnetic circuit becomes saturated, the flux is proportional to the current, hence the pull is proportional to the square of the current to be measured.

Fig. 2,495.—A calibrated scale. This means that printed scales are not employed, but each instrument has its scale divisions plotted by actual comparison with standards, after which the division lines are inked in by a draughtsman. There are makes of direct current instruments employing printed scales in which the scale deflections are fairly accurate, even though the scales are printed, but printed scales should not be used on alternating current instruments.

Ques. What are some objections to moving iron instruments?

Ans. Instruments of this type are not independent of the frequency, wave form, or temperature and external magnetic fields may affect the readings temporarily.