Thermal action of electric current. Electro-motoric action of heat
What work can a
current perform? It can be mechanical, that is, it can move tangible masses.
However, not the entire amount of E·I
erg can be converted into mechanical work; experience tells
us that every conductor through which flows electricity heats up
(Joule heat). Let us assume that we can convert the
entire working capability of a
current into heat: How many
calories would arise? The answer follows immediately from the preceding work. 1 Joule is equivalent to 0.24 cal, E·I·t
Joule are equivalent to the quantity of heat Q=0.24·EI·t
cal, that is, a current of EI Watt develops during one
second 0.24·EI cal, provided its entire energy becomes heat.
Using I = E/W, the equation Q = 0.24·EI·t cal (Joule ) can be rewritten Q = 0.24 I²·W·t, where W is the resistance of the entire circuit. This equation also applies to every separate section of a circuit. Let there be given a conductor of length L, through which flows the current I and which has the resistance W. If we subdivide it into n parts l1 ··· ln with resistances w1 ··· wn, then W = w1 + w2 + · · · + wn, that is
c·I²·W=c·I²(w1+w2+···+wn)=c·I²w1+ c·I²w2+··· + c·I²wn.
For the length li with the resistance wi, c·I²wi states the same as c·I²W does for the conduct L with the resistance W - the heat developed in li. Hence, the heat developed in L distributes itself on the sections l corresponding to their resistances. If each of the sections l1 to ln have the same resistance, the same amount of heat is developed in each of them. If the entire conductor consists of the same material and has everywhere the same cross-section, then equal sections have the same resistance. Such a conductor is called homogeneous. Hence in a homogeneous conductor the heat is distributed uniformly.
However, you deal very rarely with homogeneous conductors. For example, if you want to convert current, which comes from a distant generator, into another form of energy, you aim to avoid conversion into heat in the transmission lines to the user. Hence these cables are made out of a well conducting material (copper), have large cross-sections and are as short as possible; for example, this applies to the transmission lines between power stations and consumers.
Th same applies to telegraph lines: You only want to convert the current into mechanical energy at the receiving stations and not into heat on the way . Formerly, the cables were made out of cheap iron, however, since iron does not conduct well, they are now made out of copper with correspondingly large cross-sections.
Electric lighting depends on the development of heat by a current. The essential part of every lamp is a section which heats up while current passes through it and hence emits light. In incandescent lamps (Fig. 479), it is a very thin filament, in former times made out of carbonized cellulose, now out of a metal which is hard to melt (tungsten); in order to protect it from combustion, you enclose it in a glass bulb and create in it a vacuum or fill it with a neutral gas. In Nernst lamps (Fig. 480), the conductor is a small rod made out of rare earths (circonium oxide and yterbium earths). It must be heated in order to conduct; a flame serves this purpose, but an electrically driven heating component is employed, a platinum wire S covered with caoline which surrounds the Nernst rod. In arc lamps, the conductor consists of two, almost one another touching ends of two carbon rods (Fig. 481) and a bridge of incandescent carbon particles linking them. The bridge forms as follows: If no current flows, the carbon rods touch one another; the current separates them (by means of a mechanism it drives). While it does so, it is not cut off, but generates between the carbon rods by heating and vapourization of the carbon an incandescent layer of gas in the form of a lunar crescent - the light arc (whence arises the term arc lamp). Conduction is maintained by the strongly heated and ionized air between the carbon rods. The light comes from the white-hot carbon tips, especially (when direct current is used) from the upper positive carbon, which forms a crater (Fig. 481).
In electric lamps, the heat is only the means of their objective. All improvements aim to create as much light as possible while as little as possible heat is generated. In contrast, generation of heat is the purpose of electric heaters. In essence, they are like incandescent lamps or arc lamps. The first method is used in the heating part of the Nernst lamp; a spiral (S in Fig. 480), made out of very thin platinum wire, surrounds the incandescent part and is heated by the current until it glows; clothes flat-irons, cooking stoves, etc. are heated by installed thin wires. The second method, when an arc of light becomes the source of heat, is employed in the oven of Fig. 482, which is widely used in Metallurgy. The material is placed between the ends of the carbon rods A and B and is exposed to the temperature of several thousand degrees of the light arc.
