Unkown - "The 1995 U.S. Congress Address Book [Revision B]"

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Edited by Henry Chadwick - "Spalding's Baseball Guide and Official League Book for 1895"

The Story Of Electricity

J >> John Munro >> The Story Of Electricity




With alternating current there can be used the various forms of
single phase or polyphase current familiar in power work, but the
latter is now preferred, and in Europe and in the United States in
the latter part of 1908 the number of single phase lines was
estimated at 27 and 28 respectively, with a total mileage of 782
and 967 miles. A trolley wire or suspended conductor is used. To
employ a single phase current, motors of either the repulsion type
or of the series type are used and are of heavier weight than the
direct current motors, as they must combine the functions of a
transformer and a motor. It is for this reason that we often see
two electric locomotives at the head of a single train on lines
where the single phase system is employed, while on neighboring
lines using direct current, one locomotive of hardly larger size
suffices. With the polyphase current a motor with a rotating field
is used, and they have considerable efficiency as regards weight
when compared with the single phase and with the direct current
motor. The polyphase motor, however, is open to the objection that
it does not lend itself to regulations as well as the direct
current form, and with ingenious devices involving the arrangement
of the magnetic field and the combination of motors, various
running speeds can be had. The usual voltage for these motors is
3,000 volts, but in the polyphase plant designed for the Cascade
Tunnel 6,000 volts are to be used. They possess many advantages,
especially their ability to run at overload, and consequently a
locomotive with polyphase motor will run up grade without serious
loss of speed. The single phase system has been carried on on
Swiss and Italian railroads, notably on the Simplon Tunnel and the
Baltelina lines with great success, and the distribution problems
are reduced to a minimum. In the United States a notable
installation has been on the New York, New Haven & Hartford
Railroad, where the section between Stamford and New York has been
worked by electricity exclusively since July 1, 1908. Here the
single phase motors use direct current while running over the
tracks of the New York Central from Woodlawn to the Grand Central
Terminal. On both the New York, New Haven & Hartford and the New
York Central locomotives the armature is formed directly on the
axle of the driving wheels, so consequently much interest attaches
to the new design adopted for the Pennsylvania tunnels, where the
armatures of the direct current motors are connected with the
driving wheels by connecting rods somewhat after the fashion of
the steam locomotive, and following in this respect some
successful European practice.





APPENDIX.

UNITS OF MEASUREMENT.


(From Munro and Jamieson's Pocket-book of Electrical Rules and
Tables).





I. FUNDAMENTAL UNITS.--The electrical units are derived from the
following mechanical units:--

The Centimetre as a unit of length;
The Gramme as a unit of mass;
The Second as a unit of time.

The Centimetre is equal to 0.3937 inch in length, and nominally
represents one thousand-millionth part, or 1/1,000,000,000 of a
quadrant of the earth.

The Gramme is equal to 15.432 grains, and represents the mass of a
cubic centimetre of water at 4 degrees C. Mass is the quantity of
matter in a body.

The Second is the time of one swing of a pendulum making 86,164.09
swings in a sidereal day, or 1/86,400 part of a mean solar day.





II. DERIVED MECHANICAL UNITS.-


Area.-The unit of area is the square centimetre.

Volume.--The unit of volume is the CUBIC CENTIMETRE.

VELOCITY is rate of change of position. It involves the idea of
direction as well as that of magnitude. VELOCITY is UNIFORM when
equal spaces are traversed in equal intervals of time The unit of
velocity is the velocity of a body which moves through unit
distance in unit time, or the VELOCITY OF ONE CENTIMETRE PER
SECOND.

MOMENTUM is the quantity of motion in a body, and is measured by
mass x velocity.

ACCELERATION is the rate of change of velocity, whether that
change take place in the direction of motion or not. The unit of
acceleration is the acceleration of a body which undergoes unit
change of velocity in unit time, or an acceleration of one
centimetre-per-second per second The acceleration due to gravity
is considerably greater than this, for the velocity imparted by
gravity to falling bodies in one second is about 981 centimetres
per second (or about 32.2 feet per second). The value differs
slightly in different latitudes. At Greenwich the value of the
acceleration due to gravity is g=981.17; at the Equator g=978.1;
at the North Pole g=983.1.

FORCE is that which tends to alter a body's natural state of rest
or of uniform motion in a straight line.

