CHAPTER XI

TRANSMISSION OF HEAT-CONVECTION

78. ON account of their low conductivity, fluids (¿.e. liquids and gases) are only warmed very slowly when the heat is applied from on top. But they soon get warm when the heat is applied from below, as when water is heated in a kettle by placing it on a fire, or when air is heated

by contact with warmer soil.

The parts

which are first heated expand, and being thus rendered lighter 1 they ascend: their place is taken by colder parts of the fluid, which, in turn, are heated and also ascend. There is thus produced in the fluid an upward current which carries heat with it. This mode of transmission of heat is called convection, and may be defined as follows:

Convection is the transmission of heat by actual motion of the parts of a heated fluid.

It is clear that convection-currents can only be produced in liquids and gases, for the parts of a solid cannot move about in the manner described.

79. Convection in Liquids.-The convection of heat in liquids may be illustrated by the following experiments.

Fig. 50.

EXPT. 56.-Fill a round-bottomed flask with water, and

1 See Expt. 12, p. 25.

drop into it some crystals of magenta dye. Heat the flask over a small flame (smaller than that shown in the figure). The water just above the heated spot ascends, and its place is taken by colder water from the sides. A current of hot (and coloured) water rises up the middle of the flask, spreads out on top, and then works its way down the colder sides as indicated by the arrows.

The direction of the currents may also be shown by throwing in sawdust.

EXPT. 57.

With the aid of a lantern convection-currents in water may be shown by projection on a screen.

The water (slightly warmed) is placed in a cell with parallel glass sides (Fig. 51), and the image of this is focussed on the screen. If a piece of ice be now floated on the water, it will gradually melt, and the colder water sinks to the bottom of the cell. The descending current shows upon the screen in the form of streaks, which are due to the unequal refraction of cold and warm water (see Light, ch. vi.)

Or again, the cell may be filled with cold water and hot water may be introduced into this by means of a pipette dipping into it. If this is done very slowly and cautiously, the

hot water will be seen to rise upwards and form a layer floating on the colder water. (The image on the screen is of course inverted.) (See Light, ch. viii.)

EXPT. 58. Make a paper box by folding stout writingpaper; put a few stitches in to keep the flaps from opening, and hang the box up by threads from the four corners (Fig. 52). Half-fill the box with water and let the flame of a Bunsen burner play on the bottom of it. The water can be heated to boiling without scorching the paper.

You should not conclude from this experiment that paper is a good conductor of heat. It rather shows how rapidly heat is carried off by convection. If it were not for the water the paper would soon burn, and if the flame is too big the box does get scorched above the water-line.

80. Circulation of Water.-The following experiment shows how a continuous circulation of water can be maintained by convection.

EXPT. 59.-A glass tube of the form shown in Fig. 53 is

-10

Fig. 53.

filled with water up to the open neck on top, and one of the vertical branches of the tube is heated (near the lower corner)

by means of a small gas-flame or spirit-lamp. The heated water ascends and its place is taken by colder water from below.

To show up the motion of the water better, drop in through the neck a little magenta dye (as much as can be taken up on

the point of a penknife, crushed and moistened with water before dropping it in). The coloured water moves towards the left, passes down the left-hand tube, and so round as shown by the arrows. The circulation can be kept up as long as you please, but cannot well be followed after the dye has worked round to the neck.

81. Heating by Hot Water.-One of the best methods of heating large buildings is by means of hot-water pipes. The general arrangement of a hot-water system is shown in Fig. 54. The principle on which the method depends has been illustrated in Expt. 59.

The water is heated in the boiler B, and rises up through the pipe ab; it then passes through coils (C, C', etc.) which are placed in the various rooms and serve to distribute the heat wherever it is most required. Having parted with much of its heat, the water is now colder and heavier; it sinks through the return pipe cơ to the bottom of the boiler, where it is again heated and begins its journey afresh.

All three processes-conduction, convection, and radiation-come into play here. Heat is radiated out into each room from the pipes and coils; these are painted a dull black so as to increase their emissive power (p. 103), and the separate pipes in each coil are provided with numerous flanges, so as to increase the radiating surface. It is by conduction that the heat passes from the furnace through the boiler into the water and, later on, from the water to the outer surface of the pipes and coils. The air above these is heated by convection; and the whole system is a capital example of a continuous water convection-current carrying heat with it from the boiler to the coils.

82. Convection in Gases.-The draught up a chimney is a familiar example of convection. Before passing into the chimney the air is heated by the fire and is thus rendered lighter than the air outside. In the same way convectioncurrents are produced in the chimneys of oil-lamps.

The upward motion of the air above any hot body (e.g. a red-hot poker) may be observed by holding near it a piece of smouldering brown paper (or 'touch-paper' made by soaking brown paper in saltpetre solution and then drying it).

The quivering appearance of objects seen over lime-kilns and chimney-stacks is due to the irregular refraction of light through the heated air rising up. In this way convectioncurrents in air may be well shown by using a lantern as in Expt. 57 (excepting that here no focussing is necessary).

EXPT. 60.-Hold a spirit-lamp in a strong beam of light from the lantern, and examine its shadow on the screen. Wavy streaks of light around and above the flame show the ascending currents of hot air.

Examine in the same way the shadow of a red-hot poker or