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In the early 1800s, scientists Thomas Johann Seebeck of Germany and Jean Charles Athanase Peltier of France, working independently of each other, made discoveries related to the effect of heat on electrical current. Their findings, the Seebeck Effect and the Peltier Effect, respectively, were built upon in the 1850s by British physicist William Thomson, later known as Lord Kelvin. Integrating their work and his own insights, Thomson described what came to be called the Thomson Effect. The last of this trio of interrelated thermoelectric effects is illustrated in the tutorial below.
Depicted above is a length of wire to which a Galvanometer, which measures electrical current, is attached. No external power source is connected to this wire. However, if you slowly move the Bunsen burner using the Burner Position slider, you will generate a flow of electrons (depicted by yellow particles) in the wire that travel ahead of the moving burner; this current is registered on the galvanometer. If you reverse the direction of the burner, the current flow will also reverse.
What causes this current to flow is not the flame itself (in fact, heat alone creates resistance to a current), but rather the difference in temperature that the flame creates along the conductor, what’s known as a thermal gradient. You could achieve the same effect by placing an ice cube on the conductor. The important thing is that one point along the conductor is warmer and another point is colder, and that thermal energy, in the form of electrons, will travel from hotter to colder. The electric potential difference is proportional to the temperature difference.
Now explore what happens when we introduce an applied voltage, by way of a Battery, to this scenario by clicking on the Applied Current radio button. (The electrons, going in the opposite direction of conventional current, travel from the negative terminal to the positive.) The Bunsen burner, positioned in the middle of the circuit, causes electrons in the circuit to flow away from it, in both directions. Some of these electrons flow in the same direction as the current generated by the battery; these liberate heat, making that section of the conductor hotter (as indicated by the reddish color). On the other side of the flame, where the electrons set into motion by the heat are swimming against the current, heat is absorbed, making that section of the conductor cooler (denoted by the blue color).
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