from the substance surrounding the coil, whether this substance be air, water or brine, and the temperature of the latter is gradually being reduced to nearly the temperature of the liquid boiling inside of the coil. The evaporating temperature depends upon the pressure in the cooling coil, low temperature in the cooler requires a correspondingly low pressure in the cooling coil and a higher temperature in the cooler a corresponding increase in the evaporating pressure, which latter is controlled by the regulating valve. lic buildings, the carbonic acid system is decidedly preferred on account of its safety and noiseless operation. Illustrations 2 and 3 show a 45 and a 20 ton carbonic acid compressor, each operated by means of a Corliss engine of proportionate size, both installed in Chicago department stores. The system employed in these particular plants is known as the brine system. This fluid, which has a very low freezing point, is cooled by means of a dircet expansion coil and is then pumped through a series of pipe coils placed in FIG. 4. BRINE PUMPS, D. P. CONDENSER AND COOLER HOUSE BACK OF CONDENSER. In practice this pressure is kept at from 18 to 30 atm., and it is so regulated that its boiling pressure, and so the evaporating temperature, is from 10 to 20° lower than the temperature of the surrounding substance; for instance, if a temperature of +32° F. is to be maintained in the cooler, the evaporating pressure is regulated by the regulating valve to register about 25 atm. on the gauge, which will make the liquid boil in the cooling coil at a temperature of +10° F. For work in a closed building, such as department stores, hotels and other pub side of a large number of coolers The cold brine picks up the heat from the surrounding substance and returns to the cooling coil to be cooled over again. The cooling coil is constructed on the double pipe system and is contained in the cooling house. Fig. 4 shows the brine pumps and condensers which are a part of the plant shown in Fig. 3. Water is cooled by allowing it to flow over a series of cooling coils inside of which the carbonic acid boils or evaporates, when it reaches the bottom of the tank, it is cooled to 38°. Practical Electricity-No. III By G. H. F. Before entering upon a description of the methods by which the sizes of the wires for the several classes of circuits and branches is determined, we will refer to two styles of circuits not described in the previous article, one is called the "Tree system," the other the "Closet system." The "tree system," Fig. 15, is so called from its resemblance to the trunk and branches of a tree, the main circuit repre so that when it is desirable to cut in or out any particular circuit the proper switch must be closed or opened as the case may be, in the closet system the switches for the various circuits are all located at some central point and enclosed in a shallow closet so they may be secured from meddlesome fingers through closing and locking a door, the convenience of this arrangement appeals to everyone and the fact that the switches senting the trunk and the several branch circuits the limbs or boughs of the tree, this system embraces the necessary safety devices, switches, etc., and is mainly used in large buildings, stores, offices, factories, etc. Where wires are broken thus a switch is placed, [] represents a safety plug or fuse. The closet system differs from the tree system in the location of the switches, in fact it is practically the only difference between them. In the tree system the switches are located wherever convenient and may be widely separated FIG 15. are located as stated gives rise to the name "closet system." Suppose we have a large building, as for instance a public hall. The lights are divided up into a number of circuits, so the required number may be cut in or out, a few only being necessary for illuminating the stairways and entrance halls. These are cut in, but the main illumination is reserved until the proper moment and whether the lamps be arranged in clusters, or rows, or, as is usually the case, a combination of these, the circuits are all controlled from some central point, the switches being grouped as shown in Fig. 16. A A are the main circuit wires or mains. we have spent an amount of money unnecessarily and added to the weight without deriving any benefit. So with our wires; if too small, we lose energy BBB are the circuits for the border through resistance, besides running the lights. CC are the circuits for the center lights. risk of overheating; if too large, we have added much to the cost of installation without good reason, therefore we must M M M are switches controlling each always calculate the size of our wires to circuit. do only the work required. SSS are safety plugs. The switches M, safety plugs S and bus bars D D are enclosed in a shallow closet as stated. The mains A A lead to either the main switch board or other 'mains not shown. The above is only a suggestion for arranging the border lights and center clusters for a hall or meeting room, the lights are all placed close to the ceiling and may be arranged as shown, or in any other manner most suitable for the effect desired. The closet containing the switches, etc., is usually placed at a convenient height from the floor where it may be easily reached by the person in charge. Innumerable combinations may be provided for in this manner and the wiring of the circuits may be either concealed, moulding or bracket work. The closets and doors should be lined with some good non-combustible material, preferably asbestos board about an eighth of an inch thick. In some cases special circuits are called for which may be continuously "live" and the lamps controlled by switches within their sockets. The most important point involved in running circuits is the size of the wire. This is established by the number of lights to be used on that particular circuit, or in other words, the size of the wire depends upon the amount of work. to be done, just as is the case if we are to run a line of steam or water pipe, we know the amount of work to be done by the steam, or the supply of water desired, is governed by the size of our pipe. If too small we lose efficiency through an excess of friction, if it is too large, then The capacity of a wire to conduct current depends upon the area of its crosssection. Consider the end of a piece of wire cut off squarely, then when looking at the end we have a circular figure of a given diameter and the area of this circular figure may be easily calculated. Owing to the small size of the circle we do not use feet and inches in our calcu lations, but a unit called a "mil." If we divide an inch into one thousand equal parts, each part will be the one-thousandth of an inch. This may be expressed as a common fraction or as a decimal fraction thus .001. In all calculations relating to electric light wiring we use this unit, not as a measure of the diameter of the wire, but of its circular area; therefore we call the unit a "circular mil," and the "circular mil" is the product of multiplying the diameter in mils by itself, in other words, if we have a wire of a certain number of thousandths of an inch in diameter, we say it is so many "mils" in diameter, and to obtain the area or "circular mils" we square the diameter, that is, multiply it by itself. Suppose our wire measures six one thousandths of an inch in diameter, then we would say we have a diameter of six "mils" and the circular area would equal six multiplied by six or thirty-six "circular mils." The ability of a wire of any size to conduct or convey a current of electricity is called its "carrying capacity," this carrying capacity is rated in amperes and depends upon the circular mils of the wire. A number of gauges are used for measuring the diameter of wire; the one which is considered as the standard in this country being the Brown and Sharp, |