Winter 2010 Issue
Figure 1. Monitoring and measuring solar energy, 1942. From Forty Years of Radio Research, by George C. Southworth. Note that this photo itself is referred to in the book at “Reprinted by permission from Scientific Monthly 82, 55-66, 1956.”
It was during a field trip to the local phone office when my classmates and I received our second Bell Labs experience, an early demonstration of the touch-tone and picture telephones. Of the former, we’d be challenged to see who could dial faster, the person using a touch-tone phone or a rotary. The rotary challenger put up a good fight, for everyone knew how to speed up the dialing process by forcing the dial in reverse, but it was a fight to be lost. The touch-tone phone just couldn’t be beat. Also, the picture phone, which seemed pure science fiction, would become a highlight of the New York World’s Fair in 1964. The connections of these innovations to our story may not be obvious, but the technology behind them was rooted in decades of mathematics and scientific research taking place at Bell Labs during the first half of the 20th century. Ever vigilant to provide the best phone service in the world, the many innovations and scientific breakthroughs at Murray Hill touched our lives in many unexpected ways.
In his book Forty Years of Radio Research, wireless pioneer George C. Southworth provides a rare glimpse inside Bell Labs, including the work of two associates, Dr. Harry Nyquist and Dr. J. B. Johnson1. In 1928 Johnson theorized that all electronic circuits generated noise depending on their absolute temperature and the band of frequencies under consideration. Nyquist theorized that “Johnson noise” is a form of one-dimensional black-body radiation existing even in systems of such circuits. Although the weakest link would appear to be the “first circuit noise,” such as that of the antenna in a radio receiving system, Southworth wrote, “mature thought showed that the first circuit was really the medium to which the antenna was coupled. In particular cases the medium might include particular objects in interstellar space … one of the most obvious heavenly bodies was the sun.”
In those days receiver noise wasn’t considered much of a problem, but once radar came on the scene, with weak signals awash in circuit noise, things changed quickly. Indeed, the signals of interest were so weak that even noise from celestial bodies, such as the sun and even the Milky Way Galaxy, had the potential to interfere with radar operations. It was J. S. Hey who first associated certain problems with British radar to noise from the sun rather than enemy jamming.
In 1942, Southworth pondered the work of an earlier physicist, Max Plank, who in 1901 revolutionized the world of physics with his quantum theory for black-body radiators. This theory predicted “the total amount of [solar] energy falling on the earth” and “specified the amount of power contained in each unit bandwidth being sent out.” Although the amount of energy predicted to fall within the microwave region would be small, well below the noise level of a typical 1940s receiver, Southworth wondered if a “double-detection” receiver developed for waveguide research and “groomed” for low noise might do the job. Southworth connected this 9400-MHz receiver to a small parabolic dish using a section of waveguide and directed one of his associates, A. P. King, to aim the antenna toward the sun as shown in figure 1.
Almost immediately a small but definite
increase in noise was detected, as indicated by a panel-mounted milliamp
meter. However, there was more. Southworth knew that if the received
energy could be measured, Plank’s theory could be used to predict the
temperature of the object radiating it. Thus it was that on June 29, 1942
the first centimeter radio emissions from the sun were not just detected
and measured, but the temperature of the sun was determined by radio. It
is interesting to note that Robert H. Dicke, the same Princeton physicist
who later confirmed the “Big Bang” noise detected by Penzias and Wilson,
made a related instrument with the ability to switch the receiver between
the antenna and a reference of known temperature, thus providing a means
of calibration. Known as the Dicke Radiometer, this instrument allowed for
much greater accuracy and was used by Dicke himself to measure the
temperature of the sun and moon in 1945.
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