When a comparison is made of modern Chemical, physiological and industrial methods with those commonly practised only about a hundred years ago, it is quickly realised that without the expansion thermometers very little progress could have been made, temperature measurement and control having not only become essential features in practically every field of activity, but in a great many cases, having been the basis of the investigations, results of which are now seen wherever modern production methods are in use, or where the lessons of scientific research are being applied.
We must go back nearly 400 years, however, to find the first recorded account of an attempt to measure temperature, for until then, only visual or sensory evidence was used in its estimation – the bubbling of water when boiling point was reached, the formation of ice indicating freezing point; the change in the colour of the dough as bread was baked, and so on. Then in 1592, Galileo invented his air thermoscope, using the expansion and contraction of nearly all substances under the influence of heat and cold respectively as the basis of the appliance.
He used air as the expanding and contracting medium, and employed a glass-tube with a bulb at one end the other end being open. To prepare it for use, the open end was immersed in a vessel containing water and the bulb gently heated. The air in the tube expanded and some of it bubbled out through the water. The bulb was then allowed to cool and the consequent contraction of the air inside caused water to be drawn up into the tube. From then on, if the temperature of the air surrounding the bulb rose, the water would be driven down the tube, to rise again as the temperature fell.
It was not until some years later that any real form of temperature measurement was attempted when, in 1626, Sanctorious made a thermometer working on the same principle, having a tube with a number of bends to provide a greater length. This was fastened to a board on which a rough scale was inscribed and the apparatus is generally accepted as the first clinical thermometer, because its main object seems to have been to measure the heat of the body.
This, curiously enough, does not seem to have been connected with physiological research, but had as its object the establishment of a system of calibration in which the heat of the body was to be one reference point. Variations of atmospheric pressure affected the air-thermoscope adversely, and in 1654, Boyle, often described as the “Father of Chemistry”, improved on it by introducing hermetical sealing of the tube and using liquid (alcohol) as the expanding medium instead of air, the instrument reaching a form from which it was departed very little up to the present time.
During the latter part of the seventeenth century, research continued with the object of establishing a system of calibration which would enable comparative readings between thermometers to be taken.
As before, the heat of the body was taken as one reference point, but the desirability of other fixed points became apparent, and in 1694, one Renaldeni used the freezing and boiling points of water for this purpose, but these were not generally accepted until Fahrenheit established his scale in 1709. It is interesting to note, however, that the first scale made use of the freezing point of water and the bodys temperature as the reference points and had only twelve divisions. Later on these were sub-divided into eight, giving ninety-six divisions on the scale, the highest of which corresponded approximately to the recognised normal body temperature to-day.
Fahrenheit is also considered to be responsible for the introduction of mercury as a filling, so that by the early years of the eighteen century, the direct reading expansion thermometer had been evolved in a form differing very little in general design from that in common use today.
With such means of temperature measurement becoming available to chemists, physicists, physiologists and others, it was natural that investigation into the application of the thermometer in wider fields should take place. The early experiments, using body heat as a reference point, had shown that a relation existed between a persons temperature and his or her general state of health, but it was not until the middle of the nineteenth century that Wunderlich, after some years of studying the problem, published, in 1868, a treatise setting forth the results of his research and it is generally considered that the use of a clinical thermometer became an established part of the medical routine after its appearance.
The clinical thermometer of those days was very different from the small, handy and easily read type familiar to readers of this article. They took the form of engraved stem instruments, sometimes up to about 12″ in. long, with a range of 90* to 110* F., each degree being sub-divided into 1/5* F., a scale so open that reading must have been very difficult.
Apart from this, readings had to be taken with the thermometer in position as no maximum registering device was incorporated, and where the temperature under the armpit was required, a bent thermometer was used. The medical practitioner of those days was consequently burdened with a large wooden case containing a set of two thermometers, one straight and one bent, to do the work achieved by the small 4″ in. instrument with which we are familiar.
