In 1966, moving-coil meters were introduced with a new type of galvanometer, the d'Arsonval, which used a capacitive sensor to detect the deflection of the pointer. The electron deflection in the coil was detected by a capacitive plate which moved parallel to the coil. The capacitance change was read by a separate circuit and was linear to the deflection of the pointer. D'Arsonval meters still exist today, but they have a lower cost and are more rugged than traditional d'Arsonval meters. Movable-coil meters, which are still in use, are far more sensitive than D'Arsonvals, but they are more expensive, larger, heavier, and have a shorter life expectancy.
A later design incorporated a moving-coil galvanometer encased in glass, with a vacuum tube or diode as the current-sensing element. This was the precursor to the digital multimeter, although the diode was not replaced by a solid-state element until the 1950s. These digital-style multimeters have a dial marked in ohms to show what range of resistance can be measured. They are still the most popular type of multimeter for the amateur.
The need for a device that was fast and rugged led to the development of the moving-coil galvanometer. This is the device that would eventually become the standard for measuring current. At the time, the encasement of the moving coil was a huge problem, so early moving-coil meters used a spinning rotor mounted in a brass case and spun at high speed. The rotor was a very delicate part and a serious hazard, so they also were extremely expensive and fragile.
The first practical all-electric, d.c. voltage-measuring device was the variable resistor, invented by the German physicist Hans Geissler in 1881. The variable resistor consisted of a circuit containing two electromagnets and a movable pointer. The electromagnets were connected to a Wheatstone bridge, and their currents were detected by a second Wheatstone bridge. The deflection of the pointer was proportional to the sum of the currents in the two electromagnets, which varied the resistance of a pair of resistors in the bridge circuit. The bridge circuit was arranged so that the resistance of the fixed resistor was proportional to the voltage being measured. The resistance was also proportional to the current in the unknown circuit, so the bridge could be calibrated to give the unknown's current in the bridge circuit. This circuit was used to measure up to 20 V at full scale in the late 19th and early 20th century. By means of a coupled circuit the bridge could be used to measure either current or voltage. In addition, the variable resistance could be calibrated to give either voltage or current. 827ec27edc