Observation of Reflection and Refraction with a Thermopile

What is a thermopile? A thermopile is a set of thermocouples that has been designed and arranged to measure little quantities of radiation or radiant heat. With a thermopile and galvanometer, Scientists and researchers can repeat Herschel’s experiment more strikingly than a thermometer. And with simple apparatus they can show that, when heat is reflected, the angle of reflection is equal to the angle of incidence. We can also show the first law of reflection; that the incident and reflected rays are in the same plane as the normal to the reflector at the point of incidence.

If heat is radiant energy, then its intensity should fall off as the inverse square of the distance from a point source. We can then check that it does so by setting up an electric lamp, with a compact filament, in the dark room preferably with black lamp walls. When we put a thermopile at different distances from the lamp, the deflection of the galvanometer is found to be inversely proportional to the square of the distance.

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Illustrated Description: If we wish to do this experiment with radiation that includes no visible light, then we must modify it. Instead of the lamp, we can use a large blackened tank of boiling water, A, and we fit the thermopile, B, with a conical mouthpiece, blackened on the inside. The blackened prevents any radiation from reaching the pile by reflection at the walls of the mouthpiece. We now find that the deflection of the galvanometer, G, does not vary with the distance of the pile from the tank, provided that the tank occupies the whole field of view of the cone. The area S of the tank from which radiation can reach the thermopile is then proportional to the square of the distance d. And since the deflection is unchanged when the distance is altered, the total radiation from each element of S must therefore fall off as the inverse square of the distance d.

How Does a Thermopile Work?

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The working principle of a thermopile depends on the electromotive force, which appears between the junctions of two different metals, when one junction is hot and the other cold. The modern thermopile is designed to Coblenz. It consists of many junction between fine wires, are of silver and diameter. Their junctions are attached to thin discs of tin, about 0.2mm thick, and about 1mm square. One set of discs is blackened and mounted behind a slit, through which radiation can fall on them; the junctions attached to them become the hot junction of the thermopile. The other, cold, junctions are shielded from the radiation to be measured; the discs attached to them help to keep them cool, by increasing their surface area.

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Older types of thermopile are made from bars of metal about a millimeter thick. They are slow to war, up when radiation falls upon them, but are more rugged than the modern type.

When radiation falls on the blackened discs of a thermopile, it warms the junction attached to them, and sets up an E.M.F. this E.M.F. can be measured with a potentiometer, or for less accurate work, it can be used to deflect a galvanometer, connected directly to the ends of the thermopile.