Verwandte Artikel zu The Measurement of Electric Currents: Electrical Measuring...
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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. 1893. Excerpt: ... Fig. 25 shows the Author's station voltmeter. Instead of quadrants, it has half. circles, two pair being used for symmetry. A gravity control is employed, and the Fig. 25. moving system is so shaped that the index traverses the whole scale between, say, 1,800 and 2,200 volts. This instrument is useless for low pressures. In describing these instruments, continuous currents have been alone considered; and it will be best to discuss alternate current instruments by themselves. Alternate-current Voltmeters And Amperemeters. Many instruments already described will do equally well for continuous and alternate currents. This is the case more especially with amperemeters. Of course all instruments with permanent magnets are out of the question for alternate-current work. A soft iron amperemeter which is so arranged that the iron does not produce hysteresis errors with continuous, will generally work with alternate currents, if the iron is laminated so as to avoid Foucault currents. In order that such an instrument shall work correctly, it is necessary that the force at every part of the period should be proportional to the square of the current. If the induction is strictly proportional to the current the reading is thus correct. If the induction is appreciably affected by hysteresis there is an error. If there are any Foucault currents in the iron core, or in the brass work of the instrument, the iron is not fully magnetized, and the reading is too low. In order that a voltmeter should be available for alternate currents it must be made with no appreciable time-constant, else it reads too low. It can, of course, be calibrated for a particular frequency; but that is not of much use, as stations do not keep accurately enough to stated speeds. Various methods have...
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