Function

• The main way that a voltmeter operates is by passing an electric current through some resistance. As a result, if you think of a high-resistance ammeter, you also have a voltmeter's function in mind. When designing a voltmeter, it is important to minimize circuit disturbance -- which means that the instrument should be able to work with a minimum of current. One way to do this, for example, would be to place a microammeter in a series circuit with high resistance.
  
  

Types

• There are six primary types of voltmeters: moving coil galvanometers, microammeters (as mentioned above), vacuum tube voltmeters (VTVM's), oscilloscopes, digital voltmeters (DVM's) and potentiometers.
  
  

Features

• The moving coil galvanometer suspends a coil of fine wire within a strong magnetic field. Once the current enters the field, the indicator will turn and push a small spring. The amount the indicator turns is proportional to the amount of current. To make this work as a voltmeter, series resistance is necessary, so that the size of the turn is then proportional to the amount of voltage applied.
  The microammeter works by reading the "Ohms/Volt" ratio on the front of the device. If you read the resistance from the device and multiply by the voltage range, this will tell you the resistance, and will let you calculate the voltage.
  The VTVM relies on a transistor circuit, or tube, to increase the voltage coming in. This increases sensitivity and input immediancy.
  The oscilloscope deflects a ray within a CRT (cathode-ray tube). The ray consists of an electron beam traveling within the vacuum found inside the tube. The beam is either deflected by a magnetic field from a coil on the outside of the tube or by voltage on plates on the inside of the tube. Comparing this unknown voltage with a known reference point gives the voltage inside the tube.
  The DVM uses an electronic circuit as an integrator, starting with a constant level of input voltage to measure the change in output when a new current is ramped through the device. The reference voltage and ramp-up time for the new voltage is multiplied together and then divided by the time to ramp back down from the new voltage, to give the unknown voltage.
  The potentiometer uses the null-balance method to compute voltage. Resistance is changed by the wiper until the detector reads zero voltage. The equation used is: Unknown voltage equals known voltage, divided by potentiometer resistance from one of the end terminals to the other, then multiplied by potentiometer resistance going from the wiper to an end terminal.
  
  

Considerations

• It is important to calibrate a voltmeter frequently to check its accuracy. The Weston Cell is just one of many devices used to calibrate voltmeters effectively.
  
  

History

• In 1819, scientist Hans Oersted sent electrical current through a wire that was resting by a compass needle, and he observed that the needle would jump each time the current went through. He wrapped a coil of wire around the compass and observed that, the stronger the current, the more the needle would jump away from north.
  This was too crude a method to make any sort of precise measurements. Near the end of the 1800's, Arsene D'Arsonval made an adjustment to this method: using a smaller coil that was attached to the compass needle's base, and encircled by a round magnet, D'Arsonval was able to get more differentiation in his readings. This setup is still used in modern analog voltmeters.