The first effect and lower the test tube into a beaker of hot water. is that the water in the small tube will drop to B. What is the cause of this? The second effect is that the water will then begin to rise and will finally run over the top C. Why? Empty the test tube, and let it become dry and cool. Introduce a short column of water into the middle of the small tube, hold it in a horizontal position, and push in the stopper as before. Clasp the test tube in the hand and watch the position of the water index. The above demonstrations show that solids, liquids, and gases expand on being heated. The amount of this expansion depends upon the change in temperature. Demonstration. - Place a piece of beeswax on a tin plate and hold it over a source of heat. What change in its physical condition is produced? This shows that when enough heat is applied, a body may change from a solid to a liquid. 246. Measurement of Temperature. The idea of the temperature of a body that we receive from our sensations is so dependent upon other things than the temperature, that it is frequently incorrect. What seems a high temperature to one person may seem a low temperature to another, and the same temperature seems different to us at one time from what it does at another. If one hand is wet and the other dry and both are held in the current of warm air coming from a register, the air will feel warm to the dry hand and cold to the wet hand. In this and similar ways we find that the body is not a good instrument with which to measure temperature. The instrument that is used for this purpose is called a thermometer. The principle employed is that of the unequal expansion of bodies when heated. The most common form is the mercury thermometer, which consists of a glass tube with thick walls and a small bore, blown into a bulb at one end for holding the mercury. The air is partly driven 247. Filling the Thermometer. from the bulb by heating it, the open end of the tube is put into mercury, and some of the mercury is driven into the bulb by the atmospheric pressure when the bulb cools. By repeating the process the bulb and tube are entirely filled. The mercury is then heated to a high temperature, and the tube is sealed at the top and left air-tight. 248. The Fixed Points. Since the boiling point and the freezing point of pure water are always the same under the same pressure, these points are taken as the fixed points for the thermometer. The freezing point is determined by placing the bulb and part of the stem in snow or finely crushed ice, contained in a suitable vessel (Fig. 222). The point at which the end of the mercury column comes to rest, when close to the ice, is marked as the freezing point. The boiling point is fixed by suspending the thermometer in the steam from boiling pure water. The bulb should be at least an inch above the water and the boiler should be tall enough so that the mercury will come only just above the stopper by which it is supported. Whenever the steam is coming briskly from the escape pipe and the mercury has ceased to rise, the end of the column is marked as the boiling point, provided the barometer reads 760 mm. at the time. FIG. 222 The manometer tube m (Fig. 223) shows whether the pressure of the steam is the same as that of the atmosphere. If this is not the case, a correction has to be made since the temperature of steam rises rapidly with an increase of pressure. When the pressure is near 760 mm., an increase of 27 mm. in the pressure produces a change of 1° C. in the boiling point. 249. Graduating the Scale.-The bore of the thermometer tube should be of uniform diameter throughout its length. At ordinary temperatures, equal increases of temperature will then cause practically equal amounts of elongation of the mercury column in any part of the tube; hence, when the kind of scale has been decided upon, the length between the freezing and boiling points is divided into equal parts, called degrees. FIG. 223 250. Thermometric Scales. The scales in most general use in this country are the Centigrade, or hundred-degree scale of Celsius, which makes the freezing point of water zero (0°) and its boiling point 100°; and the Fahrenheit, which makes the freezing point 32°, the boiling point 212°, and puts the zero 32° below the freezing point. In both scales, readings that are below zero are designated by the minus sign, as -10° C. The Fahrenheit scale is the one in common use in the United States, but the Centigrade has been adopted for scientific work on account of its greater convenience. Unless otherwise mentioned, the Centigrade scale will be used in this work. 212 FAHRENHEIT 32° 100° = 251. Comparison of Centigrade and Fahrenheit Readings. Since the Centigrade scale has 100 degrees between the fixed points, and the Fahrenheit 180, it is evident that 100 C. degrees = 180 F. degrees, and hence 1 C. degree F. degree, and 1 F. degree = C. degree. C. degree. When the reading of one scale is to be transformed into the equivalent reading on the other, the differing positions of the zero point must be taken into account, as well as the different values of the degrees. These formulas may be used: CENTIGRADE FIG. 224 -40° 100 The relation between the readings of these scales can be seen by reference to Fig. 224. Readings between the fixed points are + in both scales, but while all readings below the freezing point on the Centigrade are those between 32° and 0° on the Fahrenheit are +. The glass 252. Limitations of Mercury Thermometers. bulb of a thermometer may gradually change in volume. There may also be a temporary change after use for high temperatures in which the bulb returns slowly to its original volume. A thermometer, therefore, should be frequently tested to determine what correction is necessary in reading. For very low temperatures alcohol is used instead of mercury, which freezes at 39° C. The ordinary form of mercury thermometer cannot be used for temperatures above 350° C., since its boiling point is 357° C. When the space above the mercury column, however, is filled with nitrogen under pres sure, it can be used for temperatures up to 550° C. For still higher temperatures the air thermometer or the electric pyrometer is used. 253. The Air Thermometer. A simple air thermometer (Fig. 225) can be made by thrusting the tube of an air thermometer bulb through a rubber stopper with two holes, and fitting this stopper to a test tube or bottle nearly full of colored water. A scale along the side is used for reading the height of the water column, which is introduced by driving out a few bubbles of air from the bulb by heating it. When the air cools, the water will rise in the stem (Fig. 225). The position of the water column is affected by the varying pressure of the air as well as by the tem perature, so that it FIG. 225 254. Metallic Thermometers.-A compound bar made by riveting together two thin strips of brass and iron will, when heated, form an arc of a circle, with the brass on the outside. This unequal expansion of two metals is the regulating principle in the metallic thermometer. A convenient form is made by fastening the two metals together in the form of a spiral spring, having on the outside the one with the greater rate of ex FIG. 226 |