When a Spirit Was Expected to Move the Needle
A galvanometer is an instrument that detects a small electric current and shows it as a moving needle or other visible deflection. In Victorian séances, investigators could connect a medium to such an instrument and watch for a change while asking whether an unseen presence was acting. The appeal was tangible: instead of relying only on a witness’s impression, the group could point to a reading on a dial.
But the instrument registered electricity, not its cause. A needle could move when the medium’s body changed the electrical conditions in the circuit. Moist skin conducts electricity more readily than dry skin, so perspiration could alter a reading; shifts in pressure or contact at an electrode could also affect the connection. Nervousness, warmth, or movement might therefore produce a signal without any spirit being involved.
A Measurement Is Not an Explanation
That gap mattered. The galvanometer could make an event visible, but it could not identify what had produced it. To treat a deflection as evidence of a spirit required more than noticing that it happened during a séance: investigators would need to separate the medium’s ordinary bodily responses and the equipment’s behavior from the proposed supernatural cause.
The device thus brought two very different ideas into the same room. Its needle offered the authority of measurement, while the séance supplied an interpretation. A sudden jump might feel like contact with another world—but the instrument itself could report only that something in the electrical circuit had changed.
