Indicating D-C Meters

Fundamentally, all meters measure current and in this sense are ammeters. When they have low internal resistance so as to produce no effect on a circuit's performance and are actuated through direct connection in series with the main current path, they are truly ammeters.

When they have high resistance so that they may be placed in shunt or directly across a voltage source they are called voltmeters. Their indication of voltage is based on the product of a known, calibrated internal resistance in series with the meter movement, and the current flowing through that resistance.

Most meters are constructed to use the attractive and repulsive force characteristics of magnetic structures to produce deflection of a pointer. In the most common PM type mutual force is exerted upon a movable, pivoted coil which itself becomes d-c current being measured. When a movable a magnet when it passes magnet is mounted in the field of a fixed permanent magnet and when it is pas sing current of the correct polarity, a force is exerted upon the movable magnet. In the resultant alignment of the fields, definite and proportional movement is produced against the res training force of a spring connected to the pivoted coil. Deflection is arranged to be proportional to the amount of current through the coil, so that when the meter scale over which the pointer moves is calibrated, a true indication of current is obtained.

The basic d-c meter movement uses the pivoted coil and a PM. The electrodynamometer type of instrument, which is less common than the PM type, employs the same basic moving-coil mechanism but uses a fixed electromagnetic field actuated by current which is related to or proportional to that in the moving coil.

When the current through a circuit is sent through the fixed-magnet coil and the voltage across it applied to the movable coil, this instru ment becomes a wattmeter, since deflection is then proportional to the product of voltage and current.

Dynamometers are less flexible than the PM type meters, less sensitive, and generally more expensive. Their basic arrangement, however, allows them to be used on either a-c or d-c when appropriate scale multiplier, and shunt arrangements are included in their internal circuitry.