The Thermometer

The first liquid thermometer was built in 1631 by Jean Rey a French physician and was used to take the temperature of patients. The stem was, however, open and the water evaporation effected the accuracy of reading. The first thermometer with a scaled stem was built in 1641 by the Grand Duke Ferdinard II of Tuscany, using alcohol instead of water. 

The introduction of the thermometer early in the 17th century, even in its crude forms, helped focus attention into two very important, but also very hazy at that time, concepts Heat and Hotness.  

Cardwell, for example, comments (p.2) Unfortunately there was for a long time doubt about what it (the thermometer) actually measured. As a clinical instrument it certainly reduced the subjective element in the diagnosis of fevers, but beyond this the exact significance of its reading was uncertain and interpretation of them depended on the observer s philosophy of nature.  

And Hatsopoulos and Keenan write on the same subject Inventors of the thermometer were trying to devise an instrument which would measure a quantity as yet undefined. The physiological sensations of hotness and coldness were involved, but they were inadequate. The thermometer would indicate the same temperature for a block of wood and a block of copper exposed to the same severe winter cold even though these two blocks seemed to be of quite different coldness by the sense of touch. An equilibrium concept was doubtless involved - namely, that all bodies exposed to the same cold atmosphere would ultimately attain a uniform degree of coldness, despite contrary evidence of the senses. (This remark should remind us of the arguments in support of the Zeroth law.) 

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The more accurate apparatus shown in Fig. i(c) is strongly recommended when laboratory conditions enable students to retain their own apparatus over a complete course of work. A glass tube T, bent as shown, is fixed by the rubber-bands RB to the thermometer G. The... 

Corrected Melting-points. In all the above determinations of melting-points, the values obtained are described as uncorrected, since no allowance has been made for the fact that the column of mercury in the thermometer is at a lower temperature than that in the bulb. For most purposes it is sufficient to record this uncorrected value, which is usually only slightly lower than the corrected value. 

The thermometer should be so arranged that the top of the bulb is just level with the centre of the side-arm of the distilling-flask. 

The distillation heads Fig. 22(F) and Fig. 22(G) can be fitted with thermometers having a ground-glass cone just above the bulb (Fig. 22(M)). These are expensive, and it is usually more convenient to fit a thermometer pocket (Fig. 22(N)) which consists of a small well , fitting as shown into the neck of the flask. A small volume of mercury is placed in the well just to cover the bulb of a conventional thermometer, and thus provides excellent thermal contact between the thermometer and the sides of the pocket. 

A metal cylinder A contains two small holes. Into one of these the thermometer is placed and into the other (arranged centrally) is placed the melting-point tube. The cylinder is pierced by a hori-2ontaI tunnel so the melting-point tube can be observed during heating. A lens is... 

Place in the flask 2 g. of benzophenone, 15 ml. of isopropanol and 2 5 g. of aluminium isopropoxide. This mixture has now to be heated gently under reflux so that the temperature registered by the thermometer in the column does not exceed 80°, i.e., so that only acetone distils. For this purpose, the flask should preferably be heated in an oil-bath direct heating, even over an asbestos sheet, may cause local overheating and decomposition the use of a water-bath on the other hand may make the column undesirably damp. 

After 3 hours, replace the separating-funnel by a reflux condenser, and the thermometer by a stopper. Add 11 g. of zinc dust in small portions by rapid removal of the stopper at such a rate that the liquid is first brought to the boil (usually about 2 portions of 2 g. each) and is then maintained gently boiling. A ooid too rapid addition of the zinc, otherwise the reaction will become too vigorous, and rapid immersion of the flask in ice-water will be necessary to control the effervescence. 

The furnace (Fig. 74) consists primarily of an iron box A, having 3 or 4 tubes (e.g., B) arranged in a horizontal row, and dipping slightly from the open (left-hand) to the closed (right-hand) end. The top, which usually forms a loose lid to the rest of A, carries a thermometer T, and care should be taken to ensure that the bulb of the thermometer is not touching any part of the metal box. The Carius tube encased in... 

For this purpose the thermometer has a reserve supply of mercury in the reservoir R. For working at low temperatures, more mercury is required in the bulb, and an extra supply is brought down from R for working at higher temperatures, less mercury is required in the bulb to enable readings to be made on the scale S, and therefore a suitable quantity of mercury is detached from that in the bulb and deposited in the reservoir. 

Therefore before the thermometer can be used in a particular experiment, it must be set, f.e., the amount of mercury must be adjusted so that the readings fall conveniently upon the scale S. 

If, on the other hand, the thermometer has previously been used at some temperature below the freezing-point of benzene, when the bulb is originally placed in the beaker of water at 7-8 C., the mercury will rise in the capillary and ultimately collect in the upper part of the reservoir at a. When the expansion is complete, again tap the thermometer sharply at R so that this excess of mercury drops down into b, and then as before check the success of the setting by placing the thermometer m some partly frozen benzene. In either case, if the adjustment is not complete, repeat the operations, making a further small adjustment, until a satisfactory result is obtained. 

If the thermometer is to be used to determine the elevation of the boiling-point of a liquid on the addition of a solute, it must be remembered that at the boiling-point of the pure solvent the mercury must now be about 1-2 above the bottom of the scale S, and hence for adjustment purposes the temperature of the beaker of water should be 6—7 above the boiling-point of the liquid itself, instead of 1-2 as before. 

Weigh the clean dry tube T, then add about 25 ml. of the pure solvent (i.e., sufficient to ensure that the bulb of the thermometer... 

If a small Beckmann thermometer, similar to that used in the Sucharda-Bobranski method (p. 442) is available, the above method can be readily modified by replacing the tube T (Fig. 79) by an ordinary small boiling-tube without the side-arm A. The thermometer B and the stirrer S are fitted as in Fig. 79. The volume of solvent (usually ca 5 ml.) must of course cover the bulb of the thermometer. 

The degrees of the thermometer are numbered from the top downwards, hence the apparent rise in the F.P. 

The capillary tubes used for the melting-point determinations should be both longer and wider than those normally used they should be at least lo cm. long (to decrease the chance of slipping off the thermometer during the determination) and 2 -3 mm. in diameter (in order to have sufficient semi molten material in the tube to observe accurately the process of melting). 

When three consistent readings are obtained, add a weighed pellet of the solute to T for this purpose it is important that the thermometer is not withdrawn, and the boiling is not interrupted. It is best to hold the pellet ready in a pair of forceps near the mouth of the side-arm of T, and then momentarily to remove the condenser, drop in the pellet and replace the condenser when the condenser is removed a small quantity of cold air entering the side-arm will cause slight condensation of the hot vapour, none of which will therefore escape. 

This method has the advantages that (a) the boiling is much steadier than that in the previous method and consistent readings of the thermometer can... 

For ethyl alcohol, two volumes of dicycZohexyl are mixed with one volume of the alcohol, a thermometer is introduced, and the mixture heated until it becomes clear. The solution is then slowly cooled, with constant stirring, and the temperature is determined at which the opalescent solution suddenly becomes turbid so that the immersed portion of the mercury thread of the thermometer is no longer clearly visible. This is the C.S.T. The water content may then be evaluated by reference to the following table. 

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