Thermometry

“ No indication is afforded by the thermometer of the absolute quantity of heat contained in any substance, but merely of the amount of free or sensible heat capable of producing a certain degree of expansion in a column of mercury. If a quantity of ice, at the temperature of zero, or 0°, be placed in a warm room, it will immediately begin to melt, and a thermometer plunged into it will soon indicate 32°, though at first the column of mercury stood at zero. But, strange to say, the mercury will remain stationary at the freezing-point until the whole of the ice has passed into the liquid form. ”

“ Also one can determine how much colder one day is than another, noting how many degrees the water ascends and descends. One can compare the greatest heat and cold of one year with those of another year. One can ascertain how much hotter one room is than another; one can maintain a room at an equal temperature by making the water of the thermometer stand always at a certain degree. One can test also the intensity of fevers; in short, one can know pretty nearly to what extent air is rarefied in the greatest heat, and so forth. ”

“ Solids, liquids, and gases expand as they are heated. This great general law is, however, subject to a few exceptions, the most remarkable of which is that presented by water. Let us suppose a simple experiment. Imagine a thermometer tube with its bulb and stem so filled with water that when the water is heated nearly to its boiling point it will rise to nearly the top of the long stem. Now let us cool it. As the cooling proceeds the water will descend, and this descending will continue until it attains the temperature marked on our ordinary thermometer as 39°, or more strictly 39-2/10 ”

“ But our test would not do for all liquids, because there are some that do not answer readily to it, but are extremely sluggish in the neighbourhood of their melting points, so that they seem almost solid even when liquid. We are obliged, then, to look for a better test, and we should probably observe that the most convenient would be found in the fact that an addition of heat produces a change in temperature in a body that has passed its melting point. Place a thermometer in melting snow, it marks zero until the snow is really melted, and after that it rises. ”

“ Thermometers.—It is the expansion of liquids by heat that in the thermometer gives us the measure of temperature. The liquid metal mercury is commonly used for this purpose, and answers well except in the extreme cold of the arctic regions. There, as mercury becomes solid at 39 degrees below zero, it is necessary to use a thermometer with alcohol in it, as this fluid can not be frozen by any degree of cold. The operation of the thermometer is simply this: Heat expands the fluid in the bulb, and the only way in which it can occupy more space as it expands is by rising in the tube. ”

“ The Thermometer is an instrument to indicate the temperature of the atmosphere. It is constructed on the principle that heat expands and cold contracts most substances. The thermometer consists of a capillary tube, closed at the top and terminating downwards in a bulb. It is filled with mercury which expands and fills the whole length of the tube or contracts altogether into the bulb, according to the degree of heat or cold to which it is exposed. Any other fluid which is expanded19 by heat and contracted by cold, may be used instead of mercury. ”

“ Now nothing is easier than to use the thermometer. You place it in the situation where you want to measure the heat, and the mercury goes up or down of itself until it reaches the degree which corresponds with the temperature of the place. It is much more convenient than your mamma's yard measure, which has to be moved about over the stuff, and which is very apt to slip if you do not hold it carefully. Dressmakers would be delighted to have a measure which only wanted laying upon the material, and which would unroll itself and stop short just at the proper point. ”

“ In the Réaumur scale, still largely used in Russia and Germany, the freezing-point is marked zero, and the space between this and boiling-point is divided into 80 degrees. To reduce it to Fahrenheit, multiply by 21⁄4 and add 32; to Centigrade, increase the number by one-fourth of itself. Thus: F = 9⁄5 C + 32 = 9⁄4 R + 32; C = 5⁄9 (F - 32) = 5⁄4 R; R = 4⁄9 (F - 32) = 4⁄5 C.—Maximum thermometer, one that registers the maximum temperature to which it is exposed; Minimum thermometer, one that registers the minimum temperature to which it is exposed. [Gr. thermē, heat, metron, a measure.] ”

“ The essential feature of the minimum thermometer is a small black index within the bore and also within the liquid. As a lowering temperature contracts the liquid column, the cohesion of its surface drags the index toward the bulb. When the liquid expands, however, it flows around the index without moving it. The index therefore shows the lowest temperature between settings. The minimum temperature is read at the end of the index farthest from the bulb. The temperature may be read from the minimum thermometer at any time, reading from the end of the column of alcohol, and not from the index. ”

“ I suspected that you would make these objections; and, in order to remove them, I enclosed two thermometers in the air-pump; one which stands in the glass of water, the other in the phial of ether; and you may see that the equilibrium of temperature is not destroyed; for as the thermometer descends in the water, that in the ether sinks in the same manner; so that both thermometers indicate the same temperature, though one of them is in a boiling, the other in a freezing liquid. ”

“ For in order to be used in this form as real thermometers, they must be exposed on each occasion first of all to some zero level of temperature, say, that of melting ice. If we then take them into the region of temperature we want to measure, we shall discern the difference of levels through the corresponding movement of the column. The final position of the column tells us nothing in itself. It is always the change from one level to another that the thermometer registers - precisely as does the sense of warmth in our hands in the experiment just described. ”

“ Also in different substances, when certain temperatures are attained by the application of fire or other methods of heating, they undergo a change of form. Solids, at certain temperatures, are converted into liquids; and liquids, in like manner, when heated to certain degrees, become aeriform fluids or gases. These changes are familiar to every one in the ordinary phenomena attending water. Below the temperature of 32° of the common thermometer, that substance exists in the solid form, and is called ice. Above that temperature, it passes into the liquid state, and is called water ”

