Plug in an appliance at home and the device is energized by alternating current (AC). Yet for the first eighty years of the electrical age, the electrical current of choice was Volta's direct current (DC). The story of how we came to make the choice of one current over the other tells of greed and political intrigue and jealousy rather than sound scientific principles. The lead characters in this drama are icons of American industry. But let's begin at the beginning.
Volta's "pile" of zinc and silver plates separated by slices of leather soaked in brine was the first source of a steady current moving from one metal to the other and caused considerable interest because the charges could carry energy and because the charges could be made to move over a longer period of time rather than be discharged all at once. Scientists began to experiment with this new current just to see what it could do. Humphrey Davy soon found that DC current could breakdown substances that had heretofore been thought to be elements. It was Davy who first identified oxygen and hydrogen as the component elements.
THE AC/DC DILEMMA
It was mentioned earlier in the section
on electrodynamics that a generator is a device that converts
mechanical energy into electrical energy when a coil of wire is
rotated in a magnetic field. The same device can produce an alternating
current in a circuit, or it can produce a pulsating direct current
by collecting the current with a special device called a split
ring commutator. Any mechanical source of energy can be used--wind,
water, and steam pressure come to mind. (The word "power"
used in the context of this discussion is defined to be the amount
of energy delivered to, or drawn from, a generator per unit time.
Power can be measured in Watts or horsepower, among other units.)
When power is supplied to the generator, the device converts the
power into its electrical form, losing some along the way due
to system inefficiencies. When the energy is in its electrical
form, power manifests itself as the product of voltage (energy
per unit charge) times current (charge per unit time). P = VI.
The operational voltage output of a generator is determined by
the size of the coil of wire and the speed at which the coil is
rotated. Thus, there is a practical upper limit for the output
voltage of a generator due to the centrifugal effects of any object
spinning at high speeds. A source of mechanical energy supplies
power at a rate, P. The generator coil rotates at a particular
rate, causing the output voltage of the generator to be V. Thus,
by setting the values of P and V, the value of the output current
I is determined.
Nothing in life is ever simple, it seems. As a current passes
through a wire, some of the electrical energy carried by the charges
will be lost to heat energy. We find empirically that the heat
lost by a wire follows this relationship: heat loss = I^2R. Thus,
a generator producing current at a low voltage and high current
is likely to have much of its output energy lost to heat as it
is carried along the wire. This may be acceptable if the wire
is a filament in a lamp. It is unacceptable if the wire is a transmission
line linking the generator to the consumer. In addition to high
energy losses, the wires connecting generator to consumer would
have to be very thick and would cost a lot to install.
Edison staked the future of his empire on the distribution of
electricity by direct current. He recognized that the heating
loss that manifested themselves during the delivery of energy
was a part of doing business. He envisioned an America electrified
by DC power wherein every neighborhood would have its own power
station and every building would draw its energy from the central
plant. This plan worked satisfactorily in a densely populated
New York City and in other metropolitan areas. In smaller towns,
it was clear that only those buildings situated near a power station
could be electrified.
Alternating current provides a different picture for the American
landscape. Faraday had shown in the 1830’s that if two coils
of wire were in close proximity to one another, and one wire carried
and alternating current, a current could be established in the
second coil, even though there was not electrical connection between
the coils. Faraday also noted that the ratio of the voltage in
the second coil (called appropriately enough, the secondary) to
that in the first coil (the primary) was the same as the ratio
of the number of turns of wire in the secondary to that in the
primary. This device is called a transformer and was developed
for this purpose during the decades following its discovery. Thus,
if power coming from a generator is sent to a so-called step-up
transformer, say with a winding ratio of 10:1, the voltage coming
from the secondary coil is increased by a factor of 10. But P
= VI suggests that if V goes up by a factor of 10, I goes down
by a factor of 10. HL = I^2R suggests that if I is reduced by
a factor of 10, heating losses are reduced by a factor of 10^2.
