THERMOMETER. 



THERMOMETER. 



223 



The advantages of mercury over alcohol and air, as a measure of tem- 

 perature, are that its expansions are more nearly proportional to the 

 increments of heat than those which take place in either 

 of the other fluids ; it is easily deprived of air, and its 

 power to conduct heat being considerable, the changes of 

 its volume by changes of temperature in the surrounding 

 medium take place more rapidly than those of any other 

 fluid except the gases. 



At first the scales for measuring degrees of tempe- 

 rature were arbitrary, and consequently no two thermo- 

 meters could be compared together. The scale of the 

 Florentine thermometer was determined by meriting the 

 place where the top of the spirit column stood in the 

 tube when the latter was immersed in snow, and the 

 place at which it stood at the time of the greatest heats 

 in Florence : the interval between the points was divided 

 into 60 parts. Subsequently, in this country, Mr. Boyle 

 and Sir Isaac Newton formed scales for determining the 

 expansion of the spirit or oil by making the space included 

 in each degree of the tube equal to a certain portion of 

 the whole volume. Thus, supposing the ball of the 

 thermometer and part of the tube to be divided into 

 10,000 equal 'parts, and to be wholly occupied by the oil 

 when the instrument is plunged in melted ice, Sir Isaac 

 found that by the heat of the human body the oil 

 expanded 256 such parts, and by that of boiling water 

 725 parts ; then, considering the point at which the top 

 of the column stood in the tube when the latter was 

 placed in the ice, as the zero of the scale, he divided the 



So- 



40-T 



L 



Fahrenheit's interval between this point and that at which the top of 

 Thermometer, the column stood when the ball of the thermometer was 

 placed under the arm of a man, into 12 parts. After- 

 wards by proportion he found that the distance from the ice-point 

 to that of boiling water was equal to 34 such parts (' Phil. Trans.,' 

 vol. xxiL). This method, being of difficult execution, waa soon 

 abandoned. 



The scale which has been in general use in this country since the 

 year 1724, is supposed to have been invented by Fahrenheit. It is 

 quite unknown on what ground he made choice of the fixed points on 

 his scale, or of the number of graduations between them ; but it is 

 thought that one of the fixed points was that of boiling water, and 

 that the other, which is the zero of the scale, was that at which 

 the top of the column stood when the instrument was exposed to an 

 intense cold in Iceland, in 1709. The extent of the scale between 

 this last point and that of boiling water is divided into 212 parts, 

 and the point of freezing water is at the thirty-second division 

 from the zero point. See the scale on the right of the tube in the 

 above figi 



M. Reaumur constructed a thermometer in which spirit of wine was 

 employed, and he formed a scale in a manner nearly similar to that 

 wliich had been put in practice by Sir Isaac Newton. He computed 

 the volume of the glass ball, and graduated the tube so that the space 

 between two divisions was equal to one-thousandth part of that 

 volume : he then found the zero of the scale by marking the place 

 where the top of the column stood when the thermometer was placed, 

 in water just freezing : and afterwards, plunging the instrument in 

 boiling water, he observed whether or not the spirit rose exactly eighty 

 divinions. If not, he strengthened or diluted the spirit till it did so ; 

 and the point at which the top of the spirit stood became the point of 

 boiling water. Of this instrument an account was published in the 

 'ires ' of the Academy of Sciences for 1730, but the construction 

 has been long since abandoned ; for, besides the difficulty of giving a 

 proper degree of strength to the spirit, it is well known that the 

 latter cannot be made to take the temperature of boiling water, so that 

 tin- determination of the upper point in the scale must be very erroneous. 

 That which is now called Reaumur's thermometer U an improvement 

 on the former, by M. Deluc, who determined the poults of freezing and 

 br.iling water by experiment, and divided the distance between them 

 into eighty parts, the zero of the scale being at the former point. See 

 the scale on the left of the tube in the above figure. 



A third scale, called " Centigrade," has been much in use among the 

 philosophers of the Continent within the last eighty years: it was 

 invented by C 1-iius, a Swede, and it differs from that of Reaumur or 

 . only in the distance between the points of freezing and boiling 

 water being divided into 100 parts. The length of each degree in this 

 thermometer, as well as in that of Reaumur, is greater than in the 

 scale of Fahrenheit ; and consequently the indications of temperature, 

 when the top of the spirit or mercury is between the lines of division 

 are rather uncertain, from the difficulty of estimating them accurately 

 by the eye ; also, the temperatures required to be determined being 

 often below the point of freezing-water, the employment of negative 

 signs is of more frequent occurrence with these thermometers than 

 with those of Fahrenheit. 



