Practice Assignment 09
Comparing electrical properties of metals
The success of this assignment depends on the composition and dimensions of the metal wires. Metals with low resistivity, such as copper, will have very low resistance over 1 metre, unless very thin. The resulting current may be too high for the meter. Higher resistivity wires, such as nichrome, should be no problem.
It is suggested that samples should be pre-tested, to ensure suitability. A resistance of at least 0.2 ohm over 1 metre is required. If necessary, longer lengths may be used, but samples must be of equal length for comparisons to be valid. Alternatively, a lower voltage from a D.C. power pack may be used.
A multimeter measures resistance in the same way. When set to a resistance range, voltage from an internal cell is applied across the specimen. The meter registers the current, but the scale is reversed to read 0 ohms for full scale deflection (maximum current) and infinite resistance for zero current. Digital instruments compute resistance from V/I, displaying the result.
The two meter method is used, to reinforce the relationship between V, I and R, and because educational multimeters cannot measure low resistances, their lowest range normally being 200 ohm.
Teachers should stress that the switch must only be closed long enough to take readings, and then left open. If left ‘on’, the high currents would drain the cell, heat the wire and alter its resistance.
Apparatus and Reagents
- Standard Procedure:
- SP 0009:2005 Method for measuring the resistance of metal wires
- 1.5 V cell
- voltmeter capable of measuring 1.5 V DC
- ammeter capable of measuring 10 A DC
- metre rule
- connecting wires
Test Specimens
- selection of thin wires, at least 1 metre long, of various metals (See above)