FORCE is measured by the acceleration which it imparts to mass--i.
e., mass x acceleration.

THE UNIT OF FORCE, or DYNE, is that force which, acting for one
second on a mass of one gramme, gives to it a velocity of one
centimetre per second. The force with which the earth attracts any
mass is usually called the "weight" of that mass, and its value
obviously differs at different points of the earth's surface The
force with which a body gravitates--i e, its weight (in dynes), is
found by multiplying its mass (in grammes) by the value of g at
the particular place where the force is exerted.

Work is the product of a force and a distance through which it
acts. The unit of work is the work done in overcoming unit force
through unit distance--i e, in pushing a body through a distance
of one centimetre against a forch of one dyne. It is called the
Erg. Since the "weight" of one gramme is 1 X 981 or 981 dynes, the
work of raising one gramme through the height of one centimetre
against the force of gravity is 981 ergs or g ergs. One
kilogramme-metre = 100,000 (g) ergs = 9 8 1 X 10^7 ergs. One foot-
pound = 13,825 (g) ergs, = 1 356 X 10^7 ergs.

Energy is that property which, possessed by a body, gives it the
capability of doing work. Kinetic energy is the work a body can do
in virtue of its motion. Potential energy is the work a body can
do in virtue of its position. The unit of energy is the Erg.

Power or Activity is the rate of work; the practical unit is
called the Watt--10^7 ergs per second.

A Horse-power = 33,000 ft--Ibs per minute = 550 ft--Ibs per
second, but as seen above under Work, 1 ft--Ib = 1 356 X 10^7
ergs, and under Power, 1 Watt = 10^7 ergs per sec a Horsepower =
550 X 1 356 X 10^7 ergs = 746 Watts; or, =EC/746=C^2R/746=E^2/(746
R) =HP where E = volts, C = amperes, and R = ohms.

The French "force de cheval" = 75 kilogramme metres per sec = 736
Watts = 542 48 ft--lbs. per sec. = .9863 H.P.; or one H.P. =
1.01385 "force de cheval."

DERIVED ELECTRICAL UNITS.--There are two systems of electrical
units derived from the fundamental "C.G.S." units, one set being
based upon the force exerted between two quantities of
electricity, and the other upon the force exerted between two
magnetic poles. The former set are termed electro-static units,
the latter electro-magnetic units.





III. ELECTROSTATIC UNITS.--


UNIT QUANTITY of electricity is that which repels an equal and
similar quantity at unit distance with unit force in air.

UNIT CURRENT is that which conveys unit quantity of electricity
along a conductor in a second.

UNIT ELECTROMOTIVE FORCE, or unit DIFFERENCE OF POTENTIAL exists
between two points when the unit quantity of electricity in
passing from one to the other will do the unit amount of work.

UNIT RESISTANCE is that of a conductor through which unit
electromotive force between its ends can send a unit current.

UNIT CAPACITY is that of a condenser which contains unit quantity
when charged to unit difference of potential.





IV. MAGNETIC UNITS.--


UNIT MAGNETIC POLE is that which repels an equal and similar pole
at unit distance with unit force in air.

STRENGTH OF MAGNETIC FIELD at any point is measured by the force
which would act on a unit magnetic pole placed at that point.

UNIT INTENSITY OF FIELD is that intensity of field which acts on a
unit pole with unit force.

MOMENT OF A MAGNET is the strength of either pole multiplied by
the distance between the poles.

INTENSITY OF MAGNETISATION is the magnetic moment of a magnet
divided by its volume.

MAGNETIC POTENTIAL.--The potential at a point due to a magnet is
the work that must be done in removing a unit pole from that point
to an infinite distance against the magnetic attraction, or in
bringing up a unit pole from an infinite distance to that point
against the magnetic repulsion.

UNIT DIFFERENCE OF MAGNETIC POTENTIAL.--Unit difference of
magnetic potential exists between two points when it requires the
expenditure of one erg of work to bring an (N. or S.) unit
magnetic pole from one point to the other against the magnetic
forces.





V. ELECTRO-MAGNETIC UNITS.--


UNIT CURRENT is that which in a wire of unit length, bent so as to
form an arc of a circle of unit radius, would act upon a unit pole
at the centre of the circle with unit force.

UNIT QUANTITY of electricity is that which a unit current conveys
in unit time.

