Ohm's Law says, "In metallic conductors at a constant temperature and in a zero magnetic field, the current flowing is proportional to the voltage across the ends of the conductor, and is inversely proportional to the resistance of the conductor." From this comes the equation R=V/I. The resistance (R) of a material -- in this case, metals -- determines how well the material conducts electricity. The V in the equation, sometimes shown as an E, stands for volts, and the I for the current, or amperage. The conductivity of metals can be measured by an ohmmeter, named after George Ohm whose law determines how resistance, and conductivity, are measured. The lower the resistance, the greater the conductivity of the material.
- Skill level:
Make sure you have metal samples of the same thickness. This should be easy enough if you buy your samples in the form of metal sheeting. Ask the vendor for identical sheeting sizes.
Cut your metal samples into squares that are the same size, or buy them this way. For example, make sure your metal samples are 10 cm (4 inches) on each side, making the area 100 cm square (16 inches square).
Stick lengths of bare electrical wire to either side of each sample using electrical tape. For the sake of good experimental practice, make them all the same length, say, 15 cm (6 inches).
Attach your ohmmeter to the electrical wire on your first metal sample. There are four connecting cables with alligator clips on a precision ohmmeter. Basic ohmmeters have two, but four-wire ohmmeters eliminate the resistivity of the connecting cables, as two cables provide the current while two others measure the voltage passing through the test material. This eliminates any cable resistance as a factor in the experiment. Attach the voltage sense cables on either side of the sample, as near to the test metal as possible. Put the current providing cables further out on the electrical wire.
Switch on the ohmmeter and write down the resistance, which is measured in ohms. The ohmmeter works the resistance out using the equation R=V/I, according to Ohm's Law. Conductivity is the opposite of resistance. Conductivity is measured in siemens, which are sometimes called inverse ohms. The inverse ohm is usually written as ohm^-1.
Use the equation O = L/RA to calculate the conductivity of your metal sample. L is the length of your metal sample; in this case, 10 cm (4 inches). This is divided by R, which is the resistance you measured on your ohmmeter, multiplied by the area of your metal sample; in this case, 100 cm square (16 square inches). This will give you the conductivity in ohm cm^-1 (ohm inch^-1) (meaning inverse ohm).
Tips and warnings
- If you are given a random sample of metals to test, for instance, you can work out what type of metal each sample is from its conductivity. There are electrical conductivity tables that list the conductivity of different materials.
- Use the appropriate precautions when using electricity in experiments.
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