pressure representing the friction of the pipe is lost, as there can be no useful return for it. The friction is affected by very many circumstances; it is increased in direct proportion to the length of the pipe and also in the square of the velocity of the flow of air. The pressure of the air does not affect it. The losses by friction may be quite serious if the piping system is poorly designed, and, on the other hand, extravagant expenditure in pipe may result from a timid overrating of the evils of friction. A thorough knowledge of the laws governing the whole matter, as well as a ripe experience, is necessary to secure true economy and mechanical success. The loss of power in pipe friction is not always the most serious result. When a number of machines are in use in a mine, and the pipes are so small as to cause considerable loss of pressure by friction, there will be sudden and violent fluctuations in pressure whenever a machine is started or stopped. Breakages will be common occurrences, as the changes are too quick to be entirely guarded against by the attendant; perfectly even pressure at the compressor is no safeguard against this class of accidents. The trouble arises in the pipe, and the remedy must be applied there. A system of reservoirs and governing valves will regulate these matters and allow successful work to be done with pipes that would otherwise be entirely inadmissible.

The ordinary formulas for calculating the volume of air transmitted through a pipe do not take into account the increase of volume due to reduction of pressure, i. e., loss of head. To transmit a given volume of air at a uniform velocity and loss of pressure, it is necessary to construct the pipe with a gradually increasing area. This, of course, is impracticable, and in a pipe of uniform section both volume and velocity must increase as the pressure is reduced by friction. The loss of head in properly proportioned pipes is so small, however, that in practice the increase in volume is usually neglected. The table on page 482 gives the loss of pressure by flow of air in pipes calculated for pipes 1,000 ft. long; for other lengths, the loss varies directly as the length.

The resistance is not varied by the pressure, only so far as changes in pressure vary the velocity. It increases about as the square of the velocity, and directly as the length. Elbows, short turns, and leaks in pipes all tend to reduce the pressure in addition to the losses given in the table. An elbow with a radius of one-half the diameter of the pipe is as short as can be made. LOSS BY FRICTION IN ELBOWS

With regard to the design, installation, and operation of air compressors, the following suggestions made by Mr. Alex. M. Gow, Mechanical Engineer, Oliver Iron Mining Co., and slightly enlarged, will be of interest.

Design for Avoiding Explosions.-Clearance space should be reduced to a minimum. Ingoing air should traverse as small a surface of hot metal as possible. Discharge valves and passageways should contain no pockets or recesses for the accumulation of oil. Cylinders and heads should be waterjacketed; in some cases piston water-cooling may be resorted to. Stage compression, with adequate intercooling should be employed wherever final pressure and first cost of installation will warrant. Discharge valves must be easy of access for cleaning and examination. There must be no excuse for dirty or leaky valves.

Installation of Compressor.-Air should be drawn from the coolest and cleanest place possible, and never from the engine room. Engine-room air is never cool nor clean and an open intake is a constant invitation to squirt oil in from a can. Around collieries, it is well to consider the washing of the air. Coal dust drawn in with the air, mixes with the oil and forms a substance that,

on heating, cakes and may take fire. The discharge pipe should be of ample size and have as few bends as possible. A thermometer, preferably recording, should be placed on the discharge pipe. Provision for aftercooling should be made, a water spray will answer, to be used when the thermometer indicates the necessity. The receiver should be provided with a manhole for cleaning, and a drain easy of access and ample in size. Automatic sight-feed lubricators should be depended on for regular lubrication, but in addition an oil pump may be installed for the introduction of soap and water in case of necessity.

Operation of Compressors.-High flash-test cylinder oil of the best obtainable grade should be used for regular lubrication of the air cylinders. Mr. L. A. Christian advises that the flash point be 625° F., and that the oil should be comparatively free from unnecessary volatile carbon compounds. Volatile hydrocarbons tend to reduce the flash point, and, mixing with the dust from the air, form combustible deposits in the receiver and outlet passages. Further, oil of low-flash test, on reaching the interior of the heated air cylinder will be vaporized and will pass out with the air into the receiver without affording any lubrication to the wearing surfaces. If the oil is too dense or is compounded with animal or vegetable oils, as is the case with many steam-cylinder oils, it will have a tendency to adhere to the discharge valves and passages, and, being subjected to the dry heat of the compressed air, will gradually change to a hard, brittle crust, which in time will completely choke up the air passages or will prevent the valves seating. The amount of oil to be fed into the air cylinder should be, if the machine makes less than 120 rev. per min., about 1 drop every 3 min. Kerosene should never be used to cut or eat away deposits of carbon, as is sometimes done, as its flash point is about 120° F. If the cylinders or air passages need cleaning, soapsuds made of 1 part of soft soap and 15 parts of pure water should be fed into the cylinder and the machine worked with a liberal solution instead of oil for a few hours or a day; then the blow-off valve of the receiver should be opened and the accumulation of oil and water drained off. After this treatment and before the machine is shut down, oil should be fed into the cylinder for an hour or so, in order that the valves and the parts connected with the cylinder may be coated with oil and thus prevent rust. Discharge valves must be kept tight; to this end the use of the steam engine indicator is advised. The cards may not tell much about the conditions of the valves, but one of the greatest values of the indicator is the moral effect upon the engineer. The valves should be cleaned from dust and oil and frequently examined.

