This Bailey Current Controller was used to regulate the voltage between a battery and patient during therapeutic treatments.
The “wings,” usually made of conductive carbon, can be lowered or raised into a liquid, such as salt water. By doing so, the amount of current applied to the body can be increased or decreased.
The advantage over other types of variable resistors (rheostats) is that it can be adjusted gradually and smoothly, without the dangers of sudden changes in voltage.
The manufacturer described this feature: “From full strength of the battery down to a current so feeble as to be imperceptible to the most sensitive organ, and this without any possibility of breaks in the circuit or sudden shocks to the patient.”