The year 1822 was also marked by Seebeck’s discovery of thermo-electricity. The generation of a current of electricity by a heated junction of two metals, increasing with the temperature, appeared to afford a simple and satisfactory basis for a pyrometer, and Becquerel constructed an instrument on these lines in 1826. Pouillet and others also endeavoured to measure temperatures by the thermo-electric method, but partly owing to the use of unsuitable junctions, and partly to the lack of reliable galvanometers, these workers failed to obtain concordant results. The method was for all practical purposes abandoned until 1886, when its revival in reliable form led to the enormous extension of the use of pyrometers witnessed during recent years. In 1828 Prinsep initiated the use of gas pyrometers, and enclosed the gas in a gold bulb. Later workers used porcelain bulbs, on account of greater infusibility, but modern research has shown that porcelain is quite unsuitable for accurate measurements, being porous to certain gases at high temperatures, even when glazed. Gas pyrometers are of little use industrially, but are now used as standards for the calibration of other pyrometers, the bulb being made of an alloy of platinum and rhodium. Calorimetric pyrometers, based on Regnault’s “method of mixtures,” were first made for industrial purposes by Byström, who patented an instrument of this type in 1862. This method has been widely applied, and a simplified form of “water” pyrometer, made by Siemens, is at present in daily use for industrial purposes. It is not capable, however, of giving results of the degree of accuracy demanded by many modern processes. The resistance pyrometer was first described by Sir W. Siemens in 1871, and was made by him for everyday use in furnaces. Many difficulties were encountered before this method was placed on a satisfactory footing, but continuous investigation by the firm of Siemens & Co., and also the valuable researches of Callendar and Griffiths, have resulted in the production of reliable resistance pyrometers, which are extensively used at the present time.