An elementary book on electricity and magnetism and their applications . Fig. 96. — Thom-son astatic gal-vanometer. 158 ELECTRICITY AND MAGNETISM magnet; such an instrument is called the dArsonval contains the essential principles of most of our moderncommercial ammeters and voltmeters and is constructedon just the reverse principle of the Thomson type. The in-strument consists essentially of a coil of fine wire pivoted orsuspended in a powerful magnetic field. In the most sensitiveinstruments the coil is suspended by a very fine wire or ribbon,usually of phosphor bronze, which also
An elementary book on electricity and magnetism and their applications . Fig. 96. — Thom-son astatic gal-vanometer. 158 ELECTRICITY AND MAGNETISM magnet; such an instrument is called the dArsonval contains the essential principles of most of our moderncommercial ammeters and voltmeters and is constructedon just the reverse principle of the Thomson type. The in-strument consists essentially of a coil of fine wire pivoted orsuspended in a powerful magnetic field. In the most sensitiveinstruments the coil is suspended by a very fine wire or ribbon,usually of phosphor bronze, which also serves to lead thecurrent into the coil, the current leaving the coil by a spiralwire or ribbon below. Figure 97 shows one form of galva-. Telescope Fig. 97. — DArsonval galvanometer. N and S, poles of fixed magnet. nometer which is convenient for use in the laboratory. A softiron core is supported rigidly between the poles in order toconcentrate the magnetic lines of force. The rotation of themoving coil of such a sensitive galvanometer is usually measuredby means of a telescope and scale. The observer views theimage of the stationary scale reflected in a small mirror attachedto the coil of the instrument. Since the force tending to turnthe coil is proportional to the current, to the number of wind-ings, and to the intensity of the magnetic field, it is possibleto make a very sensitive instrument by making the magnelvery powerful. The elasticity of the suspending wires controls ELECTRICAL MEASURING INSTRUMENTS 159 the position of the coil and tends to bring it back to its zeroposition. This type of galvanometer is quite independent ofthe earths magnetic field and of changes in the magneticfield around it. Perhaps its
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectmagnetism, bookyear19
