Power

 
Here, come sit in this metal cage while I grill you. (Source)

You have the power to find out the truth

You will be examining people and passing judgment on them. You'll be trained to do so, and you'll likely have a background in psychology to accurately determine the results of the polygraph. The power to make the judgment call is on you.

Like an X-ray machine, a polygraph machine is merely collecting data—it's the person who reads the data who can come up with a diagnosis.