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.
