Electrical Thermometry

Construction of Platinum Thermometers.

Figure 7 gives an enlarged view of the platinum spiral of the thermometer shown in the sulphur apparatus in figure 6. This was one of the first thermometers constructed by the writer with platinum leads and mica insulation for use at high temperatures. The fine wire of the coil was fused on to the platinum leads without employing any gold or silver solder, which might contaminate the wire, and would limit the temperature to which it could be exposed. This particular thermometer was constructed primarily for testing how closely the scales of two pieces of wire from the same reel would agree with each other. With this object the two spirals were wound in a double helix, being threaded through fine holes in a thin plate of mica AB to keep them in place, without contact at any point, but so close together that their mean temperatures would be the same even if the distribution of temperature in the tube were not quite uniform. One of the wires is shown by a dotted line in the figure to distinguish the two. Each coil has three leads, one of the leads being double in each case, for the purpose of securing automatic compensation of the resistance of the leads, and freedom from stem-exposure correction. The platinum leads were insulated and held in place by being threaded through mica discs, one of which is shown at C. This method is still generally adopted, and has the advantage of giving very perfect insulation at high temperatures, besides preventing con vection currents up and down the tube. No difference exceeding 0.01° C could be detected between the scales of the two spirals, by differential measurements up to 600° C. Owing to the double winding, the insulation of the coils could be verified with great accuracy at any moment during the observations. This pattern was intended for use with a duplex compensated bridge, made for Lea and Gaskell (Jour. Phys., 1887), giving double the sensi tivity of a single bridge, and capable of calibration without the use of external resistances. It was found however that the single bridge could be so easily calibrated against another similar bridge, and that the thermometer with a single coil had such a large excess of sensitivity, that the duplex combination was abandoned as being unnecessarily complicated and expensive.

A thermometer with a single coil, wound on a serrated plate or cross of mica, is much easier to make than the duplex pat tern, and has the advantage of superior immunity from damage if the tube is broken. It differs from the duplex pattern illustrated in the figure by having four leads instead of six, the pair for the compensator being separate from the pair connected to the coil, but otherwise ex actly similar, and similarly insulated with mica discs. The insulation of the coil cannot be tested in this pattern ex cept by inspection after removing the tube, but is not likely to give trouble unless the tube is cracked or damp. The insu lation of the leads is easily tested, and usually gives sufficient warning of any trouble of this kind. The potentiometer method, which is sometimes used for meas uring the resistance, requires that a pair of leads should be connected to each end of the coil. This method eliminates the resistance of the leads, if the balance is perfect, but does not permit a satisfactory insulation test, and depends too much on the constancy of the current, besides being inapplicable to differential measurements, or to the exact elimination of accidental thermoelectric effects. On the other hand in

the Wheatstone bridge method, the equal ity of the compensator and pyrometer leads must be carefully tested in the construction of the thermometer, but is not likely to give trouble if the resis tance of the coil is large compared with that of the leads, as is usu ally the case in thermometers employed for work of precision at moderate temperatures, such as 500° C. The leads are often connected to terminals at the head of thermometers, as indicated at M in figure 6. For work of the highest precision these termi nals are better omitted, and the leads directly soldered to a flexible cable in order to avoid possible errors from thermoelectric effects and changes of resistance of the screw terminals. For tempera tures above 600° C the protecting tube should be of porcelain, and the leads of platinum throughout that part of the tube which is exposed to high temperatures. For lower temperatures a tube of hard glass and leads of gold or silver may be employed, but it is better in any case to make the lower part of the leads of plati num in order to diminish the conduction of heat along the stem. For laboratory work a tube 30 or 4o cm. in length usually suffices, but for large furnaces the length of the protecting tube is often 5 to 1 o ft. In the latter case it is usual to protect the porcelain tubes with an external steel tube, which may be removed for delicate measurements.

Special Forms of the measurement of linear expansion it is a great advantage to employ a thermometer with the bulb or sensitive portion equal in length to the bar or column under test, so as to obtain the mean temperature of the whole length. In measuring the linear expansion of a standard metre or yard, a fine platinum wire enclosed in a glass capillary, or otherwise insulated, is employed, its length being equal to that of the bar. The same method has been applied by Callendar (Phil. Trans. A, 1887) and Bedford (Phil. Mag., 1898) to the expansion of glass and porcelain at high temperatures, employing a fine wire supported along the axis of the tube under test. An equivalent method, applied to the expansion of silica by Callen dar, is to enclose a rod of the material inside a platinum tube which is heated by an electric current. This is a very rapid and convenient process, since the mean temperature of the rod must be equal to that of the enclosing tube. Any temperature up to the melting-point of platinum is readily obtained, and easily regulated. The temperature may be obtained by observing either the resistance of the platinum tube or its linear expansion. Either method may also be employed in J. Joly's meldometer, which consists of an electrically heated strip for observing the melting points of minerals or other substances in small fragments. In observing the temperature of a long column of mercury, as in the method of equilibrating columns for determining the absolute expansion of mercury, a platinum thermometer with a bulb equal in length to the column may similarly be employed with advan tage (Callendar & Moss, Phil. Trans. A, 1911, p. r). The appli cation is here particularly important because it is practically impossible to ensure perfect uniformity of temperature in a vertical column, 6 ft. or more in length, at high temperatures.