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Electricity and Magnetism
The Moving Coil Galvanometer
A coil, with current flowing through it, placed in a magnetic field, can experience a torque. The magnitude of the torque is proportional to the current. Torque I
However, the torque also depends on the angle, , between the field and the coil. Torque sin
In order to have a meter with a linear scale, the angle between the coil and the field must remain constant.
The angle can remain constant (90°) if we have a radial magnetic field. The soft iron cylinder gives us this field shape. A meter without a soft iron cylinder would work but would have a scale which was difficult to read.