Tin- following formulas will serve to convert any given number of 

 degrees on Fahrenheit's scale into the corresponding number of degrees 

 on Reaumur's and the Centigrade scales, and vice rend. 



Let r, R, anil c express any corresponding numbers of degrees on 

 the three scales respectively : then 





=F, and 





 also, g C = R, and ^ B=c. 



N.B. When F is between zero and 32, the values of R and c are 

 negative, and express the required number of degrees below zero on 

 Reaumur's and the Centigrade scales. Also, when F, R/or c expresses 

 any given number of degrees below zero on its proper scale, it must be 

 considered as negative. 



The scale invented by De 1'Isle of St. Petersburg, in 1733, being 

 still occasionally in use, it may be necessary to mention that it is 

 formed by making the space included in each degree equal to one 

 hundred-thousandth part of the whole volume of the mercury ; the 

 zero of the scale is at the point of boiling-water, and between this 

 point and that of freezing-water the space is divided into 150 parts. 



It may be observed that the situation of the freezing-point on the 

 scale of a thermometer can be determined with great accuracy if the 

 ball and part of the tube be immersed in pounded ice ; for it is known 

 that water containing ice and snow remains of the same temperature 

 till the ice is entirely dissolved, every accession of heat to the water 

 being employed in promoting the dissolution. But the point of 

 boiling water is far from being so precisely known, since it varies with 

 the density of the atmosphere at the time of making the deter- 

 mination. Distilled water in an open vessel, and under a given 

 pressure of the atmosphere, boils at an invariable temperature, except 

 as far as the nature of the vessel may make some difference ; for if the 

 heat communicated to the water be increased, the only effect produced 

 is that of driving off a greater quautity of steam in a given time : in a 

 vessel exhausted of the air the water will boil at a temperature 

 expressed by about 70 of Fahrenheit's scale, while in a vessel con- 

 structed so as to prevent the steam from escaping it will remain in a 

 liquid state at a temperature of 400 and upwards. In order 

 therefore that the temperatures indicated by different instruments 

 may agree together, it is recommended that this point should be found 

 from water boiling in the open air at a time, if possible, when the 

 height of the mercurial column in the barometer is 30 inches, and 

 when the temperature of the air is indicated by 55 of Fahrenheit's 

 scale. 



This effect of the pressure of the atmosphere on the boiling of 

 water was noticed by Fahrenheit in 1724, and M. Deluc, in his 



the barometer ; but the English artist, Bird, was the first who applied 

 a correction on account of the state of the barometer, for the purpose 

 of fixing the point of boiling water on the scales of thermometers. 



The Royal Society having, in 1776, appointed a committee to con- 

 sider the best means of adjusting the fixed points of thermometers, 

 the formula of Deluc was verified and reduced to English measures 

 for the benefit of artists, in the event of their being obliged to make 

 the instruments under different states of the atmosphere with respect 

 to density and temperature ; and the following are some of the cor- 

 rections which are given by Sir George Shuckburgh for determining 

 the true place of the boiling-point of water. The first column contains 



Inches. 

 26 

 27 

 28 

 29 

 30 

 31 



-7-09 

 -5-27 

 -3-48 

 -1-72 





 + 1-69 



the height of the barometer in inches ; and the second, the correction 

 which is to be applied with its proper sign to the number 212 on 

 Fahrenheit's scale, in order to give the correct number of degrees at 

 which the water will boil under the pressure expressed by the height 

 of the mercurial column. The committee observe that in trying the 

 heat of liquors, the quicksilver in the tube of the thermometer should 

 be heated to the same degree as that in the ball ; or if this cannot be 

 done, a correction should be applied on that account. (' Phil. Trans.,' 

 vol. Ixvii.) 



Thermometer-tubes should have their bores very slender, and, if 

 possible, perfectly equable in the whole of their length. When there 

 is any inequality in the transverse sections, the best artists make the 

 graduations of the scale vary so that they may correspond to the equal 

 divisions of a cylindrical tube ; and in order to ascertain the relative 

 dimensions of the sections, they cause a small quantity of mercury, 

 about an inch in length, to slide along the interior of the tube, 

 measuring its length in different places ; then, since the lengths aro 

 inversely proportional to the areas of the sections, the variations of 

 the former will immediately show the corresponding variations of the 

 latter. The method of calibration, as it is called, recommended by 

 the Kew committee, will be noticed presently. It is usual to 

 give to the bore an oval form, with the broader side towards the 



