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Two minutes, if the fat be sufficiently hot, will fry oysters a pale yellow-brown. They should never take longer than this, for oysters harden and shrink if overdone in the least. For this reason the use of a pyrometer, when possible, saves mistakes and trouble.
The Trampler pyrometer is based upon the difference in the coefficients of dilatation for iron and graphite, that of the latter being about two-thirds that of the former. There is an iron tube containing a stick of hard graphite. This is placed in the medium to be examined, and both lengthen under the heat, but the iron the most of the two.
The Ducomet pyrometer is on a very different principle, and only applicable to rough determinations. It consists of a series of rings made of alloys which have slightly different melting-points. These are strung upon a rod, which is pushed into the medium to be measured, and are pressed together by a spiral spring.
But that it might act with regularity, the apparatus must be kept in perfect order; so each morning Michel visited the escape regulators, tried the taps, and regulated the heat of the gas by the pyrometer.
Here the pyrometer itself consists of a set of tubes one inside the other, and all inclosed for safety in a large tube of fireclay. The central tube or pipe brings in the water from a tank above, where it is maintained at a constant level. This tube projects from the fireclay casing into the medium to be examined, and can be pushed in or out as required.
He does not see the pyrometer the instrument which registers the temperature. Coloured electric lights give him his signals. None of our machines is ever built haphazardly. The idea is investigated in detail before a move is made. Sometimes wooden models are constructed or again the parts are drawn to full size on a blackboard.
He slipped one of the switches into place, and the pyrometer needle swung around and pointed to the degree of heat in the oven which it was supposed to register. "What are those little clocks for?" "One for each oven," Mr. Nebett answered; "the keeper of the furnaces sets them when an oven is up to the required heat. Then, you see, it is easy to tell when they have been cooling long enough."
He did not speak of heat, light, electricity, as forms of energy or "force"; he conceived them as subtile forms of matter as highly attenuated yet tangible fluids, subject to gravitation and chemical attraction; though he had learned to measure none of them but heat with accuracy, and this one he could test only within narrow limits until late in the century, when Josiah Wedgwood, the famous potter, taught him to gauge the highest temperatures with the clay pyrometer.
Is it that the cool snow is grateful after the fervours of their torrid zone, where even the pyrometer would fail to record the temperature? Is it that Dickens is responsible for the season, and that Marley's ghost has set the fashion among the younger spooks? The ghost of Hamlet's father was not so timed: he walked in all weathers.
Mathiessen and others have since enunciated the law according to which this rise of resistance varies with rise of temperature; and Siemens has further perfected his apparatus, and applied it as a pyrometer to the measurement of furnace fires. It forms in reality an electric thermometer, which will indicate the temperature of an inaccessible spot.
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