The filament current is controlled by the technologist through a device called a(n):

The filament current, which is essential for the operation of an X-ray tube, is primarily controlled by a rheostat. A rheostat is a variable resistor that allows the technologist to adjust the resistance in a circuit, thereby modifying the amount of current flowing through the filament. By increasing or decreasing the filament current, the technologist can control the temperature of the filament, which in turn controls the production of X-rays. This is crucial for achieving the desired exposure levels during imaging procedures.

In contrast, a transformer is used to step up or step down voltage in an electrical circuit but does not provide direct control over the filament current. A rectifier is responsible for converting alternating current (AC) to direct current (DC), which also does not directly manage the current flowing through the filament. A capacitor stores and releases electrical energy but is not used for controlling the filament current in an X-ray circuit. Thus, the rheostat is the appropriate device for managing the filament current.