“ The cheap German paper-scale thermometers are largely used, fitted in wooden cases, as dairy thermometers, and a larger size for brewing purposes. Alarm thermometers are often employed, in which electric contact is made and a bell rung when the temperature exceeds or falls short of a certain limit. Thermostats of various forms are made use of, in which a thermometer, by the position of the mercury in the stem, regulates the gas-supply of a burner and thus the heat of an enclosure. ”

“ The Florentine thermometers had one fixed point, that of freezing water, and contained either spirits or mercury. In 1724 Fahrenheit, in Danzig, fixed three points on the scale of the mercurial thermometer, viz. the cold produced by ice and sal-ammoniac which he called 0°, freezing water or 32°, and the heat of the human blood which he assumed to be 96°. This thermometric scale, having 180° between freezing and boiling water, and that of Celsius, with 100°, are the only ones in scientific use to-day. ”

“ THE thermometer (Tab. XI. fig. 1.) is designed for shewing the greatest degree of heat, which happens in any place during the absence of the observer. It consists of a cylinder of glass joined to a tube, and differs from common thermometers only in having the top of the stem drawn out into a capillary tube, which enters into a glass ball C, joined on to the stem at the place where it begins to be contracted. The cylinder, and part of the tube, are filled with mercury; the top of which shews the common degrees of heat as usual. ”

“ The tube is fastened to a thermometer by means of platinum wire, and then the bulb of the thermometer, with its attached tube, is immersed in strong sulphuric acid or paraffin, contained in a flask. The thermometer should be suspended midway in the liquid and heat carefully applied, so as to raise the temperature gradually and equably. It will be found that rapidly raising the heat gives a different melting-point to that which is obtained by slowly raising the heat. During the process careful watching is necessary: most substances change in hue before they actually melt. ”

“ Quantitative measurements in temperature, based upon the calorie, have a definite place in physics; and those based on the British thermal unit have a broad application in various economies. Human sensitiveness to heat does not pertain to quantity but to intensity. A large block of ice at 30° may contain more heat, quantitatively considered, than a red-hot horseshoe. If carried into a room whose air was far below freezing, the ice might warm the room to a greater degree than would the horse-shoe. ”

“ Few are aware of the errors found to exist even in the thermometers of reputable makers. The well-known change which takes place with age in every thermometer not infrequently amounts to a degree and a half Fahrenheit within two years from the time the thermometer is made. The change depending upon the temperature to which a thermometer is heated, even supposing this to be no greater than the boiling-point of water, may be three fourths of a degree. ”

“ Any change of temperature displaces the centre of gravity of the system, and the instrument rotates through an arc, moving a pencil as it does so. A perfectly continuous record is produced by the photographic thermograph. Wet and dry bulb thermometers are so arranged that a beam of light passes through an air-speck, which separates part of the mercury thread, or through the vacant part of the tube, and falls on a rotating cylinder covered with photographic paper on which it traces the curve of temperature fluctuation. ”

“ If we push the process still further, with a sufficient application of energy we can separate the vapor molecules into their original elements, oxygen and hydrogen.
Let us go back now to the freezing point of water and see what is becoming of the heat that is consumed in melting the cake of ice, and still does not produce any effect upon the mercury in the thermometer. Sensible heat, [Pg 183] as before stated, is a movement of the atoms of matter, and temperature, as it affects the thermometer, is a measure of the intensity of motion exhibited by these atoms. ”

“ Each degree rise of temperature in a mass of water would then represent the addition of the same quantity of heat. The scale thus obtained would, as a matter of fact, agree very closely with that of a mercury thermometer, but the method would be very difficult to put in practice, and would still have the disadvantage of depending on the properties of a particular liquid, namely, water, which is known to behave in an anomalous manner in other respects. ”

“ The evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. In the same manner, then, the covered wet bulb thermometer will be found invariably to read lower than the uncovered one; and the greater the dryness of the air, the greater will be the difference [Pg 25] between the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike. ”

“ We see that the sensation or feeling of warmth is not an accurate guide to the temperature of a substance; and yet until 1592, one hundred years after the discovery of America, people relied solely upon their sensations for the measurement of temperature. Very hot substances cannot be touched without injury, and hence inconvenience as well as the necessity for accuracy led to the invention of the thermometer, an instrument whose operation depends upon the fact that most substances expand when heated and contract when cooled. ”

“ If a mixture of A and B be melted and then allowed to cool, a thermometer immersed in the mixture will indicate a gradually falling temperature. But when solidification commences, the thermometer will cease to fall, it may even rise slightly, and the temperature will remain almost constant for a short time. ”

“ In regard to the warmth of air, the thermometer is not an exact index of its temperature. For all bodies are constantly radiating their heat to cooler adjacent surfaces until all come to the same temperature. This being so, the thermometer is radiating its heat to walls and surrounding objects, in addition to what is subtracted by the air that surrounds it, and thus the air is really several degrees warmer than the thermometer indicates. A room at 70° by the thermometer is usually filled with air five or more degrees warmer than this. ”

“ The making of accurate thermometers is a task attended with many difficulties, the principal one being the liability of the zero or freezing point varying constantly, so much so, that a thermometer that is perfectly correct to-day, if immersed in boiling water, will be no longer accurate ”