why is the graduations in an ammeter for measuring ac currents is different from that of ammeters for dc?

Question

explain about the fact that markings in an ammeater or voltmeter for measuring ac currents and voltages are squeezed at the beginning and end of the scale.

Answer 1

Because of the different way that the meters work. A DC meter consists of a coil of wire mounted in a magnetic field, and the torque on the coil is directly proportional to the field, so the graduations are uniform. AC meters work differently (a coil in a constant magnetic field won't work because the average torque is zero), and there are at least two ways of making them (neither of which leads to uniform graduations):
- You can use a small diode to convert the AC current to be measured into DC. The current through the diode does not increase uniformly with voltage.
- You can use a soft iron slug which is pulled into a coil by the magnetic field generated by the coil. The pull on the slug will increase uniformly, but the mechanical linkage between the slug and the indicating needle does not ordiriarily lead to uniform motion.

Answer 2

The previous answer, while correct, is not complete.

Some ammeters, the current ranges on many (most?) multimeters for instance, measure the voltage developed across a small series resistor and use an active rectifier which has a linear relationship between its output and input, therefore allowing the meter's linear scale to be used for a.c.