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Thermo-Electricity Experiments


By Thomas W. Benson





It was first noted by Seebeck, of Berlin, that when a juncture of dissimilar metals was heated a current of electricity would flow. This phenomena was studied by dimming, Sturgeon, Pellaci and many others, for it gave promise of providing a means for developing electricity direct from heat, without the use of steam engines and generators.

Altho no practical application has been made of the discovery, beyond its application to pyrometry for heat measurements, several interesting devices can be easily constructed to demonstrate the phenomena.

The most powerful currents are generated by heating junctures of the more crystalline metals, such as antimony and bismuth, the other metals lying between these in regard to the strength of the current generated with a given rise in temperature.

The following list gives the thermo-electric order of the more common metals:

Bismuth
Platinum
Mercury
Lead
Tin
Gold
Silver
Copper
Zinc
Iron
Antimony

Any two of these metals when joined together and the junction heated results in a current flowing thru the exterior circuit from the metal highest on the list to that lower. The reverse is true should the junction be cooled. The further apart the metals are in the table the greater will be the electromotive force.

It is seen that bismuth and antimony will give a so-called "couple" having the highest E.M.F. These are rather difficult to obtain and handle, so for experimental purposes we will substitute iron and German- silver wire. These two metals are employed in the construction of thermocouples for electric pyrometers, and will serve our purpose admirably.

The electro-dynamic effect of heat generated currents can be shown by a device similar to that illustrated in Fig. 1. Three sides of the wire rectangle are composed of iron, the fourth of German-silver. The frame is suspended over a permanent magnet and heat applied to one corner as shown. The frame will turn thru an angle of 90 due to the currents flowing in the loop.

A large number of turns of wire could be made to form a heat compass. The details of such a device are shown in Fig. 2. A small bundle of soft iron wire is bound with paper glued into place. Take a number of lengths of iron and German-silver wire and wrap them in the same direction on the core, in alternate coils as shown. Twist the ends of the wire together so that every other joint is on top.

The ends of the more distant coils being connected together by a loop of wire, thus forming a sling to support the compass. When this apparatus is suspended over a hot copper or brass plate, the currents flowing around the windings will cause it to act as a compass and the core will place its axis in a line with the natural magnetic field of the earth.

A form of thermo-electric motor can be constructed as shown in Fig. 3, to demonstrate electro-magnetic rotation. Here we have a ring of iron wire attached to which are a number of vertical German-silver wires, bent over at the top and joined at the center to a pivot. The latter rests in a tiny depression in the pole of a horseshoe magnet, allowing the cage to turn freely.

By applying a small flame to the frame at the juncture of the G.S. and iron wires between the legs of the magnet a current will flow thru the vertical wires and rotation will result from magnetic repulsion. The device will attain quite a high speed if carefully constructed.

A thermo-battery can be cheaply made that will deliver a fairly heavy current in the following manner:" Take" a 1" pine board 6" square and drill with small holes at the intersection of lines drawn in both directions 1/4" apart. The board is then impregnated by placing in melted paraffin. Alternate 2" lengths of iron and G.S. wire are pushed into the holes and their ends tightly twisted together on both sides of the board, the end wires being led to binding posts.

When the battery is stood with one side in the sun, the other being shaded, an appreciable E.M.F. will be generated. If laid on a block of ice in the sun it will ring a vibrating bell!

A somewhat similar form of thermo-pile, comprising antimony and bismuth bars, in the hands of Forbes and Milloni led to the beautiful discovery of the polarization of heat.

Andrews noted that a current was generated when platinum wires are inserted in a bead of fused borax, potassium chlorid, chlorids of potassium and strontium, iodid of potassium, sulfate of soda, and even when boracic acid is used. These results, however, are due to a thermo-chemical action and formed the basis for the work of Edison and others on the heat cells. The action differs from the true thermic action of dissimilar metals.

Conversely we have electro-thermic effects whereby heat and cold result from an electric current passing between dissimilar metals. The production of cold by this means is rather startling and is known as the Peltier effect.

Apparatus to demonstrate the latter effect can be assembled as shown in Fig. 5. A large thistle tube has connected to its lower end a U-tube with a small bore, by means of a length of rubber tubing. The U-tube is partly filled with colored water.

A cork that fits the mouth of the thistle tube has six holes bored thru it into which are inserted short lengths of iron and G.S. wire, about five strands of wire being put in each hole. These are twisted together to form thermo-couples and the cork inserted in the mouth of the thistle tube. A little paraffin wax melted over the cork will seal the whole effectively.

When the positive and negative poles of a battery (gravity type preferred) are connected to the iron and G.S. wire respectively, the junctures inside the tube will drop in temperature and the air contracting will cause the water in the open leg of the U-tube to drop. For best results the current should be regulated by a rheostat so that it is not sufficient to heat the wire used in the couples.

A current having a fairly high E.M.F. and low amperage gives the best results. The exact cause of the current generated in a thermo-couple is still an unsolved problem. It is not due to any oxidization or chemical effect for it has been found that a bar of the same metal with different temperature of its opposite ends will give a slight current, the cold portions becoming negative, the hot parts positive. A thoro understanding of the phenomena demands more complete knowledge of matter structure and the inter-atomic forces.

A fuller realization of the forces in action may revolutionize present methods of current generation and perhaps refrigeration. Thus there is a very fine field open for experimentation by the industrious student. By experiment we learn many things here-tofore undreamt of.



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