To correct for change of zero the thermometer ought to be plunged from time to time into melting ice, and its reading noted. The amount of alteration thus becomes known, and the requisite correction may be applied, which is of course constant throughout the scale. 23. Other sources of error. If a thermometer have its fixed points determined in a vertical position it must always be used in this position: in like manner if these points are determined in a horizontal position of the instrument, then it must always be used horizontally. The reason of this is that for the same temperature the same instrument will give a higher reading in a horizontal than in a vertical position, since in the latter the hydrostatic pressure of the column of mercury will tend not only to compress the particles of mercury into less volume, but also to enlarge the capacity of the bulb. For a similar reason the reading of an unprotected thermometer in vacuo will be different from its reading in air. 24. Again, when the volume of mercury in the stem of a thermometer is exposed to a temperature different from that of the bulb, a correction must likewise be made on this account. For instance, if the bulb and the column of mercury up to freezing-point mark have the temperature of boiling water, while the remainder of the column is exposed to the atmosphere, which we may imagine to be at 32°, then the instrument will not indicate 212°. It would have done so had the whole of the mercury been heated up to the boiling point, but this is not the case, for nearly 180 degrees, or the distance between the freezing-point mark and the extremity of the column, is exposed to the atmosphere, and may be taken to have the same temperature as it has. In order to find the correction which we ought to apply, let us denote by unity the whole volume of the mercury when it is all at the temperature 32°: unity will therefore also denote the internal capacity at this temperature of the glass envelope up to freezing-point mark. Now it will be afterwards shewn (Art. 52) that this mercury when raised to 212° will have the volume 1.0182 nearly, and also (Art. 40) we may perhaps suppose that the internal capacity of the glass up to freezing-point mark will have, when raised to 212°, the volume 1.0026: this however depends upon the nature of the glass. Hence a volume of mercury equal to 1.0182 1.0026, or .0156, will exist in the tube above the freezing-point mark provided that the whole column of mercury be heated up to 212°; and it will under these circumstances occupy 180 degrees of the bore. The question to be answered is, how many degrees of the bore will this portion of mercury occupy when both it and the tube containing it are at 32°. Evidently the absolute volume of this. portion of mercury at 32° will be .0156 × I 1.0182 = .01532. Hence if we imagine the bore of the tube to preserve a constant volume for all temperatures, the rise may easily be found. For if the volume .0156 occupy 180 divisions, the volume .01532 will occupy 180 X .01532 or 176.8 divisions. But the bore of the tube in which this rise takes place does not preserve a constant volume throughout, but, being only at 32°, is really of smaller capacity than it was at 212°, in the ratio of 1 to 1.0026: the rise of 176°.8 will therefore have to be increased in this proportion, and will become 177°.2, or the mercury will indicate 209°.2 as the boiling point of water; a correction of +2°.8 must therefore be applied. This example will convey to the reader sufficiently well the method to be employed in finding this correction, but C it ought to be borne in mind that it is much better when practicable to avoid the necessity for it, by exposing the whole column of mercury as well as the bulb to the influence of the temperature which we wish to estimate. 25. Lastly, even when a mercurial thermometer has been constructed with the greatest accuracy after the method indicated in the preceding articles, so that the freezing point is denoted by 32° and the boiling point by 212°, while each degree denotes precisely the 180th part of the capacity of the bore between these two points, it does not follow that the instrument will give an intermediate temperature with absolute exactness. For, in the first place, it does not follow that the expansion in volume of the mercury above that of the glass envelope up to freezing-point mark for a true rise of 90° must be precisely half of its expansion for a true rise of 180°. In the next place, it ought to be borne in mind that the rise of 90° takes place in a bore of which the temperature is only 122°, and of which therefore the capacity is smaller than when the temperature is 212°, in the proportion of 1.0013 to 1.0026. Both of these circumstances will introduce errors, and Regnault has found that when the graduation is extended much above 212°, the difference between the mercurial and the standard air thermometer becomes very considerable at high temperatures, and also varies with the nature of the glass. But between 32° and 212°, and for a range extending not too far beyond these points, a mercurial thermometer well graduated may be considered to be a tolerably good though not a strictly accurate instrument. Since the best thermometers made in this country are all formed of the same kind of glass, it would be desirable that a few of these should be compared with an air thermometer at temperatures between 32° and 212°. The writer of this work hopes that he may ultimately be able to make this comparison. OTHER THERMOMETERS. 26. Alcohol Thermometer. It is well known that mercury freezes at about — 38° Fahr., while it boils at about 660° Fahr. A mercurial thermometer cannot therefore be used below the former point or above the latter. But while the superior limit of its accurate employment is considerably below the higher temperature, its indications may probably be relied upon very nearly to the point at which the mercury freezes. It is, however, often desirable to register still lower temperatures, and in order to do so a thermometer filled with absolute alcohol is employed. Such an instrument is not capable of being constructed with the same amount of exactness as a mercurial thermometer; but yet if it be carefully made, and used with caution, very good results may be obtained. An alcohol thermometer ought before graduation to be marked off at 32°, and at some higher temperature by comparison with a mercurial standard thermometer. The freezing point of mercury ought also, if possible, to be made use of as a fixed point; and it has been ascertained that this, like the freezing point of water, is of constant temperature, its true value, on Fahrenheit's scale, being - 37°.9. An alcohol thermometer may be used for very low temperatures, since this fluid has not yet been frozen. When a very accurate determination is desired, this thermometer should be kept in a vertical position, bulb downwards, for some time before it is read. The reason of this is that alcohol, unlike mercury, wets the capillary glass tube which contains it, and is also very volatile great care ought therefore to be taken that there is no liquid above the main column, whether condensed or adhering to the sides of the tube. 27. Maximum and Minimum Thermometers. Maximum. It is often of importance to know not merely the present temperature, but likewise the highest or lowest point to which an instrument has been exposed. Meteorologists, for instance, should be able to register every evening and morning the highest and lowest temperatures of the atmosphere. This, when not accomplished by a continuous photographic registration of temperature, is done by maximum and minimum thermometers. In Rutherford's maximum thermometer the stem is placed in a horizontal position, and the bore contains a small index made of iron or graphite, which the mercury pushes before it when it expands through increase of temperature; but when it retreats this index is left behind, since there is no cohesion between it and the mercury. In Professor Phillips' maximum thermometer this index is part of the mercurial column itself, which, as in Fig. 3, is separated from the main body of the 20 10 0 JO 20 30 40 50 GO 70 80 90 100 110 120 130 140 150 Fig. 3. fluid by a little air. When the mercury expands, the elastic force of the air pushes the index on before it; but this is kept in its position when the mercury again contracts. By this arrangement there is no risk of the index soiling the mercury or becoming entangled with it. It has also been found that when the bore is sufficiently narrow the instrument may be used in a vertical position, bulb downwards, and it is thus of service in chemical operations. Both of these maximum thermometers when read must be reset by shaking the index down towards the mercurial column as far as it will go. Negretti and Zambra's maximum thermometer is exhibited in Fig. 4. When used the stem of this instrument ought to be inclined downwards. The bore is nearly choked at A by |
