. The principles and practice of roentgenological technique . Fig. 19.—Aloving coil voltmeter. to move freely in the narrow annular gap be-tween the pole pieces and the iron core. Itsmotion is controlled by two springs (s) simi-lar to watch springs, one above and below. Alarge, non-inductive resistance is connectedin series with the coil, if the instrument isto be used as a voltmeter. (Fig. 21.) But if it is to be an ammeter, the coil isshunted with a conductor of very low re-sistance to carry the largest current wi^outundue heating. (Fig. 20.) 3. Electro-static instruments. This type ofinstru
. The principles and practice of roentgenological technique . Fig. 19.—Aloving coil voltmeter. to move freely in the narrow annular gap be-tween the pole pieces and the iron core. Itsmotion is controlled by two springs (s) simi-lar to watch springs, one above and below. Alarge, non-inductive resistance is connectedin series with the coil, if the instrument isto be used as a voltmeter. (Fig. 21.) But if it is to be an ammeter, the coil isshunted with a conductor of very low re-sistance to carry the largest current wi^outundue heating. (Fig. 20.) 3. Electro-static instruments. This type ofinstrument is used only as a voltmeter forwhich purpose it is ideally suited and its actiondepends upon the repulsion between twocharged Fig. 20.—Ammeter. D is a conductor of low resist-ance though the current flows in parallel to thecircuit R disposed about the core. M—Arma-ture of magnet. 4. Hot-wire instruments. The principle ofthese instruments is expansion of a long, thinwire of high resistance (usually a platinumsilver alloy) when connected across a movement of a pointer over a scale bythe increase in length of the heated wire indi-cates with accuracy the voltage (direct or
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectradiogr, bookyear1920
