Evolution of the Thermometer/Chapter 4

by Henry Carrington Bolton

IV. Fahrenheit and the First Reliable Thermometers.

The eminent French physicist, Guillaume Amontons, lost his hearing when a schoolboy, and like the philosopher of old who destroyed his eyesight for fear visual impressions should disturb his speculations, the Frenchman declined surgical and medical assistance lest the admission of common noises to his brain should interfere with his profound studies in mathematics and mechanics. He became skilled in surveying, able in architecture and distinguished in pure mathematics, and he made important improvements in the hygrometer, the barometer, and the thermometer.

Amontons constructed the first veritable air-thermometer which was not at the same time a barometer; it consisted of a narrow glass tube four feet long, open above, ending below in a large bulb connected by a U-shaped bend; in this bulb air was confined by a column of mercury so adjusted that when the apparatus was immersed in boiling water, the barometer standing at 28 inches, the mercury in the tube stood 45 inches above the level in the bulb, thus making the pressure of the heated air 73 inches. The tube was graduated from 73 inches down in inches and lines (pouces and lignes), so that on this scale 51.5 inches were equal to 0° C., and 73 inches to 100° C. Readings were corrected by a barometer. This instrument registered the changes in the elastic force of air produced by heat, and was styled by Amontons the "universal thermometer;" but its great length, the difficulty of transporting it, and the inaccuracy due to friction of mercury in the glass tube, prevented it from being adopted.

Amontons is often credited, especially by French authors, with the discovery that the boiling-point of water is constant under like conditions as respects pressure, etc., but this cannot be sustained for it is certain that Renaldini anticipated him in proposing the boiling-point of water as a fixed point, and that Boyle and Papin demonstrated the influence of pressure long before.

The French physicist constructed a mercury thermometer provided with a double scale ascending from 49 to 59 and descending from 14 to 24; in this were the following correspondences: 59° = solidification of tallow ; 54° = cellars of the observatory, Paris; 52° = freezing of water. This instrument had no advantages over those in use.

Commenting on the experiments of Sir Isaac Newton with the iron pyrometer, Amontons devised similar ones. Assuming that the temperature increased in arithmetical progression throughout a rod of iron 59 inches long, he obtained the following results.

Temperatures Obtained With Amonton's Pyrometer.

Thin glass melted at	4	pouces 4 lignes.
Lead melted at	8	pouces 6 lignes.
Gunpowder ignited at	8	pouces 6 lignes.
Tin melted at	11	pouces
Alloy of 3 tin and 2 lead melted at	12	pouces
Water boiled at	22	pouces
White wax melted at	30	pouces 8 lignes.
Tallow melted at	39	pouces
Butter melted at	42	pouces

In his paper published in the "Mémoirs de Paris" (1703, p. 50) Amontons makes the oracular statement "heat is the soul of nature, and it is very important for physicists to be able to measure it accurately."

Fourteen years later, Jakob Hermann, a mathematical physicist of Basle, seeking to make thermometrical readings independent of corrections for barometric pressure, proposed to close the tubes of Amontons' air thermometers by fusing the upper ends; this form of instrument he described in a work entitled: "Phoronomia," published at Amsterdam in 1716. This instrument showed directly the elasticity of the confined air, and thereby the temperature, a correction being made for the temperature of the column of mercury.

Frequent allusions to the thermometer of Philippe de la Hire are met with in works on meteorology, and his instrument was famous not because of its novel construction, but owing to the long series of observations conducted with it and published in the Memoirs of the Academy of Sciences, beginning with the year 1670. It was a Florentine thermometer having a scale the value of which has never been accurately determined; La Hire considered it sufficient to state that it had two fixed points, the temperature of a deep cellar in the observatory (48°) and that of the air in an open room when it was freezing in the vicinity.

Amontons sought to compare his standard thermometer with that of La Hire, but could not obtain permission from the authorities to place his instrument alongside of it in the observatory; after his death a superficial comparison was made. Lambert says the zero of La Hire's scale was the temperature of a mixture of snow and salt, and the 100 degrees was the temperature of melted tallow about to solidify.

It is, of course, inexpedient to attempt to chronicle in this volume every printed note on the thermometer, many of which did not bring their authors much fame, and did little to advance the instrument to perfection. But brief reference may be made to a "Dissertation" by Gabriel Philippe de la Hire, (Mémoires Acad. Science, Paris, 1706, p. 432), (son of Philippe just named) in which he mentions an instrument invented by Nugent in 1706, resembling that of Huyghens, and to a short article on the "Construction of Thermometers," by Elias Cammerarius in 1712 (Ephem. Acad. Nat. Curiosa, C. i and 2, p. 370), before we consider the eminent services to thermometry rendered by Fahrenheit.

