1. Moving iron;
  2. Moving coil;
  3. Solenoid or plunger;
  4. Magnetic vane;
  5. Hot wire;
  6. Electrostatic;
  7. Astatic;
  8. Inclined coil;
  9. Fixed and movable coil.

Again, they are divided according to their use into two classes:

  1. Portable type;
  2. Switchboard type.

Milli-ammeters or milli-voltmeters are instruments in which the scale is graduated to read directly in thousandths of an ampere or thousandths of a volt respectively.

Ques. Describe the moving iron type instrument.

Ans. The arrangement of the working parts are shown in [fig. 620]. A soft iron needle N, is pivoted inside of a coil C, and is held out of line with the axis of the coil by means of a permanent magnet M, when the instrument is idle. In this position, a pointer P, which is attached to the needle, stands at the zero mark of the scale S. If a current be passed through the coil, magnetic lines of force are set up in its center, which tend to pull the needle into line with them, and therefore with the axis of the coil. This pull is resisted by the permanent magnet M, and the amount of deflection of the needle from the zero position depends upon the strength of the current or the voltage according as the coil is wound to indicate amperes or volts.

Fig. 620. Moving iron type instrument. The essential parts are: N, soft iron needle; C, coil; M, permanent magnet; P, pointer; S, scale. Current passing through the coil acts on the needle, causing it to turn against the restraining force due to the influence of the permanent magnet.