In both the moving-coil and moving-iron meter movements, the current being measured flows through the coil. Except for this similarity, the coils in each type of movement are different. When current flows through the coil of the moving-coil meter movement, a magnetic field is produced that causes the coil to rotate. For the coil to rotate easily, it must be as light as possible. To make the coil light, it is wound on an aluminum frame. Furthermore, the coil is made from very fine wire, and when compared with the coil in the other meter movements, it contains very few turns to keep it as light as possible.
The coil in this type of movement remains stationary, and the magnetic field about the coil moves an iron vane. Because this iron vane is relatively heavy, a strong magnetic field is required to move it. Therefore, the coil of a moving-iron meter movement contains many turns of wire to produce this strong magnetic field.
The iron vanes of the moving-iron meter movement are placed inside the coil. However, the shape of the coil is different for the concentric-vane meter movement and the radial-vane movement. The coil for the concentricvane movement is constructed so that it can accept semicircular vanes and the coil for the radial-vane movement is constructed so that it can accept rectangular plates.
When a pointer is attached to the moving element of the meter movement, and a calibrated scale is put behind the pointer tip, the pointer will swing with the meter movement according to the amount of current flowing and stop at the proper place on the scale to show the current flow.
Because of the delicate nature of the meter movement, and the need to have it respond to current flow without having to overcome external forces, the pointer is generally made of thin aluminum to keep it light.
The tips of the pointer usually come in three shapes: spade, knife, and lance. The spade pointer is used when high visibility is needed, particularly at a distance. However, the broad width of the tip makes it difficult to differentiate scale index marks that are close together on high-accuracy scales. The thin, sliver-like knife pointer is best for high accuracy readings, but it is difficult to see at a distance. The lance pointer is a compromise shape that gives good accuracy with acceptable visibility.
When knife-edge pointers are used for high accuracy, parallax error must be considered. This is the error in reading when the pointer appears to be pointing off the true reading because it is not being read exactly head-on. Good instruments contain a parallax mirror to show the error. The reading should be made when the mirrored reflection of the pointer is directly in line with the pointer. In this way, the meter will not be read at an angle. These are sometimes referred to as mirrored-scale meters.
Counterweights and Retaining Pins
Although the pointer in a meter is very light, the extremely delicate sensitivity of the meter movement is such that the pointer must be properly balanced so that it does not interfere with the accuracy of the movement.
When the meter is manufactured, counterweights are attached to the pointer assembly, and meticulously adjusted to balance the pointer on the pivot so that the weight of the pointer will neither aid nor oppose the motion of the meter movement.
To limit the range over which the meter movement can travel, and to protect it from being overdriven, retaining pins are placed on both sides of the pointer to keep the pointer from going too far off the bottom or top ends of the scale. The pins thus limit any undue pressure being applied to the delicate meter movement spring.