. Practical physics. ill produce a deflec-tion of 45 degrees at Washiwgton in a smallcompass needle placed at its center. Thelegal definition of the ampere is, however,based on the chemical effect of a was given in § 305. Nearly all current-measuring instruments consist essentiallyeither of a small compass needle at the center of a fixed coil, asin Fig. 2G1, or of a movable coil sus-pended between the poles of a fixedmagnet in the manner illustratedroughly in Fig. 262. The passageof the current through the coil pro-duces a deflection, in the first case,of the magnetic needle with re
. Practical physics. ill produce a deflec-tion of 45 degrees at Washiwgton in a smallcompass needle placed at its center. Thelegal definition of the ampere is, however,based on the chemical effect of a was given in § 305. Nearly all current-measuring instruments consist essentiallyeither of a small compass needle at the center of a fixed coil, asin Fig. 2G1, or of a movable coil sus-pended between the poles of a fixedmagnet in the manner illustratedroughly in Fig. 262. The passageof the current through the coil pro-duces a deflection, in the first case,of the magnetic needle with ref-erence to the fixed coil, and, in thesecond case, of the coil with refer-ence to the fixed magnet. If theinstrument has been calibrated togive the strength of the currentdirectly in amperes, it is called anammeter; otherwise, a galvanometer(Fig. 263). 311. The commercial ammeter. Fig. 264 shows the con-struction of the usual form of commercial ammeter. Thecoil c is pivoted on jewel bearings and is held at its zero. Fk 263. A lecture-tableealvanometer 258 ELECTRICITY IX MOTION position by a spiral spring j^. When a current flows throughthe instrument, if it were not for the spring p the coil wouldturn through about 120°, oruntil its N pole came oppo-site the >S pole of the magnet(see Fig. 264). This zeroposition of the coil is chosenbecause it enables the scaledivisions to be nearly conductor «, called ashunt, carries nearly all thecurrent that enters the in-strument at B, only an exceed-ingly small portion of it going-through the moving- coil shunt is usually placedinside the instrument unlessinterchangeable shunts aredesired.
Size: 1248px × 2003px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectphysics, bookyear1922
