Page 827 - Basic Electrical Engineering

A pointer is attached to the spindle (shaft) of the moving coil which moves
over a graduated scale. The scale is calibrated in terms of amperes or volts
depending upon whether the instrument is designed for the measurement of

current or voltage, respectively. Since the deflection is directly and linearly
related to the current flowing through the coil, the scale of the instrument will

be a linear one. That is to say, the calibrations will be equally spaced as
shown in Fig. 11.16.

Figure 11.16 Constructional details of a PMMC-type measuring instrument

Fig. 11.16 shows the inside of a PMMC instrument with a U-shaped

permanent magnet. For producing high flux density, magnetic materials such
as Alnico (alminium-nickel-cobalt) are used so that magnets of smaller size

could be used and the overall size of the instrument could be reduced.
The weight of the moving system is made very low by choosing a light

aluminium drum as the armature on which coils are placed. For higher
sensitivity, the requirement is that the ratio of torque and weight of the

moving system should be high. The requirements are, therefore, for a strong
magnetic field, reduced size, and reduced weight of the moving system. The
range of deflection of the pointer has to be made high, ranging from 120° or

more so as to achieve better and correct recording of meter readings. The two
hair springs made of phosphor bronze provide the control torque. They also