CURRENT CURVES OF 3-PHASE BALANCED SYStem.

a three-phase balanced system. Now, the direction and magnitude of current in each one of the three phases may be determined at any given instant by drawing a line such as A B at any point, such as for instance 150°. Let us consider that the current in one direction is positive, as represented by the curves above the line; and in the opposite direction negative, as represented by the curves below the line. Then, at A B, the currents in phases I and 2 are in a positive direction and equal, while the current in phase 3 is in the negative direction. Again, at C D, the current in phase 2 is zero, while that in phase I is positive and exactly equal to that in phase 3, which is negative. In the same way, by passing lines through the curves, similar to A B and C D, it is possible to investigate the direction and value of currents at any desired point.

The algebraic sum of the ordinates of the three curves is equal to zero at any point. For instance, at C D, the ordinate

Operation of Cream Separator Question: How does a cream separator work?-W. B.

Answer: Centrifugal force is used to separate the cream from the milk. The receptacle for the milk is arranged with a spindle passing through the casing, so that it can be revolved at the rate of from 1,000 to 1,500 revolutions per minute. The centrifugal force throws the heavier milk near the outer part of the receptacle, where it is drawn off through a pipe for the purpose. The lighter cream, remaining near the center, is drawn out through another pipe. There is also a supply pipe so that the original milk can be run in while the machine is in operation.

Manufacture of Citric Acid

Question: How is citric acid made?—H. K. Answer: Citric acid is made on a large scale from the juice of the lime and the lemon, chiefly in the months of November and December. The juice is fermented for some time, to free it from mucilage; then boiled, filtered, and neutralized with powdered chalk and a little milk of lime. The precipitate of calcium citrate so obtained is decomposed with dilute sulphuric acid; and the resulting

solution of citric acid is separated by filtration, evaporated to remove calcium sulphate, and then concentrated in

vacuum pans.

Velocity of Steam

Question: Could you give me the velocity of steam at different pressures-say 100 lbs., 200 lbs., 300 lbs., etc., up to 750 pounds, per square inch; or the rule or way to figure it out? The sizes of the holes are 3% inch, 2 inch, and 5% inch in diameter.-A. H.

Answer: The flow of steam of greater pressure into an atmosphere of less pressure increases as the difference of pressure is increased, until the external pressure becomes only about 4-7 of the inside pressure. The flow is neither increased nor diminished by a difference of pressure greater than this ratio. When steam of varying initial pressures is discharged into the atmosphere, the atmosphere. being not more than 4-7 of the initial pressure, the velocity of outflow at constant density is given by the formula V=3.595 V h, in which V= the velocity of outflow, in feet per second, for steam of the initial density; h = height, in feet, of the column of steam of the given absolute initial pressure of uniform density, the weight of which is equal to pressure on the unit of base. Applying the above formula, the velocity for steam at 100 pounds absolute pressure will be nearly 900 feet per second. If the orifices are small, the velocity will not vary except with changes in the pressure.

Number of Lamps-Heating of Conductors

Question 1: What is the maximum number of incandescent lamps (220-ohm) which may be operated from one machine of 110 volts? Is the number limited only by internal resistance of machine and external resistance of mains?

Question 2: Is the heating effect upon a conductor, such as a trolley wire, greater or less when carrying an alternating current than

a direct current?-P. H.

Answer 1: A 16-candle-power incandescent lamp at 110 volts has a resistance of about 220 ohms when hot, and takes a current of 1⁄2 ampere. The number of

lamps that can be run is limited by the capacity of the generator. If the capacity of the generator is 100 amperes, you can operate 200 lamps from it. It is not a matter of internal resistance of the machine, but rather of current-carrying capacity of the armature conductors and of commutation without excessive sparking. Of course, the mains should have sufficient carrying capacity to handle the current without overheating.

Answer 2: The heating effect upon a conductor of an alternating current is the same as that of a direct current; that is, if a direct current of 100 amperes is flowing in a wire, the heating effect will be the same as that of an alternating current whose virtual or effective value is 100 amperes. This must be the case, because the virtual value of an alternating current is the value of an unvarying direct current which would produce the same heating effect.

Clearance Loss in Ammonia Compressor

Question 1: I should like to have you calculate the loss in ammonia compressor from following data: Compressor double-acting; size of cylinder, 15 inches by 23 inches' stroke; high pressure, 210 pounds; back pressure, 28 pounds; engine makes 60 r. p, m.; clearance, 4 inch. This is very large, in fact too large a clearance, and the loss must be great. The machine is rated at 125 tons' refrigeration in 24 hours. With this large clearance, how much would it reduce refrigeration in 24 hours? Explain fully the loss per stroke, and per 24 hours.