Accumulations of water and oil must be blown from the receiver and an internal examination made at stated intervals.

The thermometer on the discharge pipe should be watched like the steam gauge. Before it reaches 400° F., the after cooling spray should be put on, and all the water-supply pipes and the discharge valves examined.

The engineer in charge should be thoroughly instructed as to the possibility of an explosion, the dangers attendant upon the use of any but the prescribed oil, and the effect of leaky discharge valves. He should be instructed in the use of the steam-engine indicator and required to submit cards at stated intervals. He should record in the engine-room log the daily conditions of the machines under his charge. He should be given a wholesome respect for an air compressor, with imperative instructions to keep it clean, inside as well

as out.

ELECTRICITY

PRACTICAL UNITS

In electric work, it is necessary to have units in terms of which to express the different quantities entering into calculations. The unit quantity of electricity flowing through a circuit is called a coulomb. A coulomb is the quantity of electricity that will deposit from a solution of silver nitrate through which it flows .001118 gram of silver.

Strength of Current.-The strength of current flowing in a wire may be measured in several ways. If a compass needle is held under or over a wire it will be deflected and will tend to stand at right angles to the wire. The stronger the current, the greater is the deflection of the needle. If the wire carrying the current is cut and the end dipped into a solution of silver nitrate,

silver will be deposited on the end of the wire toward which the current is flowing, and the amount of silver deposited in a given time will be directly proportional to the average strength of current flowing during that time. When the current flowing in a wire is spoken of, the strength of the current is meant.

The unit used to express the strength of a current is called the ampere. If a current of 1 amp. be sent through a bath of silver nitrate, .001118 g. per sec. of silver will be deposited. The expression of the flow of current through a wire as so many amperes is analogous to the expression of the flow of water through a pipe as so many gallons per second. If 1 amp. flows through a circuit for 1 sec., the quantity of electricity that has passed through the circuit during the 1 sec. is 1 coulomb; that is, 1 coulomb is equal to 1 amp. for 1 sec.

Electromotive Force.-In order that a current may flow through a wire, there must be an electric pressure of some kind to cause the flow. In hydraulics, there must always be a head or pressure before water can be made to flow through a pipe. It is also evident that there may be a pressure or head without there being any flow of water, because the opening in the pipe might be closed; the pressure will, however, exist, and, as soon as the valve closing the pipe is opened, the current will flow. In the same way, an electric pressure or electromotive force (often written E. M. F.) may exist in a circuit, but no current can flow until the circuit is closed or until the wire is connected so that there will be a path for the current.

The practical unit of electromotive force is the volt. It is the unit of electric pressure, and fulfils somewhat the same purpose as head of water and steam pressure in hydraulic and steam engineering. The electromotive force furnished by an ordinary cell of a battery usually varies from .7 to 2 volts. Daniell cell gives an electromotive force of 1.072 volts. A pressure of 500 volts is generally used for street-railway work, and, for incandescent lighting, 110 volts is common.

A

Resistance -All conductors offer more or less resistance to the flow of a current of electricity, just as water encounters friction in passing through a pipe. The amount of this resistance depends on the length of the wire, the diameter of the wire, and the material of which the wire is composed. The resistance of all metals also increases with the temperature.

The practical unit of resistance is the ohm. A conductor has a resistance of 1 ohm when the pressure required to set up 1 amp. through it is 1 volt. In other words, the drop, or fall, in pressure through a resistance of 1 ohm, when a current of 1 ampere is flowing, is 1 volt. 1,000 ft. of copper wire .1 in. in diameter has a resistance of nearly 1 ohm at ordinary temperatures.

Ohm's Law. The law governing the flow of current in an electric circuit was first stated by Dr. G. S. Ohm, and is known as Ohm's law. It may be briefly stated as follows: The strength of the current in any circuit is equal to the electromotive force divided by the resistance of the circuit.

EXAMPLE 1.-A dynamo D generating 110 volts, is connected to a coil of wire C that has a resistance of 20 ohms; what current will flow, supposing the resistance of the rest of the circuit to be negligible?

SOLUTION.-As E-110 volts and R=20 ohms,

by Ohm's law I=110÷20=5.5 amp.

EXAMPLE 2.-If the resistance of the coil C is 6 ohms, what electromotive force must the dynamo generate in order to set up a current of 15 amp. through it?