In the year 1714, Christian Freiherr von Wolf, Chancellor of the University of Halle, and professor of mathematics and philosophy in the same, received a visit from Fahrenheit, a maker of philosophical apparatus in Amsterdam, who presented him with two thermometers made by himself, which agreed so perfectly in registering temperatures that the learned professor was amazed; he considered the unusual phenomenon so remarkable that he wrote a paper for the Acta Eruditorum in the same year, ascribing the concordance to certain singular properties of the alcohol. Nothing can illustrate more cogently the imperfections of the best instruments then in scientific hands, and the character of those in ordinary use can be imagined; nothing can better demonstrate the immense advance made by Fahrenheit, and even those who deprecate the wide use of his illogical scale must pay tribute to the value of his services in developing reliable thermometers.

Daniel Gabriel Fahrenheit was born at Danzig, 24th May, 1686, the son of a well-to-do merchant. After receiving private instruction at home he attended the gymnasium, but when fifteen years old he had the misfortune to lose both his parents in one day(14 Aug., 1701), and was then sent to Amsterdam to enter a business house. There he completed his apprenticeship of four years, but forsook commerce in order to follow his inclination to study physical science and to travel; he became interested in meteorology and acquired great skill in constructing thermometers. In 1714 he visited glass-works in Berlin and Dresden to supervise the manufacture of the tubes for his instruments, and on this journey he called on Professor von Wolf in Halle, as stated.

Returning to Amsterdam he established himself as a maker of philosophical instruments; at that period three distinguished men of science honored Holland, Dr. Hermann Boerhaave, professor of medicine and chemistry in Leyden, Pieter van Musschenbroek, professor of mathematics and physics in Utrecht, and Willem Jacob van's Gravesande, astronomer and mathematician at the Hague, and these refer in their writings to Fahrenheit and his thermometers. When he visited England some time prior to 1724, he was well received and honored by election to membership in the Royal Society. Fahrenheit died unmarried in the land of his adoption 16th September, 1736, at the age of fifty years; he was buried in the Klosterkirche in the Hague.

Fahrenheit's practical work in thermometry began as early as 1706; at first he used alcohol only, but afterwards became famous for his mercury thermometers. In 1709 he sent his instruments to distant places, Iceland and Lapland, and took them in person to Sweden and Denmark. For eighteen years Fahrenheit kept secret his method of manufacture for commercial reasons, but between 1724 and 1726 he published five brief papers in the Philosophical Transactions. Many of the experiments date, however, from 1721.

In the first of these he gives the specific gravity and boiling-points of five liquids, alcohol, rain-water, nitric acid, potash-lye, and sulfuric acid, taken at 48° of his scale; this temperature, he explains, is half-way between that of the intense cold obtained by a mixture of water, ice and sal-ammoniac, or common salt, and that of the blood of man.

The second paper, on the freezing of water in vacuo, contains the interesting observation that water can remain liquid below its freezing-point; incidentally Fahrenheit describes his thermometer.

The third paper contains the specific gravity of 29 substances, solid and liquid, the determination having been made with the balance and with the new hydrometer described in paper No. 4. This instrument was the first hydrometer of constant volume; it had a pan for carrying weights like Nicholson's (which was patterned after it), and was the first that could be used for all liquids.

In the fifth paper Fahrenheit describes his invention of the thermo-barometer, based on the fact that the boiling-point of water is influenced by barometric pressure. Boyle had observed the lowering of the boiling-point under the receiver of the air-pump, but Fahrenheit was the first to discover the principles of hypsometry.

Fahrenheit's publications are few in number and very brief, but they show him to have been an original thinker, and his great mechanical skill in working glass enabled him to carry out his designs. His account of the thermometer is of so great interest that I give it entire.

"The thermometers constructed by me are chiefly of two kinds, one is filled with alcohol and the other with mercury. Their length varies with the use to which they are put, but all the instruments have this in common: the degrees of their scales agree with one another and their variations are between fixed limits. The scales of thermometers used for meteorological observations begin below with 0° and go to 96°. The division of the scale depends upon three fixed points which are obtained in the following manner: The first point below, at the beginning of the scale, was found by a mixture of ice, water and sal-ammoniac, or also sea-salt; when a thermometer is put in such a mixture the liquid falls until it reaches a point designated as zero. This experiment succeeds better in winter than in summer. The second point is obtained when water and ice are mixed without the salts named; when a thermometer is put into this mixture the liquid stands at 32°, and this I call the commencement of freezing, for still water becomes coated with a film of ice in winter when the liquid in the thermometer reaches that point. The third point is at 96°; the alcohol expands to this height when the thermometer is placed in the mouth, or the arm-pit, of a healthy man and held there until it acquires the temperature of the body. If, however, the temperature of a person suffering from fever, or some other disease, is to be taken another thermometer must be used having a scale lengthened to 128° or 132°. Whether these degrees are high enough for the hottest fevers I have not examined; I do not think, however, that the degrees named will ever be exceeded in any fever.

