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www.GOALias.blogspot.combody temperature? Obviously, it is notconvenient to use the laboratorythermometer for this purpose.Why does the mercury not fall or risein a clinical thermometer when takenout of the mouth?Observe a clinical thermometeragain. Do you see a kink near the bulb(Fig. 4.6).What is the use of the kink? Itprevents mercury level from falling onits own.Fig. 4.6 A clinical thermometer has a kink in itThere is a lot of concern overthe use of mercury inthermometers. Mercury is atoxic substance and is verydifficult to dispose of if athermometer breaks. Thesedays, digital thermometersare available which do notuse mercury.4.4 T4.4 TRANSFEROF HEATYou might have observed that a fryingpan becomes hot when kept on a flame.It is because the heat passes from theflame to the utensil. When the pan isremoved from the fire, it slowly coolsdown. Why does it cool down? The heatis transferred from the pan to thesurroundings. So you can understandthat in both cases, the heat flows froma hotter object to a colder object. In fact,Paheli asks: “Does it meanthat heat will not betransferred if the temperature oftwo objects is the same?”in all cases heat flows from a hotterobject to a colder object.How does heat flow? Let usinvestigate.Activity 4.6Take a rod or flat strip of a metal, say ofaluminium or iron. Fix a few small waxpieces on the rod. These pieces shouldbe at nearly equal distances (Fig. 4.7).Clamp the rod to a stand. If you do notfind a stand, you can put one end ofthe rod in between bricks. Now, heatthe other end of the rod and observe.What happens to the wax pieces? Dothese pieces begin to fall? Which piecefalls the first? Do you think that heat isFig. 4.7 Flow of heat through a metal striptransferred from the end nearest to theflame to the other end?The process by which heat istransferred from the hotter end to thecolder end of an object is known asconduction. In solids, generally, the40SCIENCE