Umair Gaines
Metals are electrical conductors, meaning they have low resistance and high conductance. The resistance of a metal is dependent on its size, shape, temperature, and the number of free electrons in its atomic lattice. Measuring the resistance of a metal is important for determining the condition of a circuit or component. This can be done using a digital multimeter, which measures resistance using either a constant current or constant voltage. The constant current technique is generally used for resistance values below 200M ohms, while the constant voltage technique is used for high resistance measurements.
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What You'll Learn
Using a digital multimeter
To measure the electric resistance of metals using a digital multimeter, follow these steps:
Firstly, ensure that the component you are testing is isolated from the circuit. Either remove the component or isolate it with an open switch. This is important because the resistance of all components connected in parallel with the component being tested will affect the resistance reading, typically lowering it.
Next, check that your multimeter is in good condition. Make sure the battery is fully charged, and look for any signs of corrosion. Check your multimeter for any cracks, and check the wires for any fraying or nicks.
Now, insert the black test lead into the COM jack, and the red lead into the VΩ jack. If your multimeter has a selector knob, turn it to the Ω setting. If your multimeter has a RANGE button, press it to select a specific fixed measurement range.
Then, connect the test leads across the component being tested. Ensure that contact between the test leads and the circuit is good. If you are testing a resistor, hold the probes against the resistor legs with the same amount of pressure you use when pressing a key on a keyboard.
Finally, observe the readout window to obtain the Ω reading. Compare the results to the manufacturer’s Ω specifications. If the readings match the component, then resistance is not a problem. If the component is a load, there should be resistance that matches the manufacturer’s specs. If the reading is infinite (I) or overloaded (OL), then the component is open. If the reading is zero, then the component is closed (if it is a load, this indicates an internal short).
It is important to note that the human body conducts electricity, so always handle only the insulated part of the probe while testing resistors. If you are measuring very low resistance, use the relative mode (REL) to automatically subtract test lead resistance.
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Two-terminal measurement method
Measuring the electric resistance of metals is important for determining the condition of a circuit or component. The higher the resistance, the lower the current flow, and vice versa.
Resistance is measured using analog or digital multimeters, which can also measure current, voltage, and more. The resistance of a component can be determined by applying either a constant current or a constant voltage. The constant current technique involves sourcing a known current through an unknown resistance and measuring the resulting voltage. This technique is generally used for resistance values below 200M ohms. The constant voltage technique, on the other hand, involves applying a known voltage across an unknown resistance and measuring the resulting current. This method is used for high resistance (1e8 to 1e16) measurement applications.
Digital multimeters support two measurement methods, and in most cases, the two-terminal measurement method is used. This method applies a constant current and measures the resistance value using the instrument's voltmeter. This is the same method used by analog multimeters. However, one disadvantage of the two-terminal measurement method is that the resistance values include the wiring between the instrument and the circuit under measurement.
When using a digital multimeter, insert the black test lead into the COM jack and the red lead into the VΩ jack. Then, connect the test leads across the component being tested, ensuring good contact between the leads and the circuit. For very low-resistance measurements, use the relative mode (REL), which automatically subtracts test lead resistance. It is important to avoid touching metal parts of test leads to avoid errors. Additionally, if a circuit includes a capacitor, discharge the capacitor before taking any resistance readings.
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Ohm's Law
Technicians can use this law to calculate the unknown value of voltage, current, or resistance when two of the values are known. For example, if voltage and current are known, technicians can calculate resistance by rearranging the formula to R = V/I.
The resistance of a material can be measured using a digital multimeter. This device measures resistance using either the constant current or constant voltage technique. The constant current technique is generally used for resistance values below 200M ohms, while the constant voltage technique is used for high resistance measurements.
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Temperature coefficient
The electrical resistance of a wire or component is defined as the ratio of the voltage applied to the electric current that flows through it. The resistance of wires, resistors, and other components often change with temperature. This effect may cause an electronic circuit to malfunction at extreme temperatures.
The "alpha" (α) constant is known as the temperature coefficient of resistance. It represents the resistance change factor per degree of temperature change. All materials have a certain specific resistance (at 20° C), and they also change resistance according to temperature by certain amounts.
Pure metals typically have positive temperature coefficients of resistance, meaning that resistance increases with increasing temperature. For the elements carbon, silicon, and germanium, this coefficient is a negative number, meaning that resistance decreases with increasing temperature.
For some metal alloys, the temperature coefficient of resistance is very close to zero, meaning that the resistance barely changes with temperature variations. This is a good property if you want to build a precision resistor out of metal wire.
The temperature coefficient for aluminum is 3.8×10-3 1/°C. This can be calculated using the formula: dρ = (2.65×10-8 ohm m2/m) (3.8×10-3 1/°C) ((100 °C) - (20 °C)).
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Resistivity
The resistivity of a material is quantified by its conductivity. The more conductive a material is, the lower its resistivity, and vice versa. For example, copper is a highly conductive material with low resistivity. On the other hand, Teflon is an insulator with low conductivity and high resistivity.
The resistivity of a metallic conductor decreases as its temperature is lowered. However, even near absolute zero, a real sample of a normal conductor will still show some resistance. In contrast, in a superconductor, the resistance drops to zero when the material is cooled below its critical temperature.
The resistivity of a material is influenced by several factors, including its temperature, cross-sectional area, length, and purity. For example, higher temperatures cause greater vibrations in the crystal lattice of a metal, leading to increased resistance. Similarly, a thin cross-section of a material will restrict current flow and result in higher resistivity.
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Frequently asked questions
Electrical resistance is the capacity of a circuit or material to oppose the flow of an electric current. It is referred to as Ohms (Ω).
Electrical resistance can be measured using analog or digital multimeters. These tools also measure current, voltage, and more, and can be used in a variety of situations.
Some factors that can affect resistance readings include foreign substances (dirt, solder flux, oil), body contact with the metal ends of the test leads, or parallel circuit paths. The human body, for example, can lower total circuit resistance.
Silver is the least resistive metal known, while calcium and alkali metals have the best resistivity-density products. However, they are rarely used for conductors due to their high reactivity with water and oxygen.
