How to Connect an Ammeter for Electrical Resistance

Written by mark stansberry
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To measure electrical resistance an ammeter can be used. For this type of measurement, you will use a known voltage and the current measured to calculate an unknown resistance. Although it is a simple operation to connect an ammeter for an electrical resistance measurement, the procedure often confuses many. And you don’t want to be confused, because if you don’t connect the ammeter correctly, you may very well damage your ammeter.

Skill level:
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Things you need

  • Ammeter
  • Power supply
  • Alligator clips
  • Resistor
  • Laboratory-safe gauge wires

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Instructions

  1. 1

    Set the ammeter to its highest range. To measure current, an ammeter has different ranges. The ammeter will only give you a current reading if the current measured is within the ammeter range setting. Because you often don't know the value of the current measured, it is considered good procedure to first set the ammeter to its highest rating because trying to measure a current higher than what the range is meant to measure could damage your ammeter.

    Typical ammeter ranges include microampere, milliampere and ampere ranges. For example, the highest ampere range setting may be used to measure currents from 0.1 amperes to 1 amperes. The next highest range may be used to measure currents from 10 milliamperes to 100 milliamperes and another range could be used to measure currents from 1 milliamperes to 10 milliamperes. And the lowest range may be used to measure currents from 100 microamperes to 1000 microamperes.

    One milliampere is one-thousandths of an ampere and one microampere is one-millionth of an ampere. The ampere is the unit of measurement for electrical current.

  2. 2

    Keep your circuit’s power off until after the ammeter is connected. This is a safety precaution. Because you have to insert the ammeter’s probe between two open-ended wires, there is a chance of electric shock if the power is on.

  3. 3

    Set-up the electrical measurement circuit. To measure an electrical resistance you will need a battery, a resistor and an ammeter. To connect them together, first grab some alligator clips with insulated safety gauge wires. Then connect the direct current power supply, ammeter and resistor in series with these wires and alligator clips.

    To connect an ammeter in series, connect the positive wire from the power supply to the positive probe of the ammeter. Then connect the ammeter’s negative probe to one end of the resistor. Connect the resistor’s other end to the power supply’s negative terminal.

    Finally turn on the power supply and set the voltage of your power supply. Anywhere from 1 Volt to 10 Volts will usually do. The lower the voltage, the safer the working environment. However, if the voltage level is too low, the current will be too low to measure and you won’t obtain a non-zero current reading. And you will need a non-zero current reading for the electrical resistance calculation.

  4. 4

    Turn on the power supply and record the current on the ammeter’s display. If the value of the resistance is relatively low in relation to the power supply voltage, you will get a current reading. If the resistance is relatively high in relation to the power supply voltage, the ammeter will read zero.

    If your first reading is zero, change the meter’s range from the ampere range to the next lower range, the milliampere range. If you still get a zero reading, select the next lower range, the microamplere range. Write down that current level. Also note the current units for the range setting.

  5. 5

    Calculate the electrical resistance. The electrical resistance is equal to the power supply voltage divided by the current. If you set the power supply to 10 Volts and your current reading was 1 milliampere, the resistance would be 10,000 Ohms. One milliampere is 0.001 amperes. So use 0.001 for your calculation. Note 10 / 0.001 = 10,000.

Tips and warnings

  • If the current level is lower than what your ammeter can measure, you will never get a reading that is non zero. In these cases, you will have to increase the power supply voltage to obtain a measurable level of current. Or you can use an ammeter that has a lower range.
  • Don’t use an ammeter or power supply unless you have been trained in electronic safety. There are electronic safety certificates available. The improper use of an ammeter or power supply could result in fire, electrical shock, injury or death. There is a lot to know about electronics safety. So never assume what you are doing is absolutely safe; it isn’t.

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