Prior to Wunderlichs researches, however, scientific working in other fields had devised means of registering the maximum temperature attained, such as the metal cored glass index as used in the Sixes maximum and minimum thermometer (invented by James Six of Colchester in 1782), and much later the separated mercury column type of index known as the Philips maximum index, invented by a professor of the University of Oxford of that name just prior to the Great Exhibition of 1851, the adoption of which to clinical thermometers took place some years later.
In the meantime, Luigi Peroni, a glass-blower of Hatton Garden, London, invented the lens front, by means of which the thin column of mercury in an open scale thermometer could be magnified considerably to facilitate reading, and this, together with the constricted bore tube in which the whole of the mercury column remains at the highest temperature reached until shaken down, remains the last important development in clinical thermometers.
Parallel with the increasing use of the thermometer by physiologists were the extensive applications being made in other branches of science. Once it was realised that the thermometer was the key to almost illimitable fields of research, physicists and chemists began to demand more and more special types of thermometer suited to the particular problems they were investigating, and as a result of growing interest in temperature measurement and control among the industrialists still further patterns were produced.
In the same way, more scientific methods of food production, farming, and domestic industries generally, emphasised still further the extent to which those improved methods relied on accurate thermometers, and by the later years of the nineteenth century the demand for thermometers had reached significant proportions.
It was about this time, in 1888 to be exact, that the late Giles Henry Zeal established himself as a thermometer maker, specialising in the production of high-grade clinical thermometers, in Turnmill Street, off Farringdon Road, London, already known as the district in which skilled glass-blowers were to be found.
With a force of about twelve skilled journeymen, he commenced operations and prospered. In 1902 his elder son, Henry Herbert Zeal came into the business, which continued to expand, and in 1921 was turned into a private limited company with G.H. Zeal as managing director, the other directors being H.H. Zeal and his younger brother, Raymond Oakley Zeal who had joined the firm in 1920.
In 1922 the continued growth of the undertaking demanded larger premises, and these were found in St. John Street, Clerkenwell, where the concern remained for some twelve years during which time a system of production had been established which catered for a far wider variety of instruments than had at first been contemplated, and that side of the business which specialised in industrial thermometers had achieved a system that might be described as approaching mass production, but retaining as essential features those processes which ensure that each instrument is treated individually at the importance stages of its construction.
As indicated above, by 1934, the St. John Street premises failed to provide adequate accommodation for the staff, which had increased to 280, and a modern ground floor factory was built at Lombard Road, off Morden Road, Merton, to which the business was duly transferred on completion.
In 1935, the firm of W. Reeves & Co., specialists in brewers instruments, was acquired and a new company, W. Reeves & Co., Ltd., was formed to continue the business which had been carried on for a number of years and had a high reputation in the industry for which it catered.
Meanwhile, the business continued to expand and additions to the new factory became necessary, work on which was to have started in 1940. The outbreak of war in 1939 delayed the completion of the plans, and it was not until 1948 that the factory, as stands to-day, was completed.
In the meantime, the thermometer and hydrometer business of A.C. Cossor & Sons (Thermometers) Ltd., was acquired, together with the factor and staff of 200 at Vale Road, Finsbury Park. This, in addition to the new factory at Merton, as been re-tooled and re-equipped and apart from one or two patented designs, are able to turn out any type of expansion Thermometer, both direct reading and dial indicating, apart from a wide variety of glass. U-tube manometers and innumerable patterns of hydrometers.
It is worth mentioning that well over 5,000 different patterns of thermometers are produced at the Merton and Finsbury Park factories, where a staff of 900 currently employed.
A further development took place in 1952, when a substantial interest was acquired in that very old-established and world- renowned concern, James Powell (Whitefriars), Ltd., whose works were originally situated in Tudor Street, London, E.C. on the site of the Whitefriars monastery. These works were founded somewhere about 1680 (about the time that Boyle had laid the foundations of the research which resulted in the instruments we have to-day) and apart from the production of thermometer tubing, in itself a highly specialised task, they have contributed in no insignificant manner to some of the most beautiful of the many lovely features of our mediaeveal churches and other buildings, namely the stained glass windows. This is a far cry from thermometer making, but one is permitted to consider that this point will not be without interest to readers of this journal.
