01-01-2018, 04:20 PM
Hi am loveneesh i would like to get details on project on moving coil galvanometer ..My friend Justin said project on moving coil galvanometer will be available here
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The moving-coil galvanometer
The construction of a moving-coil galvanometer, calibrated for use as a milliammeter, is shown in Fig. 39. A rectangular coil carrying a pointer is pivoted on jewelled bearings and free to move in the annular (= ring-like) space between a soft-iron cylinder and the cylindrical pole faces of a strong magnet.
Current is led into and out of the coil by two phosphor-bronze hairsprings wound in opposite directions to compensate for thermal expansion. These hairsprings also provide the control couple. When a current flows in the coil the resultant magnetic field in the annular space is as shown in Fig. 39.2. This sets up forces on the sides of the coil as already explained in the case of a simple electric motor (page 438). Consequently, in accordance with Fleming’s left-hand rule, equal and opposite parallel forces act respectively on the two vertical sides of the coil. These two forces together form a deflecting couple which causes the coil to rotate until the deflecting couple is just balanced by the control couple set up by the hairsprings.
The function of the soft-iron cylinder is to concentrate the magnetic flux radially in the annular space. Thus for all positions of the coil the magnetic flux density is constant and in the plane of the coil, and hence the force on the sides of the coil will be proportional to the current. This, coupled with the fact that the controlling hairsprings obey Hooke’s law (page 145) ensures that the deflection will be proportional to the current, and so gives the instrument a linear or evenly divided scale. The coil itself is wound on a light aluminium former. The function of this is to damp the movement of the coil and so make it dead beat. That is to say, the pointer swings out to a deflection and comes to rest immediately instead of oscillating to and fro. This is a case of eddy current damping and will be understood after reading chapter 43. Briefly, as soon as the coil starts to move, the aluminium former cuts magnetic flux. This sets up an induced current which opposes its motion.