UNIT ELECTRO-MOTIVE FORCE or DIFFERENCE OF POTENTIAL is that which
is produced in a conductor moving through a magnetic field at such
a rate as to cut one unit line per second.

UNIT RESISTANCE is that of a conductor in which unit current is
produced by unit electro-motive force between its ends.

UNIT CAPACITY is that of a condenser which will be at unit
difference of potential when charged with unit quantity.

Electric and magnetic force varies inversely as the square of the
distance.





PRACTICAL UNITS OF ELECTRICITY.


RESISTANCE-R.--The Ohm is the resistance of a column of mercury
106.3 centimetres long, 1 square millimetre in cross-section,
weighing 14.4521 grammes, and at a temperature of 0 degrees
centigrade. Standards of wire are used for practical purposes. The
ohm is equal to a thousand million, 10^9, electromagnetic or
Centimetre-Gramme-Second ("C. G. S.") units of resistance.

The megohm is one million ohms.

The microhm is one millionth of an ohm.

ELECTROMOTIVE FORCE--E.--The Volt is that electromotive force
which maintains a current of one ampere in a conductor having a
resistance of one ohm. The electromotive force of a Clark standard
cell at a temperature of 15 degrees centigrade is 1.434 volts. The
volt is equal to a hundred million, 10^8, C. G. S. units of
electromotive force.

CURRENT--C.--The Ampere is that current which will decompose
0.09324 milligramme of water (H2O) per second or deposit 1.118
milligrammes of silver per second. It is equal to one-tenth of a
C. G. S. unit of current.

The milliampere is one thousandth of an ampere.

QUANTITY--Q.--The Coulomb is the quantity of electricity conveyed
by an ampere in a second. It is equal to one-tenth of a C. G. S.
unit of quantity.

The micro-coulomb is one millionth of a coulomb.

CAPACITY--K.--The farad is that capacity of a body, say a Leyden
jar or condenser, which a coulomb of electricity will charge to
the potential of a volt. It is equal to one thousand-millionth of
a C. G. S. unit of capacity.

The micro-farad is one millionth of a Farad.

By Ohm's Law, Current = Electromotive Force/ Resistance,

or C = E/R

Ampere = Volt/Ohm

Hence when we know any two of these quantities, we can find the
third. For example, if we know the electromotive force or
difference of potential in volts and the resistance in ohms of an
electric circuit, we can easily find the current in amperes.

POWER--P.--The Watt is the power conveyed by a current of one
ampere through a conductor whose ends differ in potential by one
volt, or, in other words, the rate of doing work when an ampere
passes through an ohm. It is equal to ten million, 10^7, C. G. S.
units of power or ergs per second, that is to say, to a Joule per
second, or 1/746 of a horse-power.

A Watt = volt X ampere, and a Horse-power = Watts/746.

HEAT OR WORK--W.--The Joule is the work done or heat generated by
a Watt in a second, that is, the work done or heat generated in a
second by an ampere flowing through the resistance of an ohm. It
is equal to ten million, 10^7, C. G. S. units of work or ergs.
Assuming "Joule's equivalent" of heat and mechanical energy to be
41,600,000, it is the heat required to raise .24 gramme of water 1
degrees centigrade. A Joule = Volt x ampere x second. Since 1
horse-power = 550 foot pounds of work per second,

W = 550/746 E. Q. = .7373 E. Q. foot pounds.





HEAT UNITS.


The British Unit is the amount of heat required to raise one pound
of water from 60 degrees to 61 degrees Fahrenheit. It is 251.9
times greater than the metric unit, therm or calorie, which is the
amount of heat required to raise one gramme of water from 4
degrees to 5 degrees centigrade.

Joule's Equivalent--J.--is the amount of energy equivalent to a
therm or calorie, the metric unit of heat. It is equal to
41,600,000 ergs.

The heat in therms generated in a wire by a current = Volt X
ampere X time in seconds X 0.24.





LIGHT UNITS


The British Unit is the light of a spermaceti candle 7/8-inch in
diameter, burning 120 grains per hour (six candles to the pound).
They sometimes vary as much as 10 per cent, from the standard. Mr.
Vernon Harcourt's standard flame is equal to an average standard
candle.

The French Unit is the light of a Carcel lamp, and is equivalent
to 9 T/Z British units.

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