Electrical measurementsA laboratory manual . l be increased by making the coil long andnarrow. The cross-section of the long narrow coil must,moreover, be two tangential circles, their point of tan-gency being as nearly as possible on the axis of- rotationof the coil.^ The problem in hand resolves itself intofuiding the shape and position of an area having a givenmoment of inertia about a point in its plane such thatthe moment of the area about a coplanar line through 1 Gi-ays Absolute Measurements in Electricity and Magnetism, Vol. II., PartII., p. 380. MEASUREMENT OF CURBENT. 141 the point i
Electrical measurementsA laboratory manual . l be increased by making the coil long andnarrow. The cross-section of the long narrow coil must,moreover, be two tangential circles, their point of tan-gency being as nearly as possible on the axis of- rotationof the coil.^ The problem in hand resolves itself intofuiding the shape and position of an area having a givenmoment of inertia about a point in its plane such thatthe moment of the area about a coplanar line through 1 Gi-ays Absolute Measurements in Electricity and Magnetism, Vol. II., PartII., p. 380. MEASUREMENT OF CURBENT. 141 the point is a maximum. Taking the point as a pole,these conditions are while JJ r^ dr dO is a constant, Xf^ sin 0 dr dO is a maximum. 72. The Kelvin Balances. — In the balances of LordKelvin the electrodynamic action between the fixed andmovable coils is counterbalanced by adjustable weightsor sliders instead of the torsion of a helical spring. The coils are ring-shaped and horizontal. The twomovable rings E and F (Fig. 68) are attached to the ends. Fig. 68. of a horizontal balance beam which is supported by twotrunnions a and &, each hung by an elastic ligament offine wires, through which the current passes into and outof the circuit of the movable rings. These rings areplaced midway between two pairs of fixed rings, AB and61), which are connected as shown in the diagram, sothat the movable ring on either side is attracted by oneof the fixed rings and repelled by the other. When acurrent passes through the six coils in series, the beamtends to rise at F and sink at F. 142 ELECTBICAL MEASUREMENTS. The balancing is performed by means of a weight,which slides on a horizontal graduated arm attached tothe balance beam (Fig. 69). A trough is fixed to theright-hand end of the beam, and in it is placed a weightwhich counterpoises the sliding weight, shown near thecentre of the beam, when it is at the zero of the scaleand no current is passing through the balance. By thisarrangement the range
Size: 2312px × 1081px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisherbosto, bookyear1895
