The floor of the incubator, which is raised by short feet from the table
on which it stands, is perforated in the central portion by a number
of holes, and which are so situated that they lie beneath the raised
cylinder of the cold-water tray (Z). The incubation-drawer is thus
supplied continuously by a slow current of moistened air because the
air in the upper part of the drawer, i.e. in contact with the floor of
the warm-water tank, is the warmest and lightest. It therefore
tends to diffuse or pass through the narrow slits between the drawer
and the walls of the incubator, and also through the aperture in the
front wall of the egg-drawer, through which a thermometer is
laced. To replace the air thus lost, fresh air passes in through the
holes in the bottom of the incubator, and on its way must pass
through the pores of the damp canvas which dips into the water in
the zinc tray (Z).
The warm-water tank is heated by an inlet (I) and outlet (O) flue which are, however, continuous. The inlet flue opens out from a vertical chimney (C), the air in which is heated either by a gas flame or that of an oil lamp. The outlet or return flue passes back through the width of the tank and opens independently to the exterior. The vertical chimney (C) is capped by a lid (L) capable of being raised or lowered upon its orifice by the lever (L′). When the cap is resting upon the chimney all the heated air from within the latter passes through the flues and heats the water in the tank. If the cap is widely raised, practically all the heated air passes directly upwards through the chimney and none goes through the flues. If the cap be but slightly raised, part of the heated air goes through the flues and part directly escapes through the aperture of the chimney. The movement of the lever (L′) which raises the cap (L) is determined by the thermostatic capsule (S), situated within the egg-drawer.
The principle upon which this capsule is designed is that the boiling point of a liquid depends not only upon temperature but also upon pressure. A given liquid at ordinary atmospheric pressure will boil at a certain degree of temperature, which varies for different substances. But if the pressure be increased the boiling point of the liquid is raised to a higher degree of temperature. A liquid when it boils passes into a gaseous condition and in this state will occupy a very much larger volume—some two or three hundred times—than in the liquid condition. If, therefore, a hermetically sealed capsule with flexible sides be filled with some liquid which boils at a given temperature, the sides of the capsule will distend when the temperature of the air round the capsule has been raised to the boiling point of the liquid within it. The distension of this capsule can be used to raise the lever (L′). The thermostatic capsule is placed on a fixed cradle (F) and is filled with a mixture of ether and alcohol, the proportions being such that the boiling point of the mixed liquid is 100° F. Between the capsule and the lever (L′) is a vertical rod (V), articulating with the lever as close as possible to its fulcrum (M). The articulation with the lever is by means of a screw, so that the necessary nice adjustment between the height of the rod (V), the thickness of the capsule and the position of rest of the damper (L) upon the chimney, can be accurately made. The temperature at which it is desired that the liquid in the capsule shall boil can be determined by sliding the weight (W) nearer or farther to the fulcrum of the lever (L′). The farther it is moved outwards, the greater is the pressure upon the thermostatic capsule and consequently the higher will be the boiling point of its contained liquid. By means of the milled-head screw (A), the height of the lever at its outer end can be so adjusted that when the liquid of the capsule is not boiling the damper (L) closes the chimney, but that when it does boil the damper will be raised sufficiently high from it. If the weight is pushed as far as it will go towards the fulcrum end of the lever, the temperature of the egg-drawer will never rise more than 100° F. because at this temperature and under the pressure to which it is then subjected, the liquid in the capsule boils, and consequently brings about the raising of the damper. It matters not, therefore, how high the flame of the gas or lamp be turned, the temperature of the egg-drawer will not increase, because the extra heat of the enlarged flame is passing directly outwards through the chimney, and is not going through the flues in the tank. In order to raise the temperature within the incubation chamber to 102° or 103°, or any other desired degree, the weight (W) must be moved outwards along the lever (L′), about 1 in. for every degree of temperature increase desired. This thermostatic capsule works admirably, and the incubator will work for months at a time and requires no adjustment, however much, within the limits of our climate, the external temperature may vary. The capsule, like all other thermostats in which the expansible substance is a liquid, is, however, dependent upon external pressure for the point at which its contained liquid boils and therefore, for the degree of temperature prevailing within the incubator drawer. It is therefore responsive to variations in atmospheric pressure, and as the barometer may fall 1 or 2 in., this may possibly make a difference of two or three degrees in the fluctuation of temperature within the egg-drawer. It is not, of course, often that such large oscillations of the barometer occur, and as a matter of practical experience, under ordinary conditions, this incubator will work for months together without attention with only half a degree variation round the point at which it was set.