"The scales of such thermometers as are used for determining the boiling-points of liquids begin also at 0° and run up to 600°, for at about this temperature mercury begins to boil. To increase the sensitiveness of thermometers they are made with cylinders instead of spheres, so that a larger surface will be more quickly affected."

Fahrenheit then gives a full account of his method of filling thermometers with liquids, a practical feature not necessary to detail in this place. The fever thermometers were known as "Pyranthropometers."

While these scanty records are all given us by Fahrenheit himself, other details are furnished by contemporary writers; Christian von Wolf describes the instruments given him by Fahrenheit thus:

The two thermometers had cylinders in place of spheres and were filled with colored alcohol. The cylinder of one was one and three-eighths inches long (12 inches to a Paris foot), thirteen sixty-fourths inch in diameter, and the lower portion ended in a sphere; the tube was six and eleven-sixteenths inches long. The scale was six and seven-sixteenths inches long and had 26 degrees, each of which was divided into four. The second degree on the cylinder was marked "greatest cold" (0° F.), and from this to the upper end were 24 degrees, some bearing special names; the fourth was "very cold," the eighth "cold," the twelfth "moderate," the sixteenth "warm," the twentieth "very hot," and the twenty-fourth "unbearable heat." The second tube of Wolf differed little in size. Wolf tested the two instruments and found the slight difference between them of one-four hundred and sixteenth of the entire scale.

Another friendly contemporary of Fahrenheit, Dr. Hermann Boerhaave, has recorded in his "Elements of Chemistry" some particulars of the celebrated thermometers. Boerhaave, writing in 1731, ascribes the invention of the thermometer to Drebbel, cites Amontons, Mariotte, and others, and gives an account of a noteworthy experiment made by Fahrenheit, who poured spirit of nitre on ice and got a temperature of −29° F., using an instrument graduated to −79°. Boerhaave then describes "an elegant thermometer made at his request by the skilled artist Daniel Gabriel Fahrenheit" thus: "The lower cylinder of this instrument contains 11,124 parts of mercury, which in the utmost cold observed in Iceland reached to the mark 0 from whence the further degrees of heat are computed upwards. Now if this be immersed in a vessel of water gradually heated, the mercury will be found to ascend continually till the water comes to boil, at 212° or more; so that setting aside the dilatation of the glass it now possesses 11,336 spaces of which in the greatest cold it possessed 11,124, so that by this difference of heat the bulk is dilated to

1
52	−	25	."
		53

In another passage, Boerhaave relates that on comparing two of Fahrenheit's thermometers, one of alcohol and one of mercury, he found a slight discrepancy and reported it to the maker, who "ingeniously owned the failing, but did not then see the cause of it, but revolving it in his own mind he at length discovered that the very glass made in Bohemia, England, and Holland expands more or less easily by the same degree of heat," and Fahrenheit suggested that the two instruments ought to be made of one kind of glass. Thereupon the doctor adds this comment: "How circumspect does nature require us to be in order to discover truth in physical matters, and how often are we deceived by following a general rule."

Fahrenheit made his thermometers with different scales at different times, commonly known as the large, medium, and small scales, their correspondence and value being shown in the table.

I.  Large.	II.  Medium	III.  Small.	(Centigrade.)
90	24	96	35.5
0	12	48	8.8
−90	0	0	−17.8

In No. I the 0° was placed at "temperate" as in the Florentine scale; in No. II each space was divided into four equal parts, and these smaller divisions were afterwards taken as degrees, thus forming scale No. III.

The earliest thermometers were made to indicate temperature up to 96° only; it does not appear that Fahrenheit used the boiling-point of water as a fixed point, although he alludes in his first paper to the fact that Amontons had shown that water boils at a constant temperature. The origin of the numbers 32 for the freezing-point and 212 for the boiling-point of water is obscure, they may have arisen in this way: After Fahrenheit abandoned the Florentine scale −90–0–90, he arbitrarily contrived the scale o–12–24 taken from the familiar foot measure, but the spaces being too large for accurate readings each was divided into four, and thus arose the scale 0–48–96. When he made thermometers for higher temperatures the scale was merely lengthened by adding more spaces of equal size, and one of the divisions marked 212 accidentally coincided with the level of the liquid at the boiling-point of water; Fahrenheit never had any intention of dividing the interval between his zero and the boiling-point of water into 212 parts.

