'Variac' is a generic trade name for a variable transformer. This is a very common type of transformer used in a variety of applications.
Variac brushes are one of the components of a Variac that defines its flexibility. The transformer's output voltage is determined by the area of the brush that is in contact with the transformer's windings. These brushes can be adjusted very precisely, making Variacs excellent for granular control over output voltage.
The brushes inside a variable transformer are typically high-resistance components that are in contact with the transformer's windings. AVariac brushcan be brought into greater or lesser levels of contact with the transformer's coils by rotating the knob.
These brushes have a wide range of applications, from advanced testing applications to simple dimmer switches.
Variac brushes are quite simple in design, as they constitute primarily of the brush, coil and the container that houses everything. However, despite this simplicity of design, Variac brushes are often designed for use with specific transformer models. The compatible transformer models for each brush are typically mentioned in the brushs name or description.