# Fundamental of electrical automotive engineering - Ohmmeter and Ammeter Function

The ohmmeter function of the DMM is used to measure the resistance of a component of the resistance between two points in a circuit. An ohmmeter is also used to check circuits for continuity. The meter test leads are placed across, or in parallel with, the component or circuit to be measured. A battery in the meter passes a small current throught the circuit being tested. The greater the percentage of this current returning to the meter, the lower the circuit resistance. The only voltage that the meter should detect is the meter's own voltage. Any circuit voltage present while measuring resistance will adversely affect readings and may damage the meter. Measure resistance with no power applied to the circuit or component, Polarity of the test leads is not important unless has a diode.

### Unit of Resistance

The basic unit resistance is the Ohm. Large and small amounts of resistance require other units. See the table above to see the common units, their symbols and mathematical multipliers. You will see these units and symbols used in service information and on test equipment displays such as DMM. The following may help you to remember the multipliers : ''micro''=one millionth ; ''milli''=one thousandth ; ''kilo''=times one thousand : and ''mega''=times one million.

### Ammeter Function

The ammeter function of the DMM is used to measure the current in amps in the circuit. To measurement curent, the meter is connected in series and becomes part of the circuit. The low resistance of the DMM current measuring circuitry must be protected by the resistance of a load in the circuit being measured. Never connect an ammeter in parallel, and never allow the ammeter to bypass a load. Ammeters have polarity. The meter will display negative values if the test leads are reversed.

The amperage sockets for the test leads are used only for measuring current. Remember, the meter resistance is very low when plugged into these jacks. Typically, DMM have two amperage jacks, one for measuring very low current reading (milliamps), and a second for higher current (usually 10 amps or less). There are usually internal fast-blow fuses which protect the ammeter circuit. Using a jumper with a fuse rated lower than the meter fuse can provide added protection.
When measuring high current such as starter current draw, an inductive high current-clamp can be used. The clamps is placed around a single wire and senses the magnetic field around the wire. Inductive clamps have polarity and should no be placed near devices with strong magnetic fields, such as a generator.

### Unit of Current

The basic unit for measuring current is the ampere or amp. Large and small amounts of current require other units. See the table above to see common units, their symbols and mathematical multipliers. You will see these units and symbols used in service information and on test equipment display such as DMM. The following may help you to remember the multipliers : ''micro''=one millionth ; ''milli''=one thousandth; ''kilo''=times one thousand ; ''mega'' times one million.

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