Thermal conductivity

“ His data for the thermal conductivity of various metals were for long the most trustworthy at the disposal of physicists, and his determination of the ohm in terms of the specific resistance of mercury showed remarkable skill in quantitative research. ”

“ The ordinary thermal motions within any solid, liquid, or gas must cause occasional shootings out of electrons from the substance, and the motion of these electrons, under the influence of electrostatic force, must contribute to the electric conductivity of the gas; must, in fact, constitute all of it which was not due to transport of atoms of the gas carrying less than the neutralizing quantum of electrons. ”

“ Since, in a solid, the molecules are held in the same relative positions, when one end of a wire is heated the rapidly vibrating molecules at the hot end set their neighbors vibrating and these the next in turn and so on until the [Pg 174] whole wire is hot. It is a fortunate circumstance that different substances have different rates of conductivity for heat. To realize this, suppose that our clothing were as good a conductor as iron, clothing would then be very uncomfortable both in hot and in cold weather. The best conductors for heat are metals. ”

“ In fact, the conductivity of a pure metal for heat is almost independent of the temperature; while the electric conductivity varies in inverse proportion to the absolute temperature, so that a pure metal as it approaches the absolute zero of temperature tends to assume the character of a perfect conductor. ”

“ 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 ”

“ If the medium, instead of being a perfect insulator, is a conductor whose conductivity per unit of volume is , the disturbance will consist not only of electric displacements but of currents of conduction, in which electric energy is transformed into heat, so that the undulation is absorbed by the medium. ”

“ Heat is communicated to surrounding objects in three well-known ways—by conduction, by radiation, and by convection. Conduction of heat is easily understood, and is the propagation of heat through any body, and it varies very much according to the substance through which it passes. Some substances are better conductors of heat than others. Silver has a far greater conductivity than gold, and copper is a better heat-conductor than tin. Flannel is a non-conductor, or rather a bad conductor, for no substance can be termed actually a non-conductor. ”

“ The best conductors offer some resistance to the passage of the current and the best insulators conduct to some extent. If we make a comparison of electric conductors we find that the metals that conduct heat best also conduct electricity best. This, it seems to me, is a confirmation of the atomic theory of electricity so far as it means anything. If a good conductor, as silver, is subjected to intense cold by putting it into liquid air, its conductivity is greatly increased. It is well known that [Pg 48] heating a conductor ordinarily diminishes its power to conduct electricity. ”

“ As the temperature variation of conductivity is large, usually about 2% per degree, it is necessary to place the resistance cell in a paraffin or water bath, and to observe its temperature with some accuracy. ”

“ But as the conductivity persisted afterwards, even for the weakest thermo-electric currents, there is some ground for supposing that the insulating medium [Pg 107] is transformed by the passage of the current, and that certain actions, such as shock and rise of temperature, bring about a modification of this new state of the insulating body. Actual movement of the metallic particles cannot be imagined in experiments where the particles, in a layer a few millimetres thick, were fixed in an invariable relative position by extreme pressures, reaching at times to more than 100 kilogrammes per sq. ”

“ Heat produced by current when wire is too small. If a wire is made to convey a current which is too large for its electrical capacity, it will get heated, which decreases its conductivity, with the result that the heat increases until finally the wire fuses. ”

“ Extremely fine metallic powder, as a rule, offers almost perfect resistance to the passage of a current. But if we take a sufficiently short column and exert a sufficiently great pressure a point is soon reached when the electrical influence will effect a sudden increase in the conductivity. ”

“ 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. ”

“ The conductivity produced by the passage of cathode rays through a gas diminishes rapidly with the pressure. When rays pass through a gas at a low pressure, they are deflected by an electric field; when the pressure of the gas is higher the conductivity it acquires when the cathode rays pass through it is so large that the potential gradient cannot reach a sufficiently high value to produce an appreciable deflection. ”

“ The figures given as indicating the relative resistance of the above bodies to the passage of electricity must be taken as approximate only, since the conductivity of all these bodies varies very largely with their purity, and with the temperature. Metals become worse conductors when heated ”

“ When a stream of a material fluid passes along a tube from a hot part to a cold part it heats the tube, and when it passes from cold to hot it cools the tube, and these effects depend on the specific capacity for heat of the fluid. If we supposed electricity, whether positive or negative, to be a material fluid, we might measure its specific heat by the thermal effect on an unequally heated conductor. Now Thomson's experiments shew that positive electricity in copper and negative electricity in iron carry heat with them from hot to cold. ”

“ This gives an average value of the conductivity over the range, but it is better to observe the temperatures at three distances, and to assume k to be a linear function of the temperature, in which case the solution of the equation is still very simple, namely,
where e is the temperature-coefficient of the conductivity. The chief difficulty in this method lay in determining the effective distances of the bulbs of the thermometers from the axis of the cylinder, and in ensuring uniformity of flow of heat along different radii. ”

