Heat conduction

“ In measuring conduction of heat in fluids, it is possible to some extent to eliminate the effects of molar convection or mixing, but it would not be possible to distinguish between diffusion, or internal radiation, and conduction. Some writers have supposed that the ultimate atoms are conductors, and that heat is transferred through them when they are in contact. This, however, is merely transferring the properties of matter in bulk to its molecules. It is much more probable that heat is really the kinetic energy of motion of the molecules, and is passed on from one to another by collisions. ”

“ As already pointed out, the combined effect will be nearly proportional to the excess of temperature in any given case provided that the excess is small, but it is not necessarily proportional to the extent of surface exposed except in the case of pure radiation. The rate of loss by convection and conduction varies greatly with the form of the surface, and, unless the enclosure is very large compared with the cooling body, the effect depends also on the size and form of the enclosure. Heat is necessarily communicated from the cooling body to the layer of gas in contact with it by conduction. ”

“ The disadvantage is that the thermo-electric force is very small, about ten-millionths of a volt per degree, so that a small accidental disturbance may produce a serious error with a difference of temperature of only 1° between the junctions. The chief uncertainty in applying this method appears to have arisen from variations of temperature at different parts of the surface, due to inequalities in the heating or cooling effect of the stream of water flowing over the surfaces. Uniformity of temperature could only be secured by using a high velocity of flow, or violent stirring. ”

“ Hence, substances that are put around hot bodies to retard the change of temperature are often called indiscriminately non-conductors, though in fact they may act partly by preventing convection or by intercepting radiation. Practically, indeed, it is of little consequence to decide exactly how the loss of heat is prevented, but, in the full study of retentive coverings, we must not altogether lose sight of the distinction between mere conduction and general transmission. ”

“ If the heat were to be applied in either of the above cases at the lower part of the tube the result would be different, because convection would then operate in the diffusion of the heat.
302. Air as a Non-Conductor.—Heat is rapidly diffused in air by convection; but it is only when the air is free that this can be done. When the air is confined in spaces or pores, or among fibres, heat makes its way through it very slowly, for it can be diffused through it then only by conduction. The variety of ways in which air is of service to us as a non-conductor is almost endless. ”

“ Loss of heat by radiation and conduction to the air will be the greater as the flame exposes a larger surface, and as a more rapid current of cold air is brought into proximity with the flame. Loss of heat by conduction, into the burner will be the greater as the material of which the burner is constructed is a better conductor of heat, and as the mass of material in that burner is larger. Loss of heat by passage into the combustion products will also be greater as these products are more voluminous ”

“ Conduction is the transfer of heat by molecular activity In Heat—Part I, we learned that when a substance is heated it absorbs thermal energy or heat, and the molecular activity within the substance increases. The increase in molecular activity is accompanied by an increase in temperature. If only part of an object is heated, the molecular activity and temperature are increased only in that part at first. ”

“ How Heat Moves through or Is Transmitted by Matter. Heat passes by contact from the warmer to the colder molecules of a body. This action is called conduction. When one end of a bar of iron is held in a fire, the end away from the fire soon becomes too hot to hold in the hand, because heat is rapidly transferred from the hot portion of the bar to the cooler portion by conduction, showing that iron is a good conductor. ”

“ Both in liquids and gases the effects of convection currents are so much greater than those of diffusion or conduction that the latter are very difficult to measure, and, except in special cases, comparatively unimportant as affecting the transference of heat. Owing to the difficulty of eliminating the effects of radiation and convection, the results obtained for the conductivities of liquids are somewhat discordant, and there is in most cases great uncertainty whether the conductivity increases or diminishes with rise of temperature. ”

“ At higher pressures the effect of conduction was masked by convection currents. The question of the variation of conductivity with temperature is more difficult. If the effects depended merely on the velocity of translation of the molecules, both conductivity and viscosity should increase directly as the square root of the absolute temperature ”

“ Indeed, a dense mass of gas presents to the conduction of heat, which represents momentum, a wall far more impenetrable than the surrounding glass, and the diffusion of heat is almost wholly brought about by convection currents which rise from the heated surfaces. It will thus be seen that the great non-conducting power of air comes into play to prevent not only the transfer of momentum from the vanes to the glass, but also, almost entirely, any direct transfer to the surrounding mass of gas. ”