Question 2: Would a spherical piston 17 inches in diameter have the same pressure (total pressure) exerted upon it as a flat piston?

Question 3: Give formula for finding area of a segment of a sphere 17 inches in diameter at base, height 95-16 inches.-R. H. K.

Answer 1: We have made calculations on the compressor, and find that the loss due to clearance is about 7 per cent. This is indeed excessive. Under the conditions of operation which you have given, it requires 2.81 cubic feet of gas to be passed through the compressor per minute, to produce one ton of refrigeration in 24 hours. Having allowed for the clearance, the compressor handles 264 cubic feet per minute; and consequently

this equals 94 tons of refrigeration per 24 hours. This, you see, is considerably less than the rating. The loss per stroke is in the same proportion as the number of strokes per minute.

Answer 2: The pressure on the spherical piston of any diameter whatever is the same as the pressure on the flat piston of the same diameter.

Answer 3: The segment having but one base, multiply half the height of the segment by the area of the base, and the cube of the height by .5236, and add the two products.

Drop in Voltage

Question: Please explain how to figure the formula for the drop in voltage in electric wires for lighting. I know the formula, but do not know where you get it.-F. Z. W.

Answer: There are two common formulæ for figuring size of wires to supply direct current to a given number of incandescent lamps at a given distance. The first formula is based on Ohm's law:

Horse-Power of Rotary Blower Question: Please give rule for calculating the horse-power of a rotary blower. Is the following correct?-R. S. D.

C. ft. x 1,728 × Poz. XN

ΡΩ Q D + 33,000 2,125,000

V2,

in which P Pressure of the air in the delivery pipe, in pounds per square foot; = Number of cubic feet of air discharged per minute; V = Velocity of air in discharge pipe in feet per second; and

Density or weight of one cubic foot of air at the observed temperature and barometric pressure. The first term in this formula agrees practically with the formula you have given; that is, we have the factor of force P times the space per minute, which is Q, the number of cubic

Explosive Pressure of Gasoline

Question: I should like to know the increase of volume or pressure of a cubic foot of a well-mixed, properly proportioned charge of gasoline and air, just after ignition. Will a charge of air in gasoline ignite as readily under low pressure as under high?-H. P. Jr.

Answer: The constitution of the vapors of gasoline and other products of petroleum, is so complex and variable that the exact heating value of any certain vapor can be determined only by direct experiment. The heating value of a cubic foot of gasoline vapor is nearly 1,000 B. T. U., or about equal to the average natural gas. When mixed with the right proportion of air at atmospheric pressure, the maximum pressure due to explosion is about 100 pounds per square inch. The pressure attained under any circumstances, is only about one-half that which would be reached if the total heat of combustion were evolved at the instant of explosion. By compressing the mixture before explosion, much higher pressures can be reached.

The maximum pressure is not reached immediately after ignition, but a fraction. of a second afterwards, the exact time depending upon the mixture and compression. The charge will not ignite as readily under low pressure as under high.

Circuit for Extension Telephone Question: Will you please give me the outline of the circuits on an extension telephone, central-energy system, where there are but two wires running to the extension telephone?E. L. L.

Answer:

In this case, as we understand it, you have a telephone located somewhere in the room, as upon the wall, equipped complete with transmitter, receiver, and bell; and you wish to connect an extension telephone upon the line, obtaining your signals from the bell on the

w

B

A

first-mentioned phone. The accompanying sketch shows the proper connections, where A is the phone located upon the wall, and B is your extension desk telephone. If you wish a bell connected on your extension phone, it would simply be bridged across the line as shown in the connections for A.

Breaking Gauge Glass

Question: Will you please give me the cause and remedy for the following: At the last four boiler washings, I have taken out the drain nipple below the water glass, and swabbed out water glass with a small wad of waste on end of a thin pine stick. Now, in each case the glass burst. Last time, it cracked before steam was raised. The swab fits very loose, and I cannot assign any cause for it.H. B. H.

Answer: The water glass is always a very frail piece of apparatus, and especially so when at a high temperature. Often, if the glasses are not lined up perfectly straight, they will break when slightly heated, owing to expansion and internal strains. Of course, if a glass is hot, nothing cold should be allowed to touch it. The only remedy is not to touch the glass unless this is absolutely necessary, and then to use care to see that the adjustment is not altered in

any way.