SOLUTION.-In this case the third formula will

be used; that is, E-15X6=90 volts.

D

E-110 Volts

реше

R=20 Ohms

In case the current and electromotive force are known, the resistance of the circuit may be calculated by using the second formula.

EXAMPLE 3.-If the current in the previous examples were 8 amp. and the electromotive force of the dynamo 110 volts, what is the resistance of the circuit?

SOLUTION.

R=110÷8=13.75 ohms.

Electric Power. The electric power expended in any circuit is found by multiplying the current flowing in the circuit by the pressure required to force

the current through the circuit. In other words, WEI, where W is the power expended, E is the electromotive force and I is the current. When E is expressed in volts and I in amperes, then W is expressed in watts. The watt is the unit of electric power, and is equal to the power developed when 1 amp. flows under a pressure of 1 volt. The watt is equal to H. P.

Let

Then,

E electromotive force, in volts;

I= current, in amperes;
R=resistance, in ohms;
W=power, in watts;

H. P.-horsepower.

E2

W=EI=I2R=

R

The energy used in forcing a current through the wire reappears in the form of heat; the heating effect of a current flowing in a conductor being proportional to the square of the current. Furthermore,

ΕΙ W
H. P.: =
746 746

This relation is very useful for calculating power in terms of electric units. The watt is too small a unit for convenient use in many cases, so that the kilowatt, or 1,000 watts, is frequently used. This is sometimes abbreviated to K. W.

The unit of work is the watt-hour, which is the total work done when 1 watt is expended for 1 hr. For example, if a current of 1 amp. flows for 1 hr. through a resistance of 1 ohm, the total amount of work done is 1 watt-hour. A kilowatt-hour is the total work done when 1 K. W. is expended for 1 hr. It is about equivalent to 1 H. P. for 1 hr. The work done when 1 watt is expended for 1 sec. is called the joule; or 1 joule is expended in a circuit when 1 volt causes 1 amp. to pass through the circuit for 1 sec.

ELECTRICAL EXPRESSIONS AND THEIR EQUIVALENTS
(Arranged for Convenient Reference by C. W. Hunt)

The path through which a current flows is generally spoken of as an electric circuit; this path may be made up of a number of different parts. For example, the line wires may constitute part of the circuit, and the remainder may be composed of lamps, motors, resistances, etc. In practice, the two kinds of circuits most commonly met with are those in which the different parts of the circuit are connected in series and those in which they are connected in multiple or parallel.

Series Circuits. In a series circuit, all the component parts are connected in tandem, so that the current flowing through one part also flows through the other parts; view (a) represents such a circuit made up of a different number

of parts. The current leaves the dynamo D at the + side and flows through the arc lamps a, thence through the incandescent lamps 1, thence through the motor m and resistance r, back to the dynamo, thus making a complete circuit. All these parts are here connected in series, so that the current flowing through each of the parts must be the same unless leakage takes place across from one side of the circuit to the other, and this is not appreciable if the lines are properly insulated. The pressure furnished by the dynamo must evidently be the sum of the pressures required to force the current through the different parts.

The most common use of this system is in connection with arc lamps, which are usually connected in series, as shown in (b). The objections to

series. In such a system, the dynamo is provided with an automatic regulator that increases or decreases the voltage of the machine, so that the current in the circuit is kept constant, no matter how many lamps or other devices are in operation. For this reason, such circuits are often spoken of as constantcurrent circuits.

Parallel Circuits. In a parallel circuit, the different pieces of apparatus are connected side by side, or in parallel, across the main wires from the dynamo as shown in (c). In this case, the dynamo D supplies current through the mains to the arc lamps a, incandescent lamps 7, and motor m. This system is more widely used, as the breaking of the circuit through any one piece of apparatus will not prevent the current from flowing through the other parts. Incandescent lamps are connected in this way almost exclusively. The lamps are connected directly across the mains, as shown in (d). Street cars and mining locomotives are operated in the same way, the trolley wire constituting one main and the track the other, as shown in (e). By adopting this system, any car can move independently of the others, and the current in each device may be turned off and on at will without affecting devices in other parallel circuits. In all these systems of parallel distribution, the pressure generated by the dynamo is maintained as constant as practicable, no matter what current the dynamo may be delivering. For example, in the lamp system, view (d), the dynamo will maintain a constant electromotive force of 110 volts. Each lamp has a fixed resistance, and will take a certain current (110÷R amperes) when connected across the mains. As the lamps are turned on, the current delivered by the dynamo increases, the pressure remaining constant, street-railway work, the pressure between trolley and track is kept in the neighborhood of 500 volts, the current varying with the number of cars in operation. In mine-haulage plants, the pressure is usually 250 or 500 volts, the former being generally preferred as being less dangerous. Lamps may also be connected in series multiple, as shown in (e). Here the two 125-volt lamps/

In