“ Thus the air, being at 90 degrees, will rise, if compressed to a certain quantity, to 120 degrees; if it be kept in this compressed and confined state until all the extra 30 degrees of heat have been conveyed away by radiation and conduction, and the air be then allowed to escape, it will be found, on issuing, to be of 60 degrees of temperature. If a cooler be formed by a pipe under water, and air be forced in under a given compression at one end, and be made to pass along to the other, it may thereby, if the cooler be sufficiently extensive, be robbed of all its heat of compression ”

“ Similarly, fuels in hot coals and ashes can be moved, or the hot material can be scraped away to deprive them of heat.
Water is highly efficient as a cooling agent because of its great heat-absorbing capacity, in both liquid and vapor form. ”

“ A very heavy electric current is passed through the spot or area which it is desired to soften, so that, notwithstanding the rapid conduction of heat into the body of the plate, the metal is brought to a low red heat. In order that the spot shall not reharden, it is requisite that the rate of cooling shall be slower than when the heating current is cut off suddenly, the current therefore undergoes gradual diminution, under control of the operator. ”

“ A fluid, either a liquid or a gas, transmits heat mainly by the method called convection.
c. Space transmits the energy of hot objects by the method called radiation.
Fig. 130.—Solids conduct heat.
151. Conduction.—To illustrate conduction, place in a gas flame the ends of same metal wires supported as in Fig. 130. In a short time the other ends of the wires become hot enough to burn one's hand. This may be explained as follows: The hot gas flame contains molecules in violent vibration and those striking the wire set its molecules rapidly vibrating. ”

“ Heat is distributed by radiation, conduction, and convection. By the first we mean that heated bodies have the power of projecting from themselves, by means of the ether, their own vibrations. Thus the sun is constantly distributing its light and heat in all directions. Conduction takes place where the molecules of a substance nearest a fire first become heated and then impart their motion to the remainder of the mass, [326] somewhat as in a row of suspended ivory balls, the first of which, when struck, transmits its motion from ball to ball, the last one flying off. ”

“ The steam pressure in the heater may be periodically varied by the gauge in such a manner as to produce an approximately simple harmonic oscillation of temperature at the hot end, while the cool end is kept at a steady temperature. ”

“ Moreover, as heat engines are imperfect there is heat rejected to the surroundings by conduction, and produced by work done against friction, so that the heat thrown on the unavailable or waste heap is still further increased. ”

“ Condensation, whether of air by pressure, or of metals by percussion, is another powerful source of heat. Thus, iron may be so dexterously hammered as to become red-hot, and the rapid condensation of a confined portion of air will set tinder on fire.
(348.) The most violent heats known are produced314 by the concentration of the solar rays by burning glasses,—by the combustion of oxygen and hydrogen gases mixed in the exact proportion in which they combine to produce water,—and by the discharge of a continued and copious current of electricity through a small conductor. ”

“ With these the heating is accomplished entirely by convection, while with direct radiators the radiant heat rarely amounts to 40 per cent., generally not over 30, and often in practice considerably less. For a radiator in any particular location the radiant heat is constant for the same conditions of temperature of the radiator and surrounding objects, and is independent of currents of air. ”

“ The vapor of water employed in an inverse manner to that in which it is used in steam-engines can also be regarded as a means of destroying the equilibrium of the caloric. To be convinced of this we need but to observe closely the manner in which motive power is developed by the action of heat on vapor of water. Imagine two bodies A and B, kept each at a constant temperature, that of A being higher than that of B. These two bodies, to which we can give or from which we can remove the heat without causing their temperatures to vary, exercise the functions of two unlimited reservoirs of caloric. ”

“ Let us suppose now that, instead of heating one degree the air subjected to a constant pressure and able to dilate freely, we inclose it within an invariable space, and that in this condition we cause it to rise one degree in temperature. The air thus heated one degree will differ from the air compressed only by its greater volume. So then the quantity of heat that the air would set free by a reduction of volume of is equal to that which would be required to raise it one degree Centigrade under constant volume. ”