THERMO-REGULATORS OR THERMOSTATS Certain special forms of thermo-regulators, adapted to the requirements of the particular incubators to which they are at tached, have already been described. It remains now to describe other forms which are of more general application. Only those kinds will be described which are applicable to incubators. The special forms used for investigations in physical-chemistry are not described. There are various types of thermo-regulators, all of which fall into one of two classes. Either they act through the expansion of a solid, or through that of a liquid. They are adjusted so that at a certain temperature the expansion of the material chosen either cuts down the gas supply or operates a switch which cuts off the whole or part of the electricity supply to the heating units.
Reichert's Thermo-regulator.—This (fig. 9) is one of the simpler forms of gas regulator in which a column of mercury raised by expansion obstructs the gas flow. The stem (S) of the regulator is enlarged above and receives a hollow T-piece (P), the vertical limb of which fits accurately into the en larged end of the stem, and one end of the cross-limb receives the inlet gas pipe ; the other end is closed. The vertical limb of the T-piece is narrowed down at its lower extremity and opens by a small aperture. Above this terminal aperture is a lateral one of the smallest size. From the en larged end of the stem there passes out a lateral arm (A) which is connected with the outlet pipe to the burner, and lower down another arm (L), which is closed at its outer extremity by a screw (R), is also at tached. The stem and lower arm are filled with mercury and the bulb of the stem is placed in the incubator chamber, and gas allowed to pass. When the desired temperature is reached, the mercury in the stem is forced upwards until it closes the aperture of the T-piece, by screwing in the screw (R) of the lower lateral arm (L).
There are several modifications of Reichert's original form. In one of these the screw arrangement in the lower arm is replaced by a piston rod working in a narrow bore of a vertically bent limb of the arm. In another form, the other end of the cross bar of the T-piece is open and leads through a stopcock to a third arm, which opens into the enlarged upper end of the stem opposite to the outlet arm (A) ; this modification acts as an adjustable by-pass and replaces the minute aperture in the side of the vertical limb of the T-piece.
Other Types. There have been many modifications of this apparatus employing essentially the same method of regulation and differing only in minor details of construction.
Bunsen's, Kemp's and Miinke's regulators are in reality of the nature of air-thermometers, and act by the expansion and con traction of air, which raises or lowers respectively a column of mercury; this in its turn results in the clusion or opening of the gas aperture. Such forms, however, are subject to the fluence of barometric pressure and an ation of 0.5 in. of the barometer column may result in the variation of the ture to as much as 2°.
Lothar Meyer's regulator is described in the Berichte of the German Chemical So ciety, i 883, p. 1089. It is essentially a liquid thermometer, the mercury column being raised by the expansion of a liquid of low boiling-point. The liquid replaces the air in Bunsen's and other similar forms. The boiling-point of this liquid must be below the temperature required as constant.
Toluol-mercury Thermo-regulators. —Novy's thermo-regulator, which is an ap paratus of this type, is shown in fig. io. The bulb (E) is full of toluol which by its expansion forces up the mercury column thus occluding the opening (f). Toluol is used on account of its high coefficient of expansion. Apart from the use of toluol instead of mercury in the bulb the principles of the apparatus are essentially the same as Reichert's. The two ground stoppers (B) and (C) with openings (b, c) and (d) may be rotated so as to regulate both the volume of gas entering the apparatus and the quantity flowing through the Regulators Controlling Electrical Heaters.—Instruments like the above-described, which incorporate a varying column of mercury, are easily modified to control electrically-heated incu bators or water-baths. A platinum wire is fused through the glass so as to make contact with the lower part of the mercury column and the gas inlet tube (f, fig. 1o) is replaced by a platinum wire contact. The rising mercury column completes a circuit which operates a small electromagnetic switch breaking the circuit to the heating units. To ensure the satisfactory working of such con tacts as these it is necessary to avoid sparking and this may be effected by inserting a condenser (i micro farad) across the terminals of the spark gap.
Metallic Thermo-regulators.—T h e solid forms of thermostats are constructed upon the same principle as the compensa tion balance of a watch or the compensat ing pendulum of a clock. This depends on the fact that the coefficient of expansion is different for different metals. It there fore results that if two bars of different metals are fastened together along their lengths (fig. II, Z and ST) with the same rise of temperature one of these will expand or lengthen more than the other.
And since both are fastened together and must therefore accommodate themselves within the same linear area, it follows that the compound rod must bend into a curved form, in order that the bar of greater expansion may occupy the surface of greater length, i.e. the convex one. Con versely, when the temperature falls, the greater degree of con traction will be in the same bar, and the surface occupied by it will tend to become the concave one. If, then, one end of this com pound rod be fixed and the other free, the latter end will describe a backward and forward movement through the arc of a circle, which will correspond with the oscillations of temperature. This movement can be utilized by means of simple mechanical arrange ments, to open or close the stopcock of a gas supply pipe.
