The relation between electric current, magnetic fields and physical forces was first noted by
Hans Christian Ørsted who, in 1820, observed a
compass needle was deflected from pointing North when a current flowed in an adjacent wire. The
tangent galvanometer was used to measure currents using this effect, where the restoring force returning the pointer to the zero position was provided by the Earth's magnetic field. This made these instruments usable only when aligned with the Earth's field. Sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called "multipliers".
The word rheoscope as a detector of electrical currents was coined by Sir
Charles Wheatstone about 1840 but is no longer used to describe electrical instruments. The word makeup is similar to that of
rheostat (also coined by Wheatstone) which was a device used to adjust the current in a circuit. Rheostat is a historical term for a variable resistance, though unlike rheoscope may still be encountered.