OPENINGS IN THE CHAMBER.
In order to communicate with the interior of the chamber, maintain a ventilating air-current, and provide for the passage of the current of water for the heat-absorber system and the large number of electrical connections, a number of openings through the walls of the chamber were necessary. The great importance of maintaining this chamber absolutely air-tight renders it necessary to minimize the number of these openings, to reduce their size as much as possible, and to take extra precaution in securing their closure during an experiment. The largest opening is obviously the trap-door at the top through which the subject enters, shown in dotted outline in fig. 7. While somewhat inconvenient to enter the chamber in this way, the entrance from above possesses many advantages. It is readily closed and sealed by hot wax and rarely is a leakage experienced. The trap-door is constructed on precisely the same plan as the rest of the calorimeter, having its double walls of copper and zinc, its thermal-junction system, its heating wires and connections, and its cooling pipes. When closed and sealed, and the connections made with the cooling pipes and heating wires, it presents an appearance not differing from any other portion of the calorimeter.
The next largest opening is the food-aperture, which is a large sheet-copper tube, somewhat flattened, thus giving a slightly oval form, closed with a port, such as is used on vessels. The door of the port consists of a heavy brass frame with a heavy glass window and it can be closed tightly by means of a rubber gasket and two thumbscrews. On the outside is used a similar port provided with a tube somewhat larger in diameter than that connected with the inner port. The annular space between these tubes is filled with a pneumatic gasket which can be inflated and thus a tight closure may be maintained. When one door is closed and the other opened, articles can be placed in and taken out of the chamber without the passage of a material amount of air from the chamber to the room outside or into the chamber from outside.
The air-pipes passing through the wall of the calorimeter are of standard 1-inch piping. The insulation from the copper wall is made by a rubber stopper through which this piping is passed, the stopper being crowded into a brass ferule which is stoutly soldered to the copper wall. This is shown in detail in fig. 25, in which N is the brass ferule and M the rubber stopper through which the air-pipe passes. The closure is absolutely air-tight and a minimum amount of heat is conducted out of the chamber, owing to the insulation of the rubber stopper M. The water-current enters and leaves the chamber through two pipes insulated in two similar brass ferules soldered to the copper and zinc walls. The insulation between the water-pipe and the brass ferule has been the subject of much experimenting and is discussed on page 24. The best insulation was secured by a vacuum-jacketed glass tube, although the special hard-rubber tubes surrounding the electric-resistance thermometers have proven very effective as insulators in the bed calorimeter.
A series of small brass tubes, from 10 to 15 millimeters in diameter, are soldered into the copper wall in the vicinity of the water-pipes. These are used for electrical connections and for connections with the manometer, stethoscope, and pneumograph. All of these openings are tested carefully and shown to be absolutely air-tight before being put in use.
In the dome of the calorimeter, and directly over the head of the subject, is the opening for the weighing apparatus. This consists of a hard-rubber tube, threaded at one end and screwed into a brass flange heavily soldered to the copper wall (fig. 9). When not in use, a solid rubber stopper on a brass rod is drawn into this opening, thus producing an air-tight closure. When in actual use during the process of weighing, a thin rubber diaphragm prevents leakage of air through this opening. The escape of heat through the weighing-tube is minimized by having this tube of hard rubber.