Final answer:
The autotransformer is the device adjusted in an x-ray circuit to select the kilo-voltage. It controls the primary voltage of the high-voltage transformer, altering the strength of the x-ray beam.
Step-by-step explanation:
The device in the typical x-ray circuit that is adjusted to select the kilo-voltage (kV) is the autotransformer. The autotransformer is used to vary the voltage that is applied to the primary coil of the high-voltage transformer. This in turn changes the voltage that is provided to the x-ray tube, allowing the technicians to adjust the strength of the x-ray beam, which is vital for producing clear images while minimizing exposure.
Transformers operate on Faraday's law of induction, which allows for the stepping up or stepping down of voltage levels. An example of this is a step-up transformer in a portable x-ray unit that could transform a 120 V input into a 100 kV output needed by the x-ray tube.
In contrast, a rectifier is used to convert alternating current (AC) to direct current (DC), while a rheostat is a device to vary resistance and a transformer is generally used to change voltage levels without an adjustable output.