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A Closer Look

Current War
homas Edison (1847–1931) built the lighting would soon be replacing all the gas- Westinghouse’s promotion of ac led
Tfirst electric generator and electri- lights. After studying the matter, he decided to direct competition with Edison and his
cal distribution system to promote his that Edison’s low-voltage system was not dc electrical systems. A “war of currents”
new long-lasting lightbulbs. The dc gen- efficient enough. In 1885, he began experi- resulted, with Edison claiming that trans-
erator and distribution system was built in menting with ac generators and transformers mission of such high voltage was dangerous.
lower Manhattan, New York City, and was in Pittsburgh. He emphasized this point by recommend-
switched on September 4, 1882. It supplied Nikola Tesla (1856–1943) was a Croatian- ing the use of high-voltage ac in an electric
110 V dc to 59 customers. Edison stud- born U.S. physicist and electrical engineer chair as the best way to execute prisoners.
ied both ac and dc systems and chose dc who moved to the United States and worked The advantages of ac were greater since
because of advantages it offered at the time. for Thomas Edison in 1884. He then set up you could increase the voltage, transmit
Direct current was used because batter- his own laboratory and workshop in 1887. for long distances at a lower cost, and then
ies are dc, and batteries were used as a His work led to a complicated set of patents decrease the voltage to a safe level. Even-
system backup. Also, dc worked fine with covering the generation, transmission, and tually, even Edison’s own General Electric
electric motors, and ac motors were not use of ac electricity. From 1888 on, Tesla company switched to producing ac equip-
yet available. was associated with George Westinghouse, ment. Westinghouse turned his attention to
George Westinghouse (1846–1914) who bought and successfully exploited Tesla’s the production of large steam turbines for
was in the business of supplying gas for ideas, leading to the introduction of ac for producing ac power and was soon setting
gas lighting, and he could see that electric power transmission. up ac distribution systems across the nation.

GENERATORS
Soon after the discovery of electromagnetic induction the elec-

tric generator was developed. The generator is essentially an

axle with many wire loops that rotates in a magnetic fi eld. Th e Armature
axle is turned by some form of mechanical energy, such as a
water turbine or a steam turbine, which uses steam generated Field coil N S
from fossil fuels or nuclear energy. As the coil rotates in a mag-
netic field, a current is induced in the coil (Figure 6.33).

Magnetic
Connection field lines
for ac
TRANSFORMERS Terminal brushes
Current from a power plant goes to a transformer to step up the
voltage. A transformer is a device that steps up or steps down A
the ac voltage. It has two basic parts: (1) a primary or “input”
coil and (2) a secondary or “output” coil, which is close by. Both
coils are often wound on a single iron core but are always fully
insulated from each other. When there is an alternating current +
through the primary coil, a magnetic field grows around the coil
to a maximum size, collapses to zero, then grows to a maximum
size with an opposite polarity. This happens 120 times a second –
as the alternating current oscillates at 60 hertz. The magnetic
field is strengthened and directed by the iron core. The growing
and collapsing magnetic field moves across the wires in the B
secondary coil, inducing a voltage in the secondary coil. The
growing and collapsing magnetic field from the primary coil FIGURE 6.33 (A) Schematic of a simple alternator (ac genera-
thus induces a voltage in the secondary coil, just as an induced tor) with one output loop. (B) Output of the single loop turning in a
voltage occurs in the wire loops of a generator. constant magnetic field, which alternates the induced current each

The transformer increases or decreases the voltage in an half-cycle.

alternating current because the magnetic field grows and col-
lapses past the secondary coil, inducing a voltage. If a direct

current is applied to the primary coil, the magnetic fi eld grows lines and a wire loop. Thus, an induced voltage occurs from a
around the primary coil as the current is established but then direct current (1) only for an instant when the current is estab-

becomes stationary. Recall that electromagnetic induction lished and the growing field moves across the secondary coil
occurs when there is relative motion between the magnetic fi eld and (2) only for an instant when the current is turned off and
162 CHAPTER 6 Electricity 6-24