Electric energy can be converted into heat; conversely, under certain conditions, heat can act directly electro-motively (Thomas Johann Seebeck 1770-1831 1821). Electricity, which arises directly from heat, is called thermoelectricity. How originate thermoelectric currents? You connect two wires made out of different materials, for example, iron and copper, by twisting their ends together (Fig. 483) and heat the joint A with a flame to a temperature different from that of the joint B (room temperature). Then an electric current will flow through the hot joint in the direction (arrow) from copper to iron. If you heat B and keep A at the lower temperature, the current flows in the opposite direction, that is, again through the hot joint from copper to iron.
You demonstrate this with the apparatus of Fig. 484. AB is a bismuth rod, CD a copper link, in between the two is a lightly movable magnet needle. If you heat the joint DB, the needle rotates - a proof that it is surrounded by an electric current. If you heat CA, the needle rotates in the opposite direction - a proof that the current now travels in the opposite direction. The increase in temperature first generates an EMF and the current depends on its magnitude and the resistance of the circuit.
If you heat both joints (Fig. 483 A and B) to the same temperature, no current is generated; obviously there arises then at each joint an EMF of equal magnitude, but oppositely directed. - You can also connect more than two metals, for example, copper, iron and antimony (Fig. 485); if all joints have the same temperature, no current arises. Obviously, the EMF at AC is equally large and opposite to the sum of the two others, that is, AC = AB + BC.
If you send a current through a bismuth-antimony circuit, there occurs - in addition to the conductor heating in accordance with its resistance - at the soldering joint a special heat action, proportional to the duration of the passage of current as well as to the current intensity (Jean Charles Athanase Peltier 1785-1845 1834). Depending on the direction of the current, it is generation or consumption of heat: Generation, if the current direction is opposite to the thermo-current (which arises during external heating of the soldering joint), consumption (that is, cooling of the soldering joint), if the current direction coincides with that of the thermo-current. You can demonstrate the Peltier effect with an instrument, similar to the air thermometer (Fig. 467) by replacing the wire by a bismuth-antimony strip.
If you heat simultaneously two joints to the same temperature (Fig. 485) and keep the third at room temperature, there arises the same EMF as if the metal between the two equally warm ones is not present, that is, as if the two equally warm joints are only a single joint. This is the reason why you can solder two metals by a third one: their ends and the joining solder have the same temperature. This is what happens in a thermo-element. Fig. 486 displays one made out of the metals M1 and M2. Their end points A are connected directly (principal soldering joint), their end points B and C (accessory soldering joint) in a round about way via the external circuit S. However, if both are kept at the same temperature, they behave as if they were directly interconnected.
The elctromotoric force of a single thermo-element is very small (the most efficient pair, antimony-bismuth, yields at a 100ºC temperature difference 0.01 Volt); however, it increases almost proportional to the temperature difference, but it drops again beyond a certain value and eventually changes its direction. Very high temperatures demand therefore other pairs of metals. For temperatures between - 200º and + 600º, you employ mainly the constantan-iron element (constantan is a copper-nickel-alloy), up to 1500º platinum platinum-rhodium alloy (10% Rh).
If you interconnect several thermo-elements as in Fig. 487, it becomes a thermo-column yielding a larger EMF. If you heat all corners of one side of the pile, while the other side is kept at room temperature or even cooled, the electro-motoric force at the free ends of the column equals that of the sum of the individual elements. While the thermo-column converts heat energy directly into that of electric current, it is useless as a source of current, even if its number of thermo-elements is very large.
However, thermo-elements and -columns are valuable as thermometers; the soldering joint of the thermo-element (Fig. 486) is exposed to the temperature to be measured (not burning gases!), the soldering joints B and C are kept at room temperature. The feeding wires are connected to a current intensity measuring device (galvanometer). With the aid of known temperatures, known melting points, boiling points, etc., you can determine which voltages belong to given temperatures, and can, by dipping the soldering joint, for example, into a melting metal, determine from the observed voltage the temperature of the joint, that is the melting temperature.
One main advantage of the thermo-element is: Its soldered joint can be taken to otherwise inaccessible locations (for example, into small openings) and its heat capacity is small. For many purposes, a single element is sufficient (for example, in the pyrometer for porcelain ovens, regeneration ovens of glass factories, etc.); it is mostly made out of platinum and platinum-rhodium (Le Chatelier). Other purposes demand a column; that of Heinrich Rubens 1865-1022 out of iron and constantan is employed frequently (Fig. 489). The soldering joints of their thermo-electrically active wires lie along a straight line, so that you can let them coincide, for example, with a linear heat source (important for measurements of radiation in spectra). Their heat capacity is so small that the galvanometer adjusts itself immediately.
