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Voltmeters

A 1901 GE volt meter
A 1901 GE volt meter

The voltmeter is a key instrument in our world of electrical technology. They are used in everything from x-ray machines to radio transmission, computers to power utility and atmospheric science measurement to the Hubble Telescope. Voltmeters can use magnetic fields, electrostatic fields, and plasma phenomena to measure voltage. In this page we talk about PMMC or moving coil galvanometer type voltmeters which were the first voltmeters and dominated the industry for over 100 years until they were replaced by wide use of digital multimeters (DMMs).

To see the science and engineering of the voltmeter we recommend you watch our short video below, if you saw this video at the start of Unit 1 already you can move on.

SchemVoltmeter600

To the right you will see a simple schematic of the voltmeter reading the DC voltage of a circuit that consists of a battery and lightbulb. The way the voltmeter is positioned in the circuit is different from the ammeter. The voltmeter measures by comparing the difference between two points and measuring the potential. The ammeter on the other hand is positioned in series with the circuit.

VoltmeterPatentShall1888Edward Weston revolutionized the field of instrumentation when he developed the first portable electric meters and a reliable PMMC (permanent magnet moving coil instrument). Before the 1870s meters were fragile lab instruments not entirely useful for testing systems. Since before the 1870s most electrical installations were for the telegraph where there was not a need for regular measurements. Mirror galvanometers were used to help set up telegraphs. Westinghouse led many of the innovations in voltmeter design with AC pioneer Oliver Shallenberger on the team. Philip Lange and Shallenberger took the galvanometers of the time and added the needed resistors that converted them from ammeter to voltmeter. They redesigned the meters to work with Tesla’s 2-phase system, and later Dobrovolsky’s 3-phase system which ended up being today’s standard.

GEmeter600The standard volt and ammeters through most of electrical age were magnetically driven and use a robust “can” design with permanent magnet.

The industry standard of voltmeter for many decades was developed in 1886 by chemist and engineer Edward Weston in New Jersey. Weston’s innovation included a stable permanent magnet and his development of a special Manganin material. See our page on ammeters to read more about the design since ammeters and voltmeters were very close.

Historic Voltmeter
A 1901 General Electric voltmeter

Don’t burn it out!
Voltmeters measure voltage across two points in the system and are not designed to have the system’s full power run through it. A resistor ensures that the primary power supply flows through the main circuit and does not burn out the voltmeter. However if one was to place the voltmeter into the circuit in the same way as an ammeter it would destroy the voltmeter. Today’s digital multimeters have a fuse placed to protect the meters. Older meters would sustain damage and had to be repaired. Companies like General Electric had an office where technicians and engineers could send volt and ammeters to be repaired and/or calibrated.

Further reading:

The PMMC

Edward Weston