Siemens’ Calorimetric or “Water” Pyrometer.—[Fig. 66] shows this instrument in longitudinal and transverse section. It consists of a double copper vessel, the inner containing water, and the outer provided with a handle. The space between is lagged with felt, to prevent escape of heat from the water. The thermometer, b, is protected by a perforated brass tube from damage that might be caused on dropping in the hollow nickel cylinder, d. Opposite the stem of the thermometer is placed a sliding-piece c, on which a temperature scale is marked. In using the instrument, the specified quantity of water is placed in the inner vessel, and the pointer on c brought opposite to the top of the mercury column in the thermometer. The nickel cylinder, which has been heated in a crucible or muffle in the furnace, is then dropped in, and the vessel shaken to secure an equal temperature throughout the water. When the thermometer is stationary, the mark on c opposite the top of the mercury gives the temperature of the furnace, the scale on c having previously been marked from calculations made for each 50 degrees. The correctness of the reading evidently depends upon the accuracy with which c has been calibrated, an operation which involves taking into account the water equivalent of the vessel and the variation of the specific heat of nickel at different temperatures. Allowing for the sources of error attaching to the method, results by this pyrometer cannot be guaranteed to better than 2 or 3 per cent, at 900° or 1000° C., but in cases where this degree of inaccuracy is not of importance, the instrument may be used with advantage. As no calculation is necessary, the determination may be made in the workshop by any workman who exercises care in conducting the operation. Copper and iron cylinders are sometimes supplied instead of nickel, but are not to be recommended, as they decrease in weight with each test, and necessitate the use of a multiplying factor to convert the reading on c into the true temperature.
Special Uses of Calorimetric Pyrometers.—The great drawback to the calorimetric method is that each observation necessitates a separate experiment, involving time and labour. The accuracy, moreover, is not comparable with that obtainable by the use of a thermo-electric or resistance pyrometer; and practically the only recommendation is the low initial cost of the outfit. When an occasional reading of temperature, true to 3 per cent., suffices, the calorimetric pyrometer may be used; and in special laboratory determinations the method will frequently be found of value. Considering the low cost of thermo-electric pyrometers at the present time, it is probable that the calorimetric method will be entirely superseded in industrial practice, as the former method gives a continuous, automatic reading, and is capable of furnishing records. Many firms have already replaced their “water” pyrometers by the more accurate and useful appliances now available.
CHAPTER VIII
FUSION PYROMETERS
General Principles.—If a number of solids, possessing progressive melting points, be placed in a furnace and afterwards withdrawn, some may be observed to have undergone fusion whilst others would be unaffected. The temperature of the furnace would then be known to be higher than that of the melting point of the last solid melted, and lower than that of the first which remained intact. Taking, for example, a series of salts, the following might be used:-
| Salt. | Melting Point. | |
| Deg. Cent. | Deg. Fahr. | |
| 1 molecule common salt + | 650 | 1202 |
| 1 molecule potassium chloride | ||
| Common salt | 800 | 1472 |
| Anhydrous sodium carbonate | 850 | 1562 |
| ” ” sulphate | 900 | 1652 |
| Sodium plumbate | 1000 | 1832 |
| Anhydrous potassium sulphate | 1070 | 1958 |
| ” magnesium sulphate | 1150 | 2102 |
If, on inspection, it were found that the sodium sulphate had melted, whilst the sodium plumbate had survived, the temperature of the furnace would be known to lie between 900° C. and 1000° C. If a number of salts or other solids could be found with melting points ranging between 900° and 1000°, it would be possible to obtain a reading within narrower limits. The accuracy of the method in all cases is decided by the interval between the melting points of successive test materials.
Wedgwood, the famous potter, appears to have been the first to apply this method of determining the condition of a furnace, his test-pieces consisting of special clay compositions. The effect of the furnace on these was noted, and the suitability of the temperature for the work in hand deduced from the observations. Wedgwood in this manner investigated the variations in temperature at different levels in his firing-kilns, and was thus enabled to place the various wares at the positions best suited for their successful firing. Modern potters still use such test-pieces, as the information gained is not merely the degree of heat, but the effect of such heat on the articles undergoing firing. The fusion method, however, is now used to determine the temperature of all kinds of furnaces, and the chief modifications will now be described.