A Definition of Galvanometer Marking Heads

The galvanometer (often abbreviated to galvo) is a current-sensitive device that operates in a similar manner to an analogue meter. In an analogue meter, a small coil of wire is wound around a lightweight aluminum bobbin that is suspended in the gap of a permanent magnetic field by means of pivots. The coil has a thin needle attached which extends over an indicating face that is the part of the meter you see.
    When electrical energy is applied to the coil, it develops a magnetic field that will act against the field in the gap causing the coil to move the indicating needle proportionally to the current applied. Some meters are designed to be at rest at one end of the scale (such as analogue VU meters), while others are at rest in the centre of the scale (an analogue FM tuning indicator for example).
    The first type of meter is a unipolar device as it reacts only to an increase in the current applied; while the second type of meter is a bipolar device as it reacts not only to the current applied, but also the polarity of the current. When the signal is negative, the indicating needle moves in one direction away from its central position: when the signal is positive, it moves in the opposite direction away from its central position.
    Unfortunately meter movements are too small, to slow, and to delicate to allow us to attach a mirror and control laser beam deflection. We must use a more rugged type of device, a scanning type galvanometer.  Galvos can be thought of as very high speed, current sensitive, limited rotation electrical motors.  The amount of rotation (within the rotational limits of the galvo) is determined by the amount of current applied; with the direction of the limited rotation controlled by the polarity of the current applied.  Galvos (scanners) are a current-sensitive bipolar device that are at rest in the centre of their limited rotation.

Usage Note: The words galvo or galvanometer refers to the basic galvanometer itself, without an attached mirror.  Scanner refers to a galvanometer that has a mirror attached to it such that it can deflect a laser beam by applying appropriate control signals.