The ohmmeter is a meter for measuring electrical resistance in ohms. It is usually just one of several meters contained in a single unit including a voltmeter which measures voltage and an ammeter which measures current in amperes. These units are typically called multimeters or VOMs, which stands for Volt-Ohm-Milliamp because current measuring is limited to the much lower and safer milliamp range.
The German physicist Georg Simon Ohm discovered that the flow of electric current through a conductor was directly proportional to the potential difference, or voltage, and inversely proportional to the resistance to current flow. The amount of resistance to the flow of current is measured in ohms in honor of Georg Ohms.
Types of Ohmmeters
Ohmmeters, let's call them multimeters, come in two basic types: analog and digital. They both perform the same functions of measuring voltage, current and resistance. The choice of which to use depends to some extent on who the user happens to be. A hobbyist who likes to tinker in the garage may only need a basic digital model, while a design engineer or professional technician or electrician may require a far more expensive unit. Digital ohmmeters are call DMMs for Digital-Multimeter.
How an Ohmmeter Measures Ohms
Older analog ohmmeters used a small battery to apply voltage to a resistance while a galvanometer measured the current flowing through the resistance. The amount of current was indicated by a pointer attached to the galvanometer which registered the amount of current on a scale measured in ohms. A more accurate type of ohmmeter uses an electronic circuit to pass a constant current through the resistance while a separate circuit measures the voltage across the resistance.
An ohmmeter can be used to troubleshoot many household items. A ceramic fuse for instance, which may offer no visible means to determine if it's blown or not, can be tested with an ohmmeter. If an ohmmeter indicates a short circuit then the fuse is good. An open circuit indication means it's blown. Stereo cables, speaker coils, guitar pickups and other electrical devices can be tested and verified with an ohmmeter.
When using the multimeter to measure resistance, or ohms, there are a number of considerations to take into account. If using an analog ohmmeter to measure ohms, the meter bust be zeroed by shorting the test leads together and using the zero control to set the indicator needle to the zero resistance reading. This accounts for battery drain over time and must be done each time the range is changed. With a digital ohmmeter there is no need to zero the meter. Whether using an analog or digital ohmmeter always make sure there is no power applied to the circuit under test to prevent possible damage to the meter.