It is a singular thing, however, that if we adopt the fixed points 32 and 212, the actual temperature of the human body is 98° not 96°; so the Fahrenheit scale now in use is not exactly the original. Moreover the zero of the Florentine thermometer is given by him as equal to 45°F. and by others as 48°.

Uncertainty also exists as to the exact temperature selected by Fahrenheit for his zero; the proportions of ice, water, and salt (or sal-ammoniac) in the mixture he used are unknown. Rüdorff has shown that the temperature obtained by mixing 100 parts of snow and 33 of salt equals −21.3° C., and 100 parts snow with 25 of sal-ammoniac gives −15.4° C., whereas the modern Fahrenheit zero is equal to 7.8° C. Fahrenheit's original mixture must have contained the two salts in proportions now indeterminable.

According to Boerhaave, Fahrenheit's zero coincides with the greatest natural cold observed in Iceland in the winter of 1709, and this is sometimes stated to have been the origin of the lower fixed point in the scale. Surely that winter was remarkably mild in frozen Iceland, for zero is often exceeded in countries not regarded as arctic; yet Boerhaave remarks that "nature never produced a cold beyond zero." The Meteorological Yearbook for Denmark shows that the temperature fell in March 1888, at Stykkisholm, Iceland, to −8.5° F.

There is another element of uncertainty in Fahrenheit's scale. Both Musschenbroek and Boerhaave state that the bulb of Fahrenheit's thermometer contains 11,124 parts of mercury at zero and that when the bulb is placed in melting ice the metal expands 32 of these parts; but Boerhaave, in another place, says the bulb contains 10,872 parts of mercury, and in still a third passage he gives the number of parts as 11,520, which Dr. Martine apprehends is nearer the truth. These vaguely named "parts" depend upon the figures taken for the expansion of mercury; to show their derivation we must bear in mind that the ratio between the capacity of the bulb and of the stem is constant for equal increments of heat.

Let x	=	the quantity of mercury in the bulb at 32°,
180	=	number of degrees between the boiling-point and freezing-point of water,
161.7/10,000	=	expansion of mercury between these two temperatures,

then: 161.7 : 10,000 = 180 : x, and x = 11,124.

(Fahrenheit's figure 161.7 is, however, erroneous, it should be 181.53.)

A number of Fahrenheit's original thermometers are preserved in European institutions; two are in the physical cabinet, Leyden, one 653 mm. long is graduated from −4° to 600°, the other one is 232 mm. long and has a scale from −4 to 100°. Both are filled with mercury. Comparison with a modern standard thermometer shows that the freezing-point of water in the larger one is 34.2°, and in the smaller 34.1°. The Real Gymnasium of St. Peter, in Danzig, treasures one of Fahrenheit's early thermometers; it is filled with alcohol, measures 110 mm. in length, and has attached in a glass tube a paper scale graduated from 0° to 100°. Above the scale is the inscription: "Cilinder termometron Ferneid."

Fahrenheit was followed almost immediately by a host of imitators, each devising a scale differing from Fahrenheit's and from one another, none of them possessing any special advantage. In 1712 to 1713, Fahrenheit, being in Berlin, communicated his method of constructing thermometers to his teacher of higher mathematics, Prof. Barnsdorf, and the latter made instruments with a scale of his own, the relation being as follows:

Fahrenheit.		Barnsdorf.
	96		13
	18		11
	4.5		0
	0		−1

Barnsdorf taught the art to Dr. Lange, professor of mathematics in Halle, and he devised a scale of his own:

Fahrenheit.		Lange.
	96		24
	52.3		12
	48		10.8
	6.4		0
	0		−1.7

Besides these imitators of Fahrenheit there may be named Christian Kirch, professor of astronomy in Berlin, Chr. Friedr. Ludolff, Sisson, and Bergen, whose scale made 32° Fahrenheit = 6° B., and 212° Fahrenheit = 174° B. Michael Christian Hanow, of Danzig, adopted a scale in which two degrees Fahrenheit equaled one degree H.; having observed in 1740 that the alcohol in a Fahrenheit thermometer fell to ten degrees below zero, he immediately devised a scale in which the zero-point was lowered ten degrees.

These objectionable imitations of Fahrenheit's thermometers brought the genuine ones into evil repute, but the latter outlived their rivals and Fahrenheit's scale is the popular one wherever English is spoken. The weakness of Fahrenheit's scale was the three fixed points somewhat vaguely defined.