“ The heat conductivity of fluids, liquids and gases is very low, practically zero, but heat is transferred in them by convection. The particles in direct contact with the source of heat are heated above the temperature of the rest, and an increase in temperature of any liquid or gas invariably decreases its specific gravity and causes an upward current of the hot particles, which brings the colder particles in turn in contact with the hot body, thus maintaining a circulation which tends to raise the temperature of the entire mass. ”

“ We shall now extend this hypothesis by assuming that in gases also the property of conductivity is due to the presence of ions. Such ions may be supposed to exist in small numbers in all gases at the ordinary temperature and pressure; and as the temperature rises their numbers will increase. ”

“ The substances which up to the present have presented the greatest variations in conductivity are the powders or filings of metals. The enormous resistance offered by metal in a state of powder is well known; indeed, if we take a somewhat long column of very fine metallic powder the passage of the current is completely stopped. The increase in the electrical conductivity by pressure of powdered conducting substances is well known, and has had various practical applications. ”

“ As this process continues, heat moves toward those parts of the object that have a lower temperature. This flow of thermal energy as a result of changing molecular activity is heat conduction. Heat can be conducted from higher to lower temperature regions within an object, or between objects in contact when they are at different temperatures. The transfer of heat is accomplished without appreciable movement or displacement of the substances. Whenever heat is transferred through an opaque solid substance, the transfer must be by conduction. ”

“ The electrical conductivity of gases in the normal state is, as we have seen, exceedingly small, so small that the investigation of its properties is a matter of considerable difficulty; there are, however, many ways by which the electrical conductivity of a gas can be increased so greatly that the investigation becomes comparatively easy. ”

“ It has long been the custom of professional engineers to represent graphically on sheets of paper ruled in squares, various properties of matter under regularly-varying conditions. For instance, to show the electric conductivity of wire at various temperatures, horizontal strips represent degrees of temperature, and vertical strips, degrees of conductivity; and if a line connecting points corresponding to conductivity at various temperatures be convexly curved above, it shows that conductivity decreases more rapidly than temperature increases. ”

“ If we take two systems geometrically similar, and such that the conductivity for heat at any part of the first is proportional to the conductivity for electricity at the corresponding part of the second, and if we also make the temperature at any part of the first proportional to the electric potential at the corresponding point of the second, then the flow of heat across any area of the first will be proportional to the flow of electricity across the corresponding area of the second. ”

“ A transference of heat between the two currents of gas takes place by the conductivity of the fire-brick, and thus the outgoing gases are continuously cooled while the ingoing gases are heated—the transference of heat being somewhat similar to that which takes place in the surface condenser of a steam engine. ”

“ The modern theory tends to show simply that it must indeed be so. Calorific conductivity is due, in fact, to an exchange of electrons between the hot and the cold regions, the heated electrons having the greater velocity, and consequently the more considerable energy. ”

“ A second way in which the heat lost by a body has been measured at different temperatures is by heating a conductor such as a thin platinum strip by means of an electric current, and measuring the temperature to which the conductor has attained. When its temperature is steady, all the energy given to it by the current must be lost as heat, and therefore the electrical energy, which can very easily be calculated, must be equal to the heat radiated by the body minus the heat received from the enclosure. ”

“ Could the tubes be so highly exhausted that convection would be nil, then the relative amounts of heat given off by convection and radiation could be determined without the difficulties attending thermal quantitative measurements. If a source of electric impulses of high frequency and very high potential is employed, a still greater number of the tubes may be taken and the wires rendered incandescent by a current not capable of warming perceptibly a wire of the same size immersed in air at ordinary pressure, and conveying the energy to all of them. ”

“ The ultimate effect of slight heating (excess of heat produces other complications) is the restoration of the original reduced conductivity. If the application of heat transforms B into A, we may expect the substance to regain its sensitiveness, which it lost in the fatigued stage b. The receiver was now exposed to radiation, and it at once responded, exhibiting almost its original sensibility. Observe how the subsequent portion of the curve is a repetition of the curve No. 6, and how the substance arrives at the second fatigued state b′. To observe the ”

“ Change of temperature also influences conductivity.” Page 63, “When using induction machines, the moisture of the air often causes experiments to fail, especially before large audiences. The atmosphere becomes saturated with moisture, and it is often impossible to get the machine in working order.” Several desiccating devices are mentioned by the authors of this work, as used with such machines, to prevent such dissipation or conduction of electricity from the machine into space by the aqueous vapor of the atmosphere. ”

“ As the solid becomes a fluid, it loses almost entirely the power of conduction for heat, but gains in a high degree that for electricity; but as it reverts hack to the solid state, it gains the power of conducting heat, and loses that of conducting electricity. ”

“ Even when mixed with a large excess of other gases, its heat conductivity is about a hundred times that of hydrogen, and this will probably be increased many times when it is obtained pure. Taking the heat conductivity at this figure—a very moderate estimate—the mean molecular velocity of the new gas is calculated to be more than a hundred miles a second, and its density only a thousandth part that of hydrogen, while the specific heat is found to be six thousand times greater than that of hydrogen. ”