The invention: Self-contained system making it possible to record
and repeatedly play back sound without having to thread tape
through a machine.
The person behind the invention:
Fritz Pfleumer, a German engineer whose work on audiotapes
paved the way for audiocassette production
Smaller Is Better
The introduction of magnetic audio recording tape in 1929 was
met with great enthusiasm, particularly in the entertainment industry,
and specifically among radio broadcasters. Although somewhat
practical methods for recording and storing sound for later playback
had been around for some time, audiotape was much easier to
use, store, and edit, and much less expensive to produce.
It was Fritz Pfleumer, a German engineer, who in 1929 filed the
first audiotape patent. His detailed specifications indicated that
tape could be made by bonding a thin coating of oxide to strips of either
paper or film. Pfleumer also suggested that audiotape could be
attached to filmstrips to provide higher-quality sound than was
available with the film sound technologies in use at that time. In
1935, the German electronics firm AEG produced a reliable prototype
of a record-playback machine based on Pfleumer’s idea. By
1947, the American company 3M had refined the concept to the
point where it was able to produce a high-quality tape using a plastic-
based backing and red oxide. The tape recorded and reproduced
sound with a high degree of clarity and dynamic range and would
soon become the standard in the industry.
Still, the tape was sold and used in a somewhat inconvenient
open-reel format. The user had to thread it through a machine and
onto a take-up reel. This process was somewhat cumbersome and
complicated for the layperson. For many years, sound-recording
technology remained a tool mostly for professionals.
In 1963, the first audiocassette was introduced by the Netherlands-based PhilipsNVcompany. This device could be inserted into
a machine without threading. Rewind and fast-forward were faster,
and it made no difference where the tape was stopped prior to the
ejection of the cassette. By contrast, open-reel audiotape required
that the tape be wound fully onto one or the other of the two reels
before it could be taken off the machine.
Technical advances allowed the cassette tape to be much narrower
than the tape used in open reels and also allowed the tape
speed to be reduced without sacrificing sound quality. Thus, the
cassette was easier to carry around, and more sound could be recorded
on a cassette tape. In addition, the enclosed cassette decreased
wear and tear on the tape and protected it from contamination.
Creating a Market
One of the most popular uses for audiocassettes was to record
music from radios and other audio sources for later playback. During
the 1970’s, many radio stations developed “all music” formats
in which entire albums were often played without interruption.
That gave listeners an opportunity to record the music for later
playback. At first, the music recording industry complained about
this practice, charging that unauthorized recording of music from
the radio was a violation of copyright laws. Eventually, the issue
died down as the same companies began to recognize this new, untapped
market for recorded music on cassette.
Audiocassettes, all based on the original Philips design, were being
manufactured by more than sixty companies within only a few
years of their introduction. In addition, spin-offs of that design were
being used in many specialized applications, including dictation,
storage of computer information, and surveillance. The emergence
of videotape resulted in a number of formats for recording and
playing back video based on the same principle. Although each is
characterized by different widths of tape, each uses the same technique
for tape storage and transport.
The cassette has remained a popular means of storing and retrieving
information on magnetic tape for more than a quarter of a
century. During the early 1990’s, digital technologies such as audio
CDs (compact discs) and the more advanced CD-ROM (compact discs that reproduce sound, text, and images via computer) were beginning
to store information in revolutionary new ways. With the
development of this increasingly sophisticated technology, need for
the audiocassette, once the most versatile, reliable, portable, and
economical means of recording, storing, and playing-back sound,
became more limited.
Consequences
The cassette represented a new level of convenience for the audiophile,
resulting in a significant increase in the use of recording
technology in all walks of life. Even small children could operate
cassette recorders and players, which led to their use in schools for a
variety of instructional tasks and in the home for entertainment. The
recording industry realized that audiotape cassettes would allow
consumers to listen to recorded music in places where record players
were impractical: in automobiles, at the beach, even while camping.
The industry also saw the need for widespread availability of
music and information on cassette tape. It soon began distributing
albums on audiocassette in addition to the long-play vinyl discs,
and recording sales increased substantially. This new technology
put recorded music into automobiles for the first time, again resulting
in a surge in sales for recorded music. Eventually, information,
including language instruction and books-on-tape, became popular
commuter fare.
With the invention of the microchip, audiotape players became
available in smaller and smaller sizes, making them truly portable.
Audiocassettes underwent another explosion in popularity during
the early 1980’s, when the Sony Corporation introduced the
Walkman, an extremely compact, almost weightless cassette player
that could be attached to clothing and used with lightweight earphones
virtually anywhere. At the same time, cassettes were suddenly
being used with microcomputers for backing up magnetic
data files.
Home video soon exploded onto the scene, bringing with it new
applications for cassettes. As had happened with audiotape, video
camera-recorder units, called “camcorders,” were miniaturized to
the point where 8-millimeter videocassettes capable of recording up to 90 minutes of live action and sound were widely available. These
cassettes closely resembled the audiocassette first introduced in
1963.
The invention: Atechnique that measures the radioactive decay of
carbon 14 in organic substances to determine the ages of artifacts
as old as ten thousand years.
The people behind the invention:
Willard Frank Libby (1908-1980), an American chemist who won
the 1960 Nobel Prize in Chemistry
Charles Wesley Ferguson (1922-1986), a scientist who
demonstrated that carbon 14 dates before 1500 b.c. needed to
be corrected
One in a Trillion
Carbon dioxide in the earth’s atmosphere contains a mixture of
three carbon isotopes (isotopes are atoms of the same element that
contain different numbers of neutrons), which occur in the following
percentages: about 99 percent carbon 12, about 1 percent carbon
13, and approximately one atom in a trillion of radioactive carbon
14. Plants absorb carbon dioxide from the atmosphere during photosynthesis,
and then animals eat the plants, so all living plants and
animals contain a small amount of radioactive carbon.