But power cannot be used safely at these high voltages. If a second
transformer is used at the consumer end of the circuit, this time
with primary and secondary coils reversed, the voltage can be
stepped down to safer values. In this scenario, the long wire
runs through remote areas. Here, the wires a placed on high poles
away from trees and buildings. Then, at a regional substation
in a town, the voltage is stepped down to more nearly usable values
before being delivered to the customers, the voltages are kept
low near the generator and consumer so that thick insulation for
the wire is not needed.
These sites are
links to an entire course in E & M
http://theory.uwinnipeg.ca/physics/charge/node1.html
http://library.advanced.org/16600/intermediate/static
electricity.shtml
http://www.phys.ufl.edu/~phy3054/extras/contents/Welcome.html
Go to quick
bios
Westinghouse learned of Tesla’s defection from the Edison
camp and offered him employment. He had already acquired the American
patent rights to alternating current distribution from Gaulard
and Gibbs, a pair of European inventors. In time, he bought from
Tesla the patent rights to the polyphase motor to complete the
package of apparatus that he needed to compete with Edison. Now
he needed a place to test the system. He installed his first system
in Great Barrington, Massachusetts and installed another in Greensburg,
Pennsylvania. While successful, these were relatively small projects.
Westinghouse needed a home run.
The opportunity he needed came in 1887 when the city officials
in Buffalo announced a project to build a power station at Niagara
Falls. They offered a competition open to inventors of the world,
with a prize of $100,000, for a successful plan to bring electricity
to the city. While some twenty proposals were received, Edison
and Westinghouse were the two leading contenders. Westinghouse
proposed his alternating current system with high voltage lines
stretching more than twenty miles from the city. Edison envisioned
a canal more than two miles long along which power stations would
be built to supply individual factories constructed there. The
battle between these two industrial titans was fierce. Edison
hammered at the idea that the high voltages of AC power were lethal.
He established a traveling road show where his employees went
from town to town with a large metal grid that was to be connected
to an AC generator. They would routinely empty the local dog pound
and tether the animals to the grid. Zap! Edison even went so far
as to electrocute a circus elephant. The Edison camp was bolstered
by the news that in August 1890, in Auburn State Prison, a convicted
murderer, William Kemmler, became the first person to be executed
by electrocution. The deed was not done which much finesse and
eye-witness accounts told of how the man suffered.
All Westinghouse could do was point to his smaller American demonstration
projects and show that there had been no fatalities to date. The
event that tipped the scales in favor of alternating current came
in 1891 at the International Electrical Exhibition in Frankfurt,
Germany. There, a demonstration project caused AC electricity
to be sent a distance of 110 miles at 77% efficiency and with
no compromise in citizen safety. In the end, the decision to electrify
Buffalo went to the Westinghouse group. In 1893, Westinghouse
won the rights to light the Colombian Exposition in Chicago. Again,
the project was carried off safely and efficiently.
To a smaller company than Edison Electric Company, losing such
a pitched battle might have spelled financial ruin. Indeed, the
Edison group found itself on the outside looking in, having actively
shunned AC technology for so long. Westinghouse held some very
important patents in the market place and Edison would be at a
disadvantage until those patents expired. His salvation was a
device called a rotary converter, a DC generator operated by an
AC motor. This device, using alternating current from the neighborhood
power line, could provide direct current in a building to run
Edison-manufactured appliances. There were other inventions yet
to come in the Edison repertoire, inventions of the phonograph
and motion picture projector and improvements to the telephone.
But his company suffered as the electrical industry raced away
in a whole new direction than the "Wizard of Menlo Park"
had anticipated.
EPILOGUE
Science and technology, and the men and women who do science and
technology, are often labeled as logical, analytical and otherwise
devoid of passion. The story presented in these few pages belies
those images. Science is carried out by real people who try to
balance high intellectual ideals with fundamental human frailties.
In this regard, science is every bit as interesting to study as
are history or music or literature.
page last edited 12/24/05