Zhang Heng arrived in the underworld. King Yama, knowing he possessed profound wisdom, told him that numerous officials continuously arrived in the afterlife, and investigating whether they had governed with integrity or corruption was extremely troublesome. He asked Zhang Heng if there was any simple method to distinguish between them.
Zhang Heng provided him with a specially designed balance scale. When King Yama looked at it, he saw that one of the scale's pans was extremely large, while the other was very small.
"How does this balance work?" King Yama asked.
Zhang Heng explained to King Yama: "Whenever an official arrives in the underworld, simply place his official hat as a counterweight onto the small pan. The common people under his jurisdiction will instantly appear on the other side, filling the large pan. If the side with the official hat outweighs the side carrying the people, then that official must have been corrupt. Conversely, if the people's side is heavier than the hat's side, then the official was relatively upright and good."
However, Zhang Heng cautioned King Yama: "Before any official is to be tested, please do not touch the scale yourself!"
The balance remained with King Yama overnight.
The next day, King Yama told Zhang Heng: "Your balance isn't very good; I've already ordered it dismantled."
"How truly regrettable!" said Zhang Heng. "I told Your Majesty not to touch it for a reason. The fact that you found it unsatisfactory clearly means you must have tested it first using your own official hat."