very high energy. Scientists study cosmic
rays to learn more about the makeup of
the galaxy.
#More to explore
Atom • Energy • Heat • Light • Nuclear
Energy • Radio • Sound • X-rays
Radio
Radio is a way of sending sounds or
other information through the air. The
information is carried by invisible radio
Radio waves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays are all
types of electromagnetic radiation. Radio waves have the longest wavelength, and gamma
rays have the shortest wavelength.
Dish antennas pick up radio waves that
carry many kinds of communication.
18 Radio BRITANNICA STUDENT ENCYCLOPEDIA
waves. Radio waves are used for broadcasting,
or sending out, radio and television
programs. Radar, cell phones,
cordless telephones, wireless computer
networks, and other forms of communication
use radio waves, too.
How RadioWorks
Radio waves are a type of radiation.
Radiation is energy that moves. Like all
waves of radiation, radio waves have a
property called frequency. Frequency
describes the number of waves that pass
a given point each second.
Radio waves are sent out by a device
called a transmitter. The transmitter
turns talking, music, pictures, or other
information into electric signals. The
transmitter combines these electric signals
with radio waves of a certain frequency.
The waves spread out in all
directions from an antenna connected to
the transmitter.
The radio waves are picked up by an
antenna connected to a device called a
receiver. The receiver separates the electric
signals from the radio waves. Then it
turns the electric signal back into the
original sounds or pictures.
A receiver that picks up waves that carry
sound is called a radio. A radio sends the
electric signal through speakers so the
sound can be heard. Each radio station
sends out radio waves of a certain frequency.
A person changes a radio from
one frequency to another to hear different
stations.
History
In the late 1890s several people began
working on ways to send and receive
electric signals using radio waves. An
Italian scientist named Guglielmo Marconi
got much of the credit for developing
radio. In 1897 he started a company
that developed several uses for radio.
The first radio station started broadcasting
in Pittsburgh, Pennsylvania, in
1920.Within two years hundreds of
radio stations were started. Radio was a
major source of home entertainment
until the mid-1950s. Then television
became more popular.
In the early 21st century satellite radio
got many people interested in radio
again. Satellite radio was a new method
of radio broadcasting. Instead of using
antennas on the ground, satellite radio
stations sent broadcasts from satellites,
or spacecraft. Satellite radio stations
provided crisp, clear sound. They also
Families of the 1930s had no television to
watch. Instead, they gathered around the
radio to listen to their favorite programs.
BRITANNICA STUDENT ENCYCLOPEDIA Radio 19
could be heard over a much wider area
than traditional radio stations could.
#More to explore
Electronics • Marconi, Guglielmo
• Radar • Radiation
• Telecommunication • Telephone
• Television
Railroad
A railroad, also called a railway, is a type
of land transportation. In a railroad a
train travels along a path of two metal
rails, or tracks. A train is a row of
wheeled cars that are linked together.
The wheels of rail cars have a rim that
keeps them on the rails.
A vehicle called a locomotive pulls most
trains. A locomotive can be powered by
an engine that burns diesel fuel or by
electricity. The electricity comes from a
third rail or an overhead wire. The train
operator, called the driver or engineer,
usually sits in the locomotive.
Types of Trains
Trains can be divided into two types:
passenger and freight. Passenger trains
carry people. Freight trains carry goods.
Passenger trains have cars with seats.
Many passenger trains also have dining
cars, baggage cars, and even sleeping cars
with beds. The speed of many large passenger
trains is about 100 miles (160
kilometers) an hour. However, some
trains in Europe and Japan can go more
than 180 miles (290 kilometers) an
hour.
Commuter trains (trains that carry
workers into a city), subways (underground
trains), and monorails (trains
that run on one rail) are smaller passenger
trains. They carry people shorter
distances. These trains often do not have
a separate locomotive. Instead, some or
all of the cars have electric motors under
the floor. The train operator sits in the
front car.
Freight trains consist of locomotives and
freight cars. There are many types of
freight cars. Boxcars often carry packaged
goods. Open hopper cars often
carry coal. Covered hopper cars often
carry grain. Tank cars carry liquids. Car
carriers carry automobiles. Special flatcars
carry trailers or freight containers.
Some freight trains have 150 or more
cars. Because they pull so much weight,
freight trains usually move more slowly
than passenger trains.
High-speed trains leave their station in London,
England. They can go all the way to
France through a railway tunnel that runs
under the English Channel.
