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   Chapter 3 THE ELECTRICITY OF HEAT.

The Story of Electricity By John Munro Characters: 6099

Updated: 2017-11-29 00:04


In the year 1821 Professor Seebeck, of Berlin, discovered a third source of electricity. Volta had found that two dissimilar metals in contact will produce a current by chemical action, and Seebeck showed that heat might take the place of chemical action. Thus, if a bar of antimony A (fig. 22) and a bar of bismuth S are in contact at one end, and the junction is heated by a spirit lamp to a higher temperature than the rest of the bars, a difference in their electric state or potential will be set up, and if the other ends are joined by a wire W, a current will flow through the wire. The direction of the current, indicated by the arrow, is from the bismuth to the antimony across the joint, and from the antimony to the bismuth through the external wire. This combination, which is called a "thermo-electric couple," is clearly analogous to the voltaic couple, with heat in place of chemical affinity. The direction of the current within and without the couple shows that the bismuth is positive to the antimony. This property of generating a current of electricity by contact under the influence of heat is not confined to bismuth and antimony, or even to the metals, but is common to all dissimilar substances in their degree. In the following list of bodies each is positive to those beneath it, negative to those above it, and the further apart any two are in the scale the greater the effect. Thus bismuth and antimony give a much stronger current with the same heating than copper and iron. Bismuth and selenium produce the best result, but selenium is expensive and not easy to manipulate. Copper and German silver will make a cheap experimental couple:-

POSITIVE

Bismuth

Cobalt

Potassium

Nickel

Sodium

Lead

Tin

Copper

Platinum

Silver

Zinc

Cadmium

Arsenic

Iron

Red phosphorus

Antimony

Tellurium

Selenium

NEGATIVE

Other things being equal, the hotter the joint in comparison with the free ends of the bars the stronger the current of electricity. Within certain limits the current is, in fact, proportional to this difference of temperature. It always flows in the same direction if the joint is not overheated, or, in other words, raised above a certain temperature.

The electromotive force and current of a thermo-electric couple is very much smaller than that given by an ordinary voltaic cell. We can, however, multiply the effect by connecting a number of pairs together, and so forming a pile or battery. Thus figure 23 shows three couples joined "in series," the positive pole of one being connected to the negative pole of the next. Now, if all the junctions on the left are hot and those on the right are cool, we will get the united effect of the whole, and the total current will flow through the wire W, joining the extreme bars or positive and negative poles of the battery. It must be borne in mind that although the bismuth and antimony of this thermo-electric battery, like the zinc and copper of the voltaic or chemico-electric battery, are respectively

positive and negative to each other, the poles or wires attached to these metals are, on the contrary, negative and positive. This peculiarity arises from the current starting between the bismuth and antimony at the heated junction.

The internal resistance of a "thermo-electric pile" is, of course, very slight, the metals being good conductors, and this fact gives it a certain advantage over the voltaic battery. Moreover, it is cleaner and less troublesome than the chemical battery, for it is only necessary to keep at the required difference of temperature between the hot and cold junctions in order to get a steady current. No solutions or salts are required, and there appears to be little or no waste of the metals. It is important, however, to avoid sudden heating and cooling of the joints, as this tends to destroy them.

Clammond, Gulcher, and others have constructed useful thermo-piles for practical purposes. Figure 24 illustrates a Clammond thermo- pile of 75 couples or elements. The metals forming these pairs are an alloy of bismuth and antimony for one and iron for the other. Prisms of the alloy are cast on strips of iron to form the junctions. They are bent in rings, the junctions in a series making a zig-zag round the circle. The rings are built one over the other in a cylinder of couples, and the inner junctions are heated by a Bunsen gas-burner in the hollow core of the battery. A gas- pipe seen in front leads to the burner, and the wires WW connected to the extreme bars or poles are the electrodes of the pile.

Thermo-piles are interesting from a scientific point of view as a direct means of transforming heat into electricity. A sensitive pile is also a delicate detector of heat by virtue of the current set up, which can be measured with a galvanometer or current meter. Piles of antimony and bismuth are made which can indicate the heat of a lighted match at a distance of several yards, and even the radiation from certain of the stars.

Thermo-batteries have been used in France for working telegraphs, and they are capable of supplying small installations of the electric light or electric motors for domestic purposes.

The action of the thermo-pile, like that of a voltaic cell, can be reversed. By sending a current through the couple from the antimony to the bismuth we shall find the junction cooled. This "Peltier effect," as it is termed, after its discoverer, has been known to freeze water, but no practical application has been made of it.

A very feeble thermo-electric effect can be produced by heating the junction of two different pieces of the same substance, or even by making one part of the same conductor hotter than another. Thus a sensitive galvanometer will show a weak current if a copper wire connected in circuit with it be warmed at one point. Moreover, it has been found by Lord Kelvin that if an iron wire is heated at any point, and an electric current be passed through it, the hot point will shift along the wire in a direction contrary to that of the current.

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