When a plant or animal dies, its radioactivity slowly decreases as
the radioactive carbon 14 decays. The time it takes for half of any radioactive
substance to decay is known as its “half-life.” The half-life
for carbon 14 is known to be about fifty-seven hundred years. The
carbon 14 activity will drop to one-half after one half-life, onefourth
after two half-lives, one-eighth after three half-lives, and so
forth. After ten or twenty half-lives, the activity becomes too low to
be measurable. Coal and oil, which were formed from organic matter
millions of years ago, have long since lost any carbon 14 activity.
Wood samples from an Egyptian tomb or charcoal from a prehistoric
fireplace a few thousand years ago, however, can be dated with
good reliability from the leftover radioactivity.
In the 1940’s, the properties of radioactive elements were still
being discovered and were just beginning to be used to solve problems.
Scientists still did not know the half-life of carbon 14, and archaeologists still depended mainly on historical evidence to determine
the ages of ancient objects.
In early 1947,Willard Frank Libby started a crucial experiment in
testing for radioactive carbon. He decided to test samples of methane
gas from two different sources. One group of samples came
from the sewage disposal plant at Baltimore, Maryland, which was
rich in fresh organic matter. The other sample of methane came from
an oil refinery, which should have contained only ancient carbon
from fossils whose radioactivity should have completely decayed.
The experimental results confirmed Libby’s suspicions: The methane
from fresh sewage was radioactive, but the methane from oil
was not. Evidently, radioactive carbon was present in fresh organic
material, but it decays away eventually.
Tree-Ring Dating
In order to establish the validity of radiocarbon dating, Libby analyzed
known samples of varying ages. These included tree-ring
samples from the years 575 and 1075 and one redwood from 979
b.c.e., as well as artifacts from Egyptian tombs going back to about
3000 b.c.e. In 1949, he published an article in the journal Science that
contained a graph comparing the historical ages and the measured
radiocarbon ages of eleven objects. The results were accurate within
10 percent, which meant that the general method was sound.
The first archaeological object analyzed by carbon dating, obtained
from the Metropolitan Museum of Art in New York, was a
piece of cypress wood from the tomb of King Djoser of Egypt. Based
on historical evidence, the age of this piece of wood was about fortysix
hundred years. A small sample of carbon obtained from this
wood was deposited on the inside of Libby’s radiation counter, giving
a count rate that was about 40 percent lower than that of modern
organic carbon. The resulting age of the wood calculated from its residual
radioactivity was about thirty-eight hundred years, a difference
of eight hundred years. Considering that this was the first object
to be analyzed, even such a rough agreement with the historic
age was considered to be encouraging.
The validity of radiocarbon dating depends on an important assumption—
namely, that the abundance of carbon 14 in nature has been constant for many thousands of years. If carbon 14 was less
abundant at some point in history, organic samples from that era
would have started with less radioactivity. When analyzed today,
their reduced activity would make them appear to be older than
they really are.Charles Wesley Ferguson from the Tree-Ring Research Laboratory
at the University of Arizona tackled this problem. He measured
the age of bristlecone pine trees both by counting the rings and by
using carbon 14 methods. He found that carbon 14 dates before
1500 b.c.e. needed to be corrected. The results show that radiocarbon
dates are older than tree-ring counting dates by as much as several
hundred years for the oldest samples. He knew that the number
of tree rings had given him the correct age of the pines, because trees
accumulate one ring of growth for every year of life. Apparently, the
carbon 14 content in the atmosphere has not been constant. Fortunately,
tree-ring counting gives reliable dates that can be used to
correct radiocarbon measurements back to about 6000 b.c.e.
Impact
Some interesting samples were dated by Libby’s group. The
Dead Sea Scrolls had been found in a cave by an Arab shepherd in
1947, but some Bible scholars at first questioned whether they were
genuine. The linen wrapping from the Book of Isaiah was tested for
carbon 14, giving a date of 100 b.c.e., which helped to establish its
authenticity. Human hair from an Egyptian tomb was determined
to be nearly five thousand years old.Well-preserved sandals from a
cave in eastern Oregon were determined to be ninety-three hundred
years old. A charcoal sample from a prehistoric site in western
South Dakota was found to be about seven thousand years old.
The Shroud of Turin, located in Turin, Italy, has been a controversial
object for many years. It is a linen cloth, more than four meters
long, which shows the image of a man’s body, both front and back.
Some people think it may have been the burial shroud of Jesus
Christ after his crucifixion. Ateam of scientists in 1978 was permitted
to study the shroud, using infrared photography, analysis of
possible blood stains, microscopic examination of the linen fibers,
and other methods. The results were ambiguous. A carbon 14 test
was not permitted because it would have required cutting a piece
about the size of a handkerchief from the shroud.
Anew method of measuring carbon 14 was developed in the late
1980’s. It is called “accelerator mass spectrometry,” or AMS. Unlike
Libby’s method, it does not count the radioactivity of carbon. Instead, a mass spectrometer directly measures the ratio of carbon 14
to ordinary carbon. The main advantage of this method is that the
sample size needed for analysis is about a thousand times smaller
than before. The archbishop of Turin permitted three laboratories
with the appropriate AMS apparatus to test the shroud material.
The results agreed that the material was from the fourteenth century,
not from the time of Christ. The figure on the shroud may be a
watercolor painting on linen.
Since Libby’s pioneering experiments in the late 1940’s, carbon
14 dating has established itself as a reliable dating technique for archaeologists
and cultural historians. Further improvements are expected
to increase precision, to make it possible to use smaller samples,
and to extend the effective time range of the method back to
fifty thousand years or earlier.