In the construction of this type of thermostat it is obvious that the greater the difference in the coefficient of expansion of the two metals used, the larger will be the amplitude of the movement obtained. Steel and zinc are two metals which satisfy this condi tion. The coefficient of steel is the lowest of all metals and is comparable in its degree with that of glass. Substances which are not metals, such as vulcanite and porcelain, are sometimes used to replace steel, as the substance of low coefficient of expansion.
Roux Thermostat.—The bi metallic thermostat most com monly employed is one of the two forms designed by Dr. Roux.
12) . In the former type the bar itself is enclosed in a tube (T) of metal, the wall of which is per forated. Towards the open end of this tube the gas box or case (C) is fixed. In the U-shaped form it is attached to the outer surface (zinc) of one limb of the bar. The gas box is capable of adjustment with respect to its distance from the bar, by means of a screw (S) and a spiral spring (SP) , which moves the box out wards or inwards along a rod (R). This adjustment enables the degree of temperature at which it is desired that the gas shall be cut off to be fixed accurately, and within a certain more or less extended range. The inlet and the outlet pipe are disconnected from each other in the gas box by means of a piston-like rod (P) and valve (V), which slides backwards and forwards in the tubular part (T) of the box, from which the outlet pipe emerges. When the valve (V) rests upon the edge of this box, the gas is completely cut off from passing through the outlet pipe, with the exception of that which passes through an exceedingly small aperture (B), serving as a by-pass. This is just large enough to allow sufficient gas to pass to maintain a small flame. The piston-like rod and valve, when free, is kept pressed outwards by means of a spiral spring. This ensures that the valve shall follow the movements of the compound bar. When this bar bends towards the gas box owing to a fall of temperature, the valve is pushed back away from the orifice and gas in increasing quantity passes through. The temperature of the incubator begins then to rise, and the zinc bar (Z) expanding more than the steel one (ST) , the bar bends out wards and the valve once more cuts off the gas supply.
In this regulator (fig. 13) the inlet (I) and outlet (0) gas pipe open into a metal bell (B), the lower and open end of which is immersed beneath water contained in a metal tray (T). The bell is suspended upon the arm of a balance (B) and the other arm is poised by a weight (W). This weight may be made of any convenient ma terial. In the original apparatus a test-tube partially filled with mercury was used.
The weight dips into one limb of a U shaped glass tube (U),which contains mer cury. Into the other limb of this tube the gas from the meter enters through a glass tube (G) which is held in position by a well fitting cork. The internal aperture of the tube (G) is very oblique, and it rests just above the level of the mercury when the instru ment is finally adjusted. This adjustment is better made in the morning when the gas pressure in the main is at its lowest. Just above the internal aperture of the tube (G), a lateral tube (L) passes out from the limb of the U and is connected with the inlet pipe (I) of the bell. If the gas pressure rises, the bell (B) is raised and the counter-poising weight (W) is proportionately lowered. This forces the mercury up in the other limb of the U-tube and consequently diminishes the size of the oblique orifice in the tube (G). Some of the gas is thus cut off and the pressure maintained constant. Should the pressure fall, the re verse processes occur, and more gas passes through the orifice of G and consequently to the burner by the outlet tube (0). (For incubators for poultry see POULTRY AND POULTRY FARMING.) BIBLIOGRAPHY.--Charles A. Cyphers, Incubation and its Natural Bibliography.--Charles A. Cyphers, Incubation and its Natural Laws (1776) ; J. H. Barlow, The Art and Method of Hatching and Rearing all Kinds of Domestic Poultry and Game Birds by Steam (London, 1827) ; and Daily Progress of the Chick in the Egg during Hatching in Steam Apparatus (London, 1824) ; Walthew, Artificial Incubation (London, 1824) ; William Bucknell, The Eccaleobin. A Treatise on Artificial Incubation, in 2 parts (published by the author, London, 1839) ; T. Christy, Jr., Hydro-Incubation (London, 1877) ; L. Wright, The Book of Poultry (end ed. London, 1893) ; A. Forget, L'Aviculture et l'incubation artificielle (Paris, 1896) ; J. H. Sutcliffe, Incubators and their Management (London, 1896) ; H. H. Stoddard, The New Egg Farm (New York, 19oo) ; Edward Brown, Poultry Keeping as an Industry (5th ed., i9o4) ; F. J. M. Page, "A Simple Form of Gas Regulator, Journ. Chem. Soc. i. 24 (London, 1876) ; V. Babes, "Ober einige Apparate zur Bacterienuntersuchung," Central blatt fur Bacteriologie, iv. (1888) ; T. Huppe, Methoden der Bacterien forschungen (Berlin, 1889) . For further details of bacteriological incubators and accessories see catalogues of Gallenkamp, Baird & Tatlock, Hearson of London, and of the Cambridge Scientific Instru ment Company, Cambridge ; of P. Lequeux of Paris ; and of F. & M. Lautenschlager of Berlin. Those of Lequeux and of the Cambridge Company are particularly useful, as in many instances they give a scientific explanation of the principles upon which the construction of the various pieces of apparatus is based. (G. P. M.; S. L. B.)