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The Adjustment of a Reflecting Galvanometer

In adjusting a reflecting galvanometer, we have first to place it so that the magnet and mirror may swing quite freely. This can be attained by the adjustment of the levelling screws on which the instrument rests. There is generally a small aperture left in the centre of the coils opposite to that through which the light is admitted to the mirror. This is closed by a short cylinder of brass or copper which can be withdrawn, and by looking in from behind, it is easy to see if the mirror hangs in the centre of the coils as it should do.

The lamp and scale are now placed in front of the mirror, the plane of the scale being approximately parallel to the coils, and the slit through which the light comes rather below the level of the mirror.

The magnet and mirror are adjusted, by the aid of the control magnet, until the light is reflected towards the scale. The position of the reflected beam can easily be found by holding a sheet of paper close to the mirror so as to receive it, moving the paper about without intercepting the incident beam. By moving the control magnet, and raising or lowering the scale as may be required, the spot may be made to fall on the scale.

The distance between the galvanometer and scale must now be varied until the image formed on the scale is as clear and distinct as possible; and, finally, the control magnet must be adjusted to bring the spot to the central part of the scale, and to give the required degree of sensitiveness.

As we have seen, the sensitiveness will largely depend on the position of the control magnet Its magnetic moment should be such that when it is at the top of the bar which supports it, as far, that is, as is possible from the needle, the field which it alone would produce at the needle should be rather weaker than that due to the earth. If this be the case, and the magnet be so directed that its field is opposite to that of the earth, the sensitiveness is increased at first by bringing the control magnet down nearer to the coils, becoming infinite for the position in which the effect of the control magnet just balances that of the earth, and then as the control magnet is still further lowered the sensitiveness is gradually decreased.

The deflexion observed when a reflecting galvanometer is being used is in most cases small, so that the value of φ measured in circular measure will be a small fraction; and if this fraction be so small that we may neglect φ3, we may put sinφ = φ = tan φ (see p. 45) and we get i = kφ.

With a sensitive galvanometer in which the coils are close to the magnet the ratio of the length of the magnet to the diameter of the coil is considerable, and the galvanometer constant is a function of the deflexion; so that k is not constant for all deflexions in such an instrument, but depends on the angle φ. If, however, the deflexions employed be small we may without serious error use the formula i = kφ, and regard k as a constant.



Last Update: 2011-03-27