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In this way the relative dilatation of the graphite is sufficiently magnified to be easily visible. A somewhat similar instrument is the Gauntlett pyrometer, which is largely used in the north of England. Here the instrument is partly of iron, partly of fireclay, and the difference in the expansion of the two materials is caused to act by a system of springs upon a needle revolving upon a dial.
In order to meet the objection that the diffusion of the rays in aphelion do not differ sufficiently, the solar pyrometer has been so arranged that the density, i. e., the diffusion of the reflected rays, can be changed from a ratio of 1 in 5,040 to that of 1 in 10,241. This has been effected by employing heaters respectively 10 inches and 20 inches in diameter.
This method, however, is not very reliable, and was superseded by his well-known electric pyrometer. This rests on the principle that the electric resistance of metal conductors increases with the temperature. In the case of platinum, the metal chosen for the purpose, this increase up to 1,500°C. is very nearly in the exact proportion of the rise of temperature.
The first real improvement in this direction, as in so many others, is due to the genius of Sir William Siemens. His first attempt was a calorimetric pyrometer, in which a mass of copper at the temperature required to be known is thrown into the water of a calorimeter, and the heat it has absorbed thus determined.
So arranged, the pyrometer works with great regularity, indicating the least variations of temperature, requiring no sort of attention, and never suffering injury under the most intense heat; in fact the tube, when withdrawn from the furnace, is found to be merely warm.
Everything was measured the gas by a 60 light, and the air by a 300 light meter; the indicated horse power, by a steam-engine indicator; the useful work, by a Prony brake; the temperature of the water, by a standard thermometer; and that of the escaping gases, by a pyrometer. The results arrived at were as follows: Per cent.
By careful experiment and study he was even enabled to rediscover the art of painting on porcelain or earthenware vases and similar articles an art practised by the ancient Etruscans, but which had been lost since the time of Pliny. He distinguished himself by his own contributions to science, and his name is still identified with the Pyrometer which he invented.
From this it is easy to deduce the temperature to which the platinum has been raised. This pyrometer is probably the most widely used at the present time. Tremeschini's pyrometer is based on a different principle, viz., on the expansion of a thin plate of platinum, which is heated by a mass of metal previously raised to the temperature of the medium.
By means of an ingenious modification of the electrical pyrometer, named the bolometer, valuable researches in measuring solar radiations had been made by Professor Langley. Faraday's great discovery of magneto-induction was next noticed, and the original instrument by which he had elicited the first electric spark before the members of the Royal Institution in 1831, was shown in operation.
The atmospheric conditions having proved unfavorable during the investigation, maximum solar temperature was not recorded. Accordingly, the heaters of the solar pyrometer did not reach maximum temperature, the highest indication by the thermometer of the small heater being 336.5°, that of the large one being 200.5° above the surrounding air.
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