_Incubator.jpg) |
| Fig. 5.—The Bedford (Greenwood’s) Incubator. |
Greenwood’s incubator (fig. 5), named the Bedford, resembles Pearson’s in that hot-air flues (F and F′) and not hot-water pipes, traverse the water tank (T). And the method of regulation of the temperature is much the same, i.e. a thermostat (V) operating upon a lever which raises a cap (C) from off the aperture of the main flue (F) and thus allows all the heat of the flame to pass directly outwards, without passing through the series of flues (F) which horizontally traverse the water-tank. Fresh air enters through a wide circular aperture (A) which surrounds the main flue, and it thus becomes partially warmed before entering the egg-chamber. The eggs are placed upon a perforated floor (E) lying over water baths (B). The water tank (T) lies in the centre of the incubation chamber and is traversed through its central axis by the main hot-air flue (F). From this, four horizontal flues pass outwards through the water and open into small vertical flues, which in their turn communicate with the exterior.
The thermostat (V) consists of a glass tube of peculiar form. This is closed at the end of its short limb and open at its other extremity on the long limb. The bent portion of the tube is filled with mercury and between the mercury column and the closed end is a small quantity of ether. The thermostat is lodged in a box (G), which forms part of the lever (L). At one end this lever is pivoted to a fixed arm, and at the other to the vertical rod which operates the ventilating cap (C). If the temperature should rise, the ether in the thermostat expands and pushes the mercury column up along the inclined long limb. This disturbs the equilibrium of the lever (L), and it descends downwards, pulling with it the vertical rod, and thus raising the cap over the main flue. If the temperature falls the reverse series of changes occur. The temperature at which the cap will be raised can be adjusted within limits by the position of the weight (W) and by the adjustment of the degree of inclination of the thermostat.
The Proctor incubator, made at Otley, is apparently, in its main features, similar to the Greenwood.
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| Fig. 6.—The Winchcombe Incubator. |
Somewhat similar, in certain features, to the Greenwood is the Winchcombe. Its improved form, in which metal replaces the wood casing, is named the Gladstone. In it there is a combination of the hot-air and the water-tank systems of warming the incubation chamber. The wall of the incubator is double, and the space between the outer and inner wall is packed with a non-conducting material. The incubation chamber is heated above by a water-tank (fig. 6 T) which is traversed by a main vertical flue (F) and four subsidiary horizontal ones which discharge externally. The main flue, however, in passing up to enter the water tank traverses the egg-chamber, and therefore serves to warm it, as in the hot-air type of incubator, by the heat of the flue itself. Around the lower half of the flue is a water vessel consisting of two concentric containers (C), holding water. In the space between these concentric containers, fresh air passes in through the aperture (A), and before it reaches the egg-chamber it passes through coarse canvas which dips into the water in the containers, and is therefore kept permanently moist. The containers are filled from a water tank (S) outside the incubator. Air passes out from the egg-chamber through the aperture (O). The temperature is regulated by a bimetallic thermostat (see below), which operates two levers, that by their arrangement can raise or depress the cap (D) over the main flue (F). The temperature at which this occurs will be determined, within limits, by the position of the adjustable weight (W).
Tomlinson’s incubator, designed in 1880, is novel in principle.
