Voltmeter
A voltmeter is a device that measures the voltage passing between any two points, typically used to check electric circuits for power. It works because it is a high-resistance ammeter that is actually measuring the amount of current that goes through it despite resistance.
One example of a voltmeter, is a moving-coil galvanometer, which works by measuring the rotation of a coil of fine wire suspended in a magnetic field; the angular rotation is directly proportional to the current running through the coil. Series resistance is added to the coil to make the rotation proportional to the voltage.
Another type is the potentiometer, a length of uniform-resistance wire, film, or other substance combined with a "wiper" that can create a short circuit on any part of the substance; this changes the effective resistance. The wiper's position can be changed by the user until the measured resistance is balanced, with no detectable current; the uniform-resistance wire is pre-calibrated so that the user knows what the voltage must be at that position.
There are several other types of voltmeters in use today, the most common being digital voltmeters that use the natural resistance of electric circuits.
Web Resources On Voltmeter
Voltmeter
Voltmeter Design
Book Resources On VoltmeterVacuum tube voltmeters by John Francis Rider
An electrostatic generating voltmeter by Edwin William Kenefake
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