As can be seen from these drawings the whole construction of the apparatus is more or less of the refrigerator type, i. e., there is little opportunity for radiation or conduction of heat. Such a construction could be multiplied a number of times, giving a greater number of insulating walls, and perhaps reducing radiation to the minimum, but for extreme accuracy in calorimetric investigations it is necessary to insure the absence of radiation, and hence we have retained the ingenious device of Rosa, by which an attempt is made arbitrarily to alter the temperature of the zinc wall so that it always follows any fluctuations in the temperature of the copper wall. To this end it is necessary to know first that there is a temperature difference between zinc and copper and, second, to have some method for controlling the temperature of the zinc. Leaving for a moment the question of measuring the temperature differences between zinc and copper, we can consider here the methods for controlling the temperature of the zinc wall.
If it is found necessary to warm the zinc wall, a current of electricity is passed through the resistance wire W, fig. 12. This wire is maintained approximately in the middle of the air-space between the zinc wall and hair-felt by winding it around an ordinary porcelain insulator F, held in position by a threaded rod screwed into a brass disk soldered to the zinc wall. A nut on the end of the threaded rod holds the insulator in position. Much difficulty was had in securing a resistance wire that would at the same time furnish reasonably high resistance and would not crystallize or become brittle and would not rust. At present the best results have been obtained by using enameled manganin wire. The wire used is No. 28 American wire-gage and has resistance of approximately 1.54 ohms per foot. The total amount of wire used in any one circuit is equal to a resistance of approximately 92 ohms. This method of warming the air-space leaves very little to be desired. It can be instantaneously applied and can be regulated with the greatest ease and with the greatest degree of refinement.
If, on the other hand, it becomes necessary to cool the air-space next to the zinc and in turn cool the zinc, we must resort to the use of cold water, which is allowed to flow through the pipe C suspended in the air-space between the zinc and hair-felt at approximately the same distance as is the heating wire. The support of these pipes is accomplished by placing them in brass hangers G, soldered to the zinc and provided with an opening in which the pipe rests.
In the early experimenting, it was found impracticable to use piping of very small size, as otherwise stoppage as a result of sediment could easily occur. The pipe found best adapted to the purpose was the so-called standard one-eighth inch brass pipe with an actual internal diameter of 7 millimeters. The opening of a valve allowed cold water to flow through this pipe and the considerable mass of water passing through produced a very noticeable cooling effect. In the attempt to minimize the cooling effect of the mass of water remaining in the pipe, provision was made to allow water to drain out of this pipe a few moments after the valve was closed by a system of check-valves. In building the new apparatus, use was made of the compressed-air service in the laboratory to remove the large mass of cold water in the pipe. As soon as the water-valve was closed and the air-cock opened, the compressed air blew all of the water out of the tube.
Fig. 13.—Detail of drop-eight feed-valve and arrangement of outside cooling circuit. The water enters at A, and the flow is regulated by the needle-valve at left-hand side. Rate of flow can be seen at end of exit tube just above the union. The water flows out at C and compressed air is admitted at B, regulated by the pet-cock.
The best results have been obtained, however, with an entirely new principle, namely, a few drops of water are continually allowed to pass into the pipe, together with a steady stream of compressed air. This cold water is forcibly blown through the pipe, thus cooling to an amount regulated by the amount of water admitted. Furthermore, the relatively dry air evaporates some of the water, thereby producing a somewhat greater cooling effect. By adjusting the flow of water through the pipe a continuous cooling effect of mild degree may be obtained. While formerly the air in the space next the zinc wall was either cooled or heated alternately by opening the water-valve or by passing a current through the heating coil, at present it is found much more advantageous to allow a slow flow of air and water through the pipes continuously, thus having the air-space normally somewhat cooler than is desired. The effect of this cooling, therefore, is then counterbalanced by passing an electric current of varying strength through the heating wire. By this manipulation it is unnecessary that the observer manipulate more than one instrument, namely, the rheostat, while formerly he had to manipulate valves, compressed-air cocks, and rheostat. The arrangement for providing for the amount of compressed air and water is shown in fig. 13, in which it is seen that a small drop-sight feed-water valve is attached to the pipe C leading into the dead air-space surrounding the calorimeter chamber. Compressed air enters at B and the amount entering can be regulated by the pet-cock. The amount of water admitted is readily observed by the sight feed-valve. When once adjusted this form of apparatus produces a relatively constant cooling effect and facilitates greatly the manipulation of the calorimetric apparatus as a whole.
THE THERMO-ELECTRIC ELEMENTS.
In order to detect differences in temperature between the copper and zinc walls, some system for measuring temperature differences between these walls is essential. For this purpose we have found nothing that is as practical as the system of iron-German-silver thermo-electric elements originally introduced in this type of calorimeter by E. B. Rosa, of the National Bureau of Standards, formerly professor of physics at Wesleyan University. In these calorimeters the same principle, therefore, has been applied, and it is necessary here only to give the details of such changes in the construction of the elements, their mounting, and their insulation as have been made as a result of experience in constructing these calorimeters. An element consisting of four pairs of junctions is shown in place as T-J in fig. 25.
One ever-present difficulty with the older form of element was the tendency for the German-silver wires to slip out of the slots in which they had been vigorously crowded in the hard maple spool. In thus slipping out of the slots they came in contact with the metal thimble in the zinc wall and thus produced a ground. In constructing the new elements four pairs of iron-German-silver thermal junctions were made on essentially the same plan as that previously described,[6] the only modification being made in the spool. While the ends of the junctions nearest the copper are exposed to the air so as to take up most rapidly the temperature of the copper, it is somewhat difficult to expose the ends of the junctions nearest the zinc and at the same time avoid short-circuiting. The best procedure is to extend the rock maple spool which passes clear through the ferule in the zinc wall and cut a wide slot in the spool so as to expose the junctions to the air nearest the ferule. By so doing the danger to the unprotected ends of the junctions is much less. The two lead-wires of German silver can be carried through the end of the spool and thus allow the insulation to be made much more satisfactorily. In these calorimeters free use of these thermal junctions has been made. In the chair calorimeter there are on the top 16 elements consisting of four junctions each, on the rear 18, on the front 8, and on the bottom 13. The distribution of the elements is made with due reference to the direction in which the heat is most directly radiated and conducted from the surface of the body.