Practice Assignment 10
Measuring changes in electrical properties - same materials
When investigating changes in resistance with length, students should work with wires of a metal with high resistivity, such as nichrome. Low resistivity metals, such as copper, will have resistances too low to measure for shorter lengths.
When investigating the effects of diameter, thicker wires of low resistance metals are again likely to prove unsuitable. 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.
The option is provided, in the questions, for students to consider measurement of diameter themselves. For the core activity, it is suggested that diameter values are provided and students’ thoughts are focused on measuring resistances.
Old stocks of wire may be labelled in Standard Wire Gauge (swg) sizes. Students should express diameters in millimetres, not in swg.
Apparatus and Reagents
- Standard Procedure:
- SP 0010:2005 Method for measuring changes in resistance with size
- 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 wires, at least 1 metre long, of the same metal with different diameters