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The Moving Coil Galvanometer
 The term galvanometer is used to refer to a meter which can measure small currents, usually in the mA (10-3A) or mA (10-6A) range. Most ammeters, voltmeters etc today (this written 2016) are digital instruments but some analogue meters are still used notably in certain audio equipment, for VU meters. Also, some people (like, me for example) still harbour a nostalgic preference for a nice old fashioned meter like the one shown here... The photo shows an analogue multi-meter which contains a galvanometer. Although the galvanometer only measures small currents, it can be used to measure many other quantities, voltage, resistance etc by connecting different resistors in series and/or parallel with the meter itself. A coil of wire with current flowing through it, placed in a magnetic field, can experience a torque (or turning effect). The magnitude of the torque produced is directly proportional to the current flowing in  the coil. C This suggests that by measuring the torque, we can measure the current flowing. However, for the meter to have an easy-to-read, linear scale, we must be careful to use the right shape of magnetic field. This diagram shows the approximate shape of magnetic field you would have if a simple bar magnet has semi-circular shaped pole pieces added to it. If a coil of wire were to be placed in this magnetic field, in the appropriate way, it would experience a torque when current flowed through it. The orange coloured rectangle represents the end view of a rectangular coil of thin copper wire through which a small current flows. The two sides of the coil (which are perpendicular to the plane of the diagram) will have forces acting in opposite sense and so a torque is produced. As the coil rotates, the directions of the two forces remain approximately the same but the perpendicular distance between them will change. This means that the magnitude of the torque will depend, not only on the current but also on the angular position of the coil. A meter using this shape of magnetic field would have a non-linear scale because as the coil rotated, the change in torque per unit current would change (decrease). This problem can be overcome by placing a soft iron cylinder in the field (as shown in the three diagrams below), in order to a radial magnetic field pattern. The perpendicular distance between the two forces is now independent of the angular position of the coil. This diagram show a more detailed view of the coil. The control springs provide a torque which opposes the torque due to the current. (These springs also conduct the current to the coil.) So, a small current will twist the control springs through a small angle... double the current, twice the angle of rotation of the coil against the springs etc. A meter without control springs would not be a meter! This diagram show a complete typical meter. The mirror is to enable the user to place his/her position so as to look at the scale at 90° to the plane of the scale. If you cannot see the reflection of the pointer because it is hidden by the pointer itself, you're looking from the correct point of view and will see the true reading. Finally, an enlarged view of the photo of the multi-meter in which some of the parts described above can be seen... if your eyesight is good enough!
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