Great Women of Science: Dr. Hedwig Kohn, Pioneering Physicist in Radiometry and Flame Spectroscopy

 “If I have come this far, it is because I have stood on the shoulders of giants.” – Sir Issac Newton

Hedwig Kohn provided shoulders we stand upon in the field of radiometry, the study and measurement of electromagnetic radiation, including UV, visible, and infrared light, radio waves and X-rays, and flame spectroscopy, which measures the amount of light absorbed when a sample is passed through a flame to determine its chemical composition. Both are not subjects we customarily hear about, but both are stepping-stones to the sciences of plasma physics, nuclear fusion, and clinical diagnostic techniques, about which we hear lots. Without Dr. Kohn’s pioneering work, it’s doubtful we would have come so far so fast.

Sadly, I came upon Dr. Kohn’s important work not in reviewing biographies of important physicists nor in accessing sources on women in science, where her contributions are sorely neglected. It took research into the history of my home state, North Carolina, where Dr. Kohn first arrived after fleeing Germany in 1935, performing critical experimental work at Duke University, for me to discover this long, overlooked great woman scientist.

Born in 1887 in then Breslau, Poland, Hedwig began auditing courses at the University of Wroclaw in 1907, even before women were allowed to matriculate. Luckily for her, shortly thereafter, women were admitted, and she received her doctorate in physics in 1913. Upon receipt of her degree, she became an assistant to her mentor, Dr. Otto Lumer, helping him with precision radiation measurements and teaching PhD students as a junior scholar, only receiving full university recognition 27 years later in 1930. At the time, only three women had that accolade: Drs. Hedwig Kohn, Hertha Sponer (chemist and physicist who contributed to quantum mechanics and molecular physics), and an ACSH Great Woman of Science, Dr. Lisë Meitner.

The accomplishment was short-lived. In 1933, with the rise of the Nazis, she, along with numerous other Jewish scientists, was dismissed. With the help of famous physicists and female academics worldwide, she followed a circuitous route to the US, finally arriving in 1940. But first:  

• In 1935, she went to Switzerland for three months, where she measured UV light from the sun. However, without a permanent job, she was forced to return to Poland.
• In 1939, she had secured a visa to work at the University of Aberdeen. But before she could depart, England imposed a blanket ban on all “enemy aliens.”  At least 70 letters by international luminaries like Max Born and Lisë Meitner, along with the American Academy of University Women (a forerunner of which was organized by another Great Woman of Science, Ellen Swallow Richards), were written seeking a position for her, without which she could not leave Nazi-occupied Poland. 

Finally, in 1940, she secured a position at the Women’s College of North Carolina (now the University of North Carolina at Greensboro) before moving on to Wellesley and then to Duke, where she joined Dr. Sponer.

For physics students coming of academic age in the 1960s, Dr. Kohn’s work measuring electromagnetic radiation formed the basis for the textbooks, introducing students to radiometry, a technique that allows radiographs (X-rays) to measure individual bones within the skeleton. The culmination of her work in radiometry allows us to diagnose bone diseases in children, as well as changes in inflammatory arthritis, 

Generally speaking, spectroscopy investigates the interaction of radiated energy (electromagnetic radiation) and matter. In flame spectroscopy, a sample is passed through a flame, and the amount of light absorbed is measured, allowing identification and quantification of the chemical components of the sample. The technique relies on the principle of emission spectroscopy, where atoms or ions in a sample subjected to high temperatures are energized and emit light at specific wavelengths. (A similar technique, flame spectrometry, measures the amount of light emitted upon flaming the sample.) By comparison, radiometry measures the power or intensity of that emission, measured with a radiometer. 

Upon her arrival at Wellesley in 1942, Dr. Kohn set up a flame spectroscopy laboratory, replicating the feat on arrival at Duke a decade later, where she spent twelve years doing experimental work in radiometry and flame spectroscopy. From her work evolved the study of combustion science and plasma physics. 

Combustion, most often what we think of as oxygen combining with other elements as they burn, results in oxidation, a reaction in which atoms lose electrons. In flame spectroscopy, the heat of a flame excites the atoms in a sample, resulting in the absorption of light that can be measured and used to identify the individual components of a compound.  The relationship between spectroscopy and plasma physics is a bit more complex.

Plasma is the least well-known of the four states of matter, the others being gases, liquids, and solids. Plasma results when gas is energized (e.g., by heat or electric fields), and electrons are released from the parent atom, generating a mixture of neutral atoms (usually photons) and charged particles (ions and negatively charged electrons). 

Like flame spectroscopy, plasma spectroscopy allows for the identification of elements within a compound. However, its higher energy state allows for greater precision in measurements. These measurements are crucial in controlled nuclear fusion, astrophysical plasmas, and industrial plasma applications. 

Dozens of articles written about Hedwig Kohn are replete with details of her perilous escape from Nazi Germany, a feat suffered by many colleagues. However, in Dr. Kohn’s case, her biographers omit a serious assessment of her work – not one of her twenty published articles is discussed, nor is the patent she received identified, nor are the accomplishments of the twelve years of her experimental work at Duke reported. It is time to remedy this omission.