Talking of glass naturally leads us to consideration of its use in thermometer making and it is probable that the memory of the day when one constructed a crude form of thermometer in the “stinks lab” at school will form the basis of the mental picture one might conjure up when thinking of a thermometer factory, but such a picture would be misleading.
Reference has already been made to the fact that each thermometer must be regarded as an individual product, the reason for this being that the most important item of raw material, the glass tube, is rarely uniform. This is due to no lack of skill on the part of the glass-blowers who draw the tube, but to the fact that their internal diameter is measured in quantities which would be regarded as critical tolerances in most other industries and consequently the glass-blower cannot be expected to produce exactly what is required, but will, nevertheless, succeed in furnishing something surprisingly near to the dimensions to which he is asked to work.
The results of his labour are delivered to the thermometer maker in “canes” of the tube about 6 ft. long which are measured on receipt for external and internal diameter, the latter being by far the more important as it enables the capacity of the bore to be estimated. Using a microscope with a graticule divided into microns, or one-millionth of a meter, the cross-section of the bore at each end of the caneis measured and noted. The cane is then cut in half and the process repeated with the two new ends, after which the halved of the original cane are themselves halved, and more measurements taken.
A comparison of the various readings enables those responsible for the selection of tube for the various types of thermometer to sort the short lengths into groups composed of tubes of very nearly the same volumetric capacity, and by doing this it is practicable to work out, within a little, the size of the bulb required for a particular range or a particular purpose. Thus, when the blower has completed the tube ready for filling, he knows that little or no time will be wasted on adjustment.
Even so, it is probable that each tube in, say a dozen intended for instruments of one particular pattern, will vary slightly from its companions. Therefore, after it has been filled, sealed and aged artificially or naturally, it must be calibrated exact according to its own characteristics.
The calibration is carried out in carefully designed tanks in which an elaborate system of paddles and baffles ensures that variations in the temperature of the liquid it contains are reduced t o a minimum. In them are suspended standard thermometers for which an N.P.L. certificate has been obtained, and the instrument to be calibrated is hung in the tank beside the standard, the tank having had its contents raised to a suitable predetermined temperature, which is read off from the standard. When the mercury column in this instrument shows no sign of rising or falling, and not until then, the new instrument is examined, and if its mercury column is stationary a minute “point” is made on the glass-tube exactly opposite the top of the column.
This procedure is repeated at other temperatures until enough “points” have been made to indicate that expansion of the filling is even, the tube passes to the graduating shop where the scale will be applied either directly on the tube, as in the case of clinical and other engraved-on-stem thermometers, or on a separate material according to the purpose for which the thermometer is intended, but in each case proportions of the scale will correspond exactly to the linear measurement between the points.
The actual dividing is done on dividing engines, designed and constructed in the tool room at the Merton factory, which are capable of engraving either an evenly-spaced scale for mercury- filled tubes or a correctly-tapered scale for spirit-filled tubes.
In the case of instruments in which a scale separate from the tube is to be used each scale is graduated according to “points” on the tube with which it is to be used, so that when the two components arrive in the mounting shop, the mounters have a complete check to ensure a uniform standard of accuracy.
It will be seen that, all through the process of producing a thermometer, continual checks have to be made of the instrument is to do what is expected of it, and these checks do not cease at the point where the tube is mounted on its sale for in most cases, both will have to be enclosed in some kind of case for industrial use. The design of these cases frequently presents a difficult problem to overcome, as for instance where the application calls for the stem of a metal case enclosing the tube and scale that must withstand very heavy pressure.
In such instances, a well is often used to receive the stem, and these are also made in both factories, the simplest being the ordinary brass type found in many heating systems and the most elaborate being of stainless steel bore cut from solid hexagon section rods. At the Merton Works, these are constructed in lengths up to 2 ft. special machinery having been installed for the purpose.
Thus it will be seen that the thermometer maker of to-day must be prepared to engage in activities far removed from the complicated process of glass manipulation and blowing required to produce the heart of the industrial thermometer, and this is especially the case where dial-indicating instruments are made as well as the direct-reading (liquid in glass) type, although in both instances the methods which result in instruments of unquestioned quality and accuracy are basically the same.