What it shows
Two different metals joined at two points produce a small voltage when the two junctions are at different temperatures (the Seebeck effect). The voltage rises roughly in proportion to the temperature difference, which is how thermocouple thermometers work.
The science
In a metal the free electrons at the hot end have more energy and diffuse towards the cold end, building up a tiny voltage along the wire. The size of this effect differs between metals, so in a loop of two different metals the two contributions do not cancel and a net emf appears that depends on the temperature difference between the junctions. For copper and iron it is only a few tens of microvolts per degree, so a 70 degree difference gives roughly 1 mV, which a digital multimeter on its lowest range can just read. Plotting emf against temperature difference gives a nearly straight line at this range.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- 30 cm of bare copper wire and 30 cm of iron binding wire (or a steel paper clip straightened)
- A digital multimeter that reads millivolts
- A cup of ice water and a cup of hot water from a kettle
- A kitchen or lab thermometer
- Sandpaper and a small pair of pliers
Safety
For this project: An adult must handle the hot water and the kettle. Do not put the junction into a flame; hot water up to 90 degrees Celsius is enough for a clear reading.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Sand both wires clean and twist one end of the copper tightly around one end of the iron for about 1 cm to make junction A.
Twist the other ends together in the same way to make junction B, then cut junction B open and connect the two free ends to the multimeter leads instead, so the meter is in the loop.
Hold the meter junction (the leads) at room temperature and dip junction A into ice water; set the meter to its lowest millivolt range and note the reading.
Move junction A into water at about 40 degrees and record the thermometer reading and the millivolt reading together.
Repeat at about 55, 70 and 85 degrees by mixing hot and cold water, giving each reading 30 seconds to settle.
Record a table of hot junction temperature, temperature difference from room temperature, and emf in millivolts.
Plot emf against temperature difference and find the slope in microvolts per degree.
Reverse the leads to show the sign changes, and discuss why industrial thermocouples use special alloys for a bigger signal.
What you should see
The millivolt reading grows steadily with temperature difference, with a slope of a few tens of microvolts per degree for copper and iron, and reverses sign when the leads are swapped.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the Seebeck effect?
A voltage produced in a circuit of two different metals when their junctions are at different temperatures.
2Why must the two metals be different?
In a loop of one metal the thermal emfs on the two sides cancel exactly.
3Why is a thermocouple useful in a furnace?
It can measure very high temperatures and gives a fast electrical signal.
4What is the reverse effect called?
The Peltier effect, where a current through a junction heats or cools it.
Take it further
Join ten copper-iron pairs in series as a thermopile and show that the voltage is ten times larger.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely