The preceding requirements are of course general in their nature, and many minor points are of nearly as much importance, but to thoroughly treat of them would necessitate a separate article on boiler design.

There is actually no such thing as the horsepower of a boiler, for the term "horsepower" is applicable only to dynamic energy, while the power of a boiler is static energy.

However, as the boiler furnishes energy to the steam engine, it is convenient to designate its capacity by the actual measured effect on the steam engine; in other words, we use the engine as a measuring rod for the boiler.

It will be easily seen that an immense variation can take place in the rated capacity of a given boiler; the varying effect being produced by a difference in quality of coal, the skill of the fireman, the amount of horsepower developed per pound of coal burned on the grate, and the amount of draft.

The preceding causes will evidently effect the same boiler to vary its power. Now add to this the different types of heating surface arrangements, water

circulation, flues, uptakes, and stack designs, and one begins to have an idea of the difficulty of estimating the horsepower of a boiler by looking at the outside of it, as is quite possible in the case of an engine.

A boiler really has no horsepower until it is connected up and furnishing power to a prime mover, and it is then necessary to know the type, of engine, kind of coal used, and various other particulars before one can give an estimate, and it cannot be done then unless one has had previous experience with that type of boiler.

The standard of horsepower adopted by the Committee of Judges, at the Centennial Exposition, has been accepted by all prominent steam users and is endorsed by the American Society of Mechanical Engineers. It is as follows:

"An evaporation of 30 lb. of water per hour from a feedwater temperature of 100° F. into steam at 70 lb. gauge pressure, which shall be considered to be equal to 34 lb. of water evaporated from a feedwater temperature of 212° F. into steam at the same temperature."

This for good engines, according to Prof. Thurston, would be about 25 lb. of water working with 64 lb. of steam, and for the best engines 15 lb. of water per 100 lb. steam pressure per hourly horsepower.

This formula is not convenient for reducing to the familiar term of "horsepower,' ," while standing on the stoop of the boiler shop guessing at the power of the boiler.

The purpose of this article, and the accompanying chart, is to furnish an empirical method of finding the horsepower of a boiler.

Roughly, it is true, including in the estimate four principal factors, viz.: kind of firing, amount of draft, type of boiler, and prime mover.

Boilers of beating and distillery

plants, etc., are not taken into consideration. It is only intended for those boilers in which the steam is used by that type of prime mover wherein the horsepower can be indicated. The table is intended for average work and any special boiler must be calculated for its own work.

For instance, a boiler may use oil or a mixture of slack that hardly deserves the name of coal.

The conditions for which this table is intended are as follows:

Fuel, average quality of bituminous coal as may be found throughout the country; firing, such as would obtain in the ordinary machine shop, or good firing, such as in large electric light plants.

The coal per horsepower of the engine is averaged from a large number of engine trials.

It must be understood that the results obtained in horsepower are only for general use and are not supposed to be

as accurate as would be obtained by an exhaustive test.

The curves may be altered to suit any particular class of boiler or engine.

The same principle may be used to work out a series of horsepower curves, using the heating surface and evaporation of water per pound of coal in accordance with the standard metho of determining horsepower.

An example of the method of working the curves is shown by the dotted lines.

We have a Scotch boiler 50 sq. ft. grate surface, 90-ft. stack, with good firing to furnish power to a triple expansion engine. What is the probable horsepower of the boiler?

Method of procedure:

Follow a line from 90-ft. stack until it cuts oblique line marked "Scotch Boiler"; thence across to line marked "triple engine and good firing," and thence to 50 sq. ft. G. S. A line carried from this latter intersection to the edge will give the horsepower.

500 K. W. GENERATORS

THE
Trecently closed

HE Sprague Electric Company has

recently closed an interesting order for three-Lundell split-pole500 K. W. engine type generators, with speed of 100 R. P. M., and wound for 250 volts, to be direct connected to gas engines.

These three generators of 500 K. W. capacity each will be installed in the new works of the Lackawanna Iron & Steel Co., of Buffalo, and are designed for a continuous overload of 25 per cent. at a high efficiency.

The gas engines will utilize as fuel

DRIVEN BY GAS ENGINES

HAVE

od of representing alternating currents, we are now in a position to look into some of the peculiarities they exhibit as compared with direct currents. After this has been done, the laws governing the flow of alternating currents will next be taken up together with examples of simple problems that illustrate the application of these laws. If alternating and direct currents, along with a few commercial measuring instruments, are available, it is an easy matter to perform experiments to illustrate the effects mentioned in the following.

Most of the peculiarities that alternating currents exhibit, as compared with direct currents, are due more or less indirectly to the fact that an alternating current is constantly changing, whereas a continuous current flows uniformly in one direction. Whenever a current flows through a wire, it sets up a magnetic field around the wire, and since the current changes continually, this magnetic field will also change. Whenever the magnetic field surrounding a wire is made to change, an E. M. F. is set up in the wire, and this induced E. M. F. opposes the current. For example, suppose we take a bundle of iron wires and wind on them, say a few hundred turns of insulated wire having a resistance of 5 ohms. connect this coil across a 100-volt direct current circuit, a current of 20 amperes will flow, because from Ohm's Law we know that current E. M. F. hence, in this case, the Resistance' 100 current would be 20 amperes. 5 Suppose, now, that we connect the

If we

same coil to a source of alternating current giving the same voltage (100) volts effective) at, say, a frequency of 60 cycles per second. We will find that the ammeter will not register 20 amperes as before, but considerably less than 20 amperes. The resistance of the coil in ohms, or the ohmic resistance as it is sometimes called, has not been changed in any way, and the voltage applied is the same as before, but the current is less because the alternating current sets up a varying magnetism through the iron core, and the varying magnetism threading through the coil in turn sets up an induced E. M. F. that chokes back the current. The coil is said to have self-induction because it is capable of setting up lines of force through itself and giving rise to an induced E. M. F. Many devices met with in alternating-current work have self-induction. A long transmission line has a certain amount of it, as have also induction motors and transformers. Incandescent lamps have very little self-induction, because the single loop, which constitutes the filament of the lamp, is able to set up but a very small amount of magnetism around itself. A water rheostat has very little self-induction, and resistances of this kind are usually spoken of as noninductive resistances.

Coming back to the coil mentioned above, we note that a device which possesses self-induction tends to choke back, as it were, the flow of an alternating current. Direct current would flow through the resistance of 5 ohms. the same as any other resistance, but the fact that self-induction is present makes a great difference in the amount of alternating current that the given

in series with a coil M, which has a considerable amount of self-induction. The two are connected across a 100-volt

alternating-current circuit. Now, if these two devices had been connected across a direct-current circuit, and if the voltage E, between points ab were measured by means of a voltmeter and added to the voltage E, obtained between bc, the sum of the two would, of course, be equal to 100 volts. If, however, we measure these two pressures when alternating current is flowing and add the results, we get the apparently impossible result that the arithmetical sum is greater than the line voltage E.

Fig. 2 shows another condition where alternating currents exhibit a peculiarity. If we take a number of sheets of tin foil and interleave them with a corresponding number of slightly larger sheets of waxed paper and then press the whole mass tightly together, we will have an electrical condenser. Alternate sheets are connected together to form one terminal, and the intervening

sheets form the other terminal. In Fig. 2 the sheets of tin foil are represented by the lines, but the sheets of paper have been omited in order not to confuse the figure. T and T" are the two terminals of the. condenser. Note particularly that there is no electrical connection between T and T'; if a direct current were applied to the terminals, no current could flow unless the insulation between the sheets broke down. If a galvanometer G were connected in circuit and a direct E. M. F. applied, it would be noticed that just after the pressure was applied a current would flow for a short interval; also, that if the terminals a b were disconnected from the source of direct current and connected together, the galvanometer would give a deflection in the reverse direction. There is a momentary current just at the instant the condenser is charged, and a reverse momentary current when it is discharged. This action will be understood by referring to Fig. 3. Here A represents a cylinder in which a piston p may be moved. R is a reservoir