The seven wires
Seven wires on one board, and the current through each is measured in turn. The seven are chosen so that three separate comparisons are hiding inside one experiment.
Materials
Three pieces of nichrome wire about a metre long with different cross-sectional areas; a copper wire and several iron wires of the same length and cross-section as the thinnest nichrome; two dry cells; an ammeter; a switch; a board about 1 m long and 20 cm wide.
Figure to be added
What each of the seven is
| Number | The wire |
|---|---|
| 1 | The thickest nichrome wire |
| 2 | The nichrome wire of medium thickness |
| 3 | The thin nichrome wire |
| 4 | The thin copper wire |
| 5 | The thin iron wire |
| 6, 7 | Two thin iron wires of unequal length |
Method
Set up the circuit as in the figure. Touching the probe X to the end of each conductor in turn, record the current that flows. Then answer these:
- What conclusion can be drawn from the readings for wires 1, 2 and 3?
- What conclusion can be drawn from the readings for wires 3, 4 and 5?
- What can you say from the readings for wires 5, 6 and 7?
The wires must be a fair comparison
For cases 4, 5, 6 and 7 the cross-sectional areas of the wires used must be equal. Without that, a difference in reading could be the thickness rather than the material or the length, and the comparison proves nothing.
The ammeter is standing in for a resistance meter
The activity measures current, not resistance — but the cells and the circuit are the same every time, so a smaller current means a larger resistance. Each of the three comparisons changes one factor and reads the answer off the ammeter.