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Postby TGTS0907129» Concept of galvanometer

Describe the concept of galvanometer


The basic electric instrument is the galvanometer, a device with which very small electricity currents can be detected and measured. It is the d’ Arsonval or permanent-magnet moving-coil type of galvanometer. The coil C is suspended between the poles N and S of U-shaped magnet by means of a light metallic ribbon. Connections are made to the coil at the ribbon. Connections are made to the coil at the terminals marked t. The cylinder of soft iron B and the pole faces N and S are skillfully shaped so as to produce a radial magnetic field in the air gap. This has the virtue of giving a field that is constant in magnitude and always parallel to the plane of the coil as the coil rotates. These conditions are necessary if the instrument is to have a scale with uniform graduations. The mirror M is used to indicate the position of the coil, either by reflecting a beam of light onto a scale or by producing an image of a scale to be viewed through a low-power telescope.

When a current is set up in a coil which is between the poles of a magnet, the coil is acted upon by a torque, which tends to until its plane is perpendicular to the line joining the poles. If a current is set up in the coil, it will turn toward a position at right angles to the position shown. In turning, however, it must twist the metallic ribbon that supports it; hence it turns to the positions in which the torque exerted on it by magnet is just neutralized by the reaction of the twisted ribbon.      

The torque exerted on the coil by the magnet is proportional to the current in the coil and the torque of reaction of the ribbon is proportional to the angle through which it is twisted. Since these torques are equal in magnitude and opposite in the sense when the coil reaches the equilibrium position, the angle through which the coil turns is prepositional to the current in it; that is, θ ∝ I, where θ is the angular deflection of the coil. This condition is realized only in well designed instruments, in which case the galvanometer instrument, in which case the galvanometer scale is property made with equally spaced divisions. In properly, reading are made on a linear scale, since the deflection s read on the scale is proportional to the angle of deflection θ, (where θ is small).

By TGTT23071246 on 1/5/2016 2:56:10 AM
TGTS0907129 on 1/4/2016 11:28:40 PM

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