Multimeter magic

Agriculture.com Staff Updated: 01/11/2011 @ 10:54am

By Ron Van Zee


Amultimeter, or VOM (volt, ohm, milliampere) meter, is like a Swiss army knife – no farmer should be without one. As essential as a Vise-Grip, a VOM can tell you why your tractor won’t start when its battery is fully charged.

To get started with a VOM, you need to know the basic elements of electricity.

Volts (represented by the letter “E”). This is the force that causes a flow of current in a conductor. Voltage can exist even when there is no current flow in a circuit. But current cannot exist without the push of voltage.

Ohms (represented by “R”). This is a resistance or opposition of electrons flowing through a circuit. Ohms are kind of an electrical back pressure.

Ampere or milliampere (represented by “I”). This is the rate of flow (current) of electrons in a circuit.

Watts (represented by “P”). This is the rate of work being accomplished by electrons moving through a circuit.

To put your VOM to full use, you need to know the following formulas. Write them down and keep them with your VOM for ready reference.

  • I=E/R: Amperes equals volts divided by ohms.
  • E=IR: Volts equals amperes multiplied by ohms.
  • R=E/I: Ohms equals volts divided by amperes.
  • P=EI: Watts equals volts multiplied by amperes.
  • E=P/I: Volts equals watts divided by amperes
  • I=E/P: Amperes equals volts divided by watts.

How The Formulas Come Into Play

A lightbulb provides a good example of how these formulas work. A lightbulb has a tungsten filament with a specific resistance, measured in ohms (R). This bulb is designed to light to a precise brightness, or wattage (P), at a specific voltage (E). A bright lightbulb has more resistance built into the filament than a dim bulb at the same voltage. So more electricity, or amperes (I), are required to light the tungsten bulb using the same power source.

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