711: Seismograph

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The reverse only works if the subject has a nervous twitch.
Title text: The reverse only works if the subject has a nervous twitch.

[edit] Explanation

A polygraph (popularly referred to as a lie detector) measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions. The belief underpinning the use of the polygraph is that deceptive answers will produce physiological responses that can be differentiated from those associated with non-deceptive answers. Polygraphs are generally considered to be pseudoscientific and are not admissible as evidence. They're generally only used to convince suspects that interrogators will know if they lie.

A seismograph is a machine which measures and records the ground's motion during e.g. an earthquake. Older seismographs held a pen against a slowly turning roll of paper, and ground motions were amplified and recorded as spikes in the pen line.

The lie detector works by measuring physiological signals which could presumably be visualized by drawing a line on paper like a seismograph. It is assumed that when someone is lying, their physiological signatures will be sharper and more stressed. In the comic, the character on the right is hooked up to the lie detector, and apparently must answer "No" to the question of the earthquake. As long as there is no earthquake, then the subject will be telling the truth, and the polygraph signal will be more stable. But if there is really an earthquake happening, then the subject is lying, and so the polygraph will show sharper signals. This mimics the effect of an actual seismograph. It is not clear why the two characters seem to be upset with each other, other than because the scene mimics a polygraph test where the subject is trying to hide something.

The title text considers the idea of using a seismograph as a lie detector. If the subject has a nervous twitch, presumably they will twitch in some way when they are telling a lie. If they are connected to a seismograph somehow, it might pick up the twitch and thus reveal that the subject is lying.

[edit] Transcript

[Cueball is standing over another man, who is strapped into a chair with wires attached to his head and arms. The wires lead to a large lie detector on a stand next to him, which has jagged lines drawn across it.]
Sitting man: No!
scritch scritch
Pro tip: In a pinch, a lie detector can double as a seismograph.

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