In this installment of the history of atom theory, physics professor (and my dad) Dean Zollman introduces us to Pierre and Marie Curie. Marie had humble beginnings, and she and her husband often worked under less than ideal circumstances. – Kim
By Dean Zollman
As I mentioned last time, Becquerel’s discovery of “rays” coming from uranium fell rather flat on the scientific community. However, very quickly one of the most famous couples in the history of science – Pierre and Marie Sklodowska Curie – conducted research that changed how the world looked at matter and the emissions from several elements. We will take a brief look at their story in this and the next post. Many books and websites are devoted to the Curies, so if you find this brief introduction interesting, search on their names.
Marie Sklodowska (1867-1934) grew up in Russian-occupied Poland. She was an excellent student in secondary school, but at that time, Polish universities did not admit women. Her goal was to move to Paris so that she could continue her education. However, she could not afford the move. So, she worked for several years as a governess to obtain enough money to travel to Paris in 1891 and enter the Sorbonne. For most of her student days, she lived in a six-floor walkup, unheated attic. She had little money, and it is said that she fainted in the university library due to the lack of food. However, she persevered in her studies. In 1893, she graduated as the top student in physics. A year later, she was the second student in mathematics. Her plan had been to return to Poland and become a teacher. However, romance got in the way.
Pierre Curie (1859-1906) was the son of a Parisian physician. He showed an aptitude for science at an early age and was conducting groundbreaking research before he was 20. Much of his early work was on magnetism. In 1880, he and his brother Jacques (1856–1941) discovered that an electric current would be created when certain crystals were deformed.
More importantly for his later work with Marie, they determined that the inverse was also true: These crystals could be deformed when a current was passed through them. They named this phenomenon piezoelectricity. His work in magnetism also led to several discoveries including that the magnetism of a material is inversely related to it temperature. (You can destroy a magnet by heating it.) Today, this principle is still called Curie’s Law.
A Fateful Introduction
To help pay for her education, one of Marie’s professors arranged a job for her to measure the magnetic properties of some materials. (Today we call this type of work undergraduate research; then, it was a way to put bread on the table in an unheated attic.) But she did not have a laboratory in which to do this work.
A professor arranged an introduction to a researcher who was working in magnetism – Pierre Curie. The professor thought that Pierre could provide space in his lab for Marie’s work. Unfortunately, Pierre’s laboratory was little more than a cubby hole, so he could not provide the space. However, for both of them this introduction was a life changing event.
Neither Pierre nor Marie had much interest in the opposite sex. Pierre has been quoted as saying that relationships with women took time away from doing science and thus were unnecessary distractions. Before leaving Poland, Marie had been involved in an ill-fated romance. Her beau’s family had rejected her because of her low social status.
However, Pierre and Marie seemed to hit it off immediately. Both were dedicated scientists and intellectual equals. With that as a foundation, their connection to each other grew.
They were married in 1895. By then, Marie had begun her research on magnetism, so she chose a wedding dress that could be worn in the laboratory after she returned from their honeymoon. The honeymoon was a bicycle trip during which there were apparently many discussions about magnetism.
Upon returning, Marie continued her measurements. As we have seen in previous posts, scientific discoveries were occurring quickly in the late 1890s. Wilhelm Roentgen discovered x-rays at the end of 1895. In an attempt to learn more about the x-rays, Henri Becquerel accidently discovered another form of radiation in 1896. About this time, Marie finished the work on magnetism and needed to choose a topic for her doctoral dissertation.
Let’s Measure These Rays
Marie’s life had been further complicated by the birth of their first daughter, Irene, and the death of Pierre’s mother. Nevertheless Marie was intent on balancing family life and pursuing her PhD. Because a majority of the scientific discussion at the time focused on Roentgen’s discovery and very few people seemed interested in Becquerel’s work, Pierre suggested that Marie might find Becquerel rays a fruitful and less crowded pursuit.
Becquerel had discovered that these rays caused photographic plates to be darkened, but he had not made quantitative measurements. He had also noticed that the rays were able to cause electrically charged objects to discharge. A natural conclusion was that one could measure the amount of rays being emitted with an electrical device.
However, the current was very small, so one needed a device to measure very low currents. Pierre and Jacque had invented just such a device – an electrometer using piezoelectricity. The electrometer has two metal plates separated by a small distance and connected to a battery. Because the metal plates are separated by air, no current flows in the device. However, a material that emits electrically charged particles will knock electrons off the air atoms. These electrons will create a current and cause a rotation in the crystal. More particles from the material results in a greater current. Thus, by comparing currents they could compare the relative amount of emissions from different substances. (I cheated and explained the device using a concept that was not known the Curies – electrons in atoms.)
At first the device yielded results that were no better than previous ones. When Pierre improved the piezoelectric component, sensitivity improved greatly. So, an excellent measurement of the activity could be determined from Pierre’s device.
Later, Marie coined the term radioactivity to describe the activity of the material. By placing some of the material that emitted Becquerel’s rays in the gap between the two metal plates, they could compare the radioactivity of different materials by measuring the rotation of the crystal. However, this device was not easy to work with. It took Marie 20 days to learn how to make accurate measurements.
Newly Discovered Elements
Eventually, Marie was able to measure the activity of pure uranium. She used this measurement as the standard with which she compared all others. She found that most materials were not radioactive. Her first discovery was that thorium was radioactive.
Then, came a major breakthrough; she placed an ore, pitchblende, into the measuring apparatus. Pitchblende is the ore that contains uranium. But the pitchblende showed a much higher activity than uranium or thorium. Something in addition to uranium and thorium must have been present in pitchblende.
With many tedious measurements, Marie was able to discern that in the ore two different radioactive elements must be present. She named the first one polonium after her native Poland and the second, radium. This was the first time that elements had been discovered based on properties other than chemistry.
Physicists of the late 19th century were willing to accept the discovery of the new elements, but chemists claimed that this was a theoretical discovery. No one had yet isolated either polonium or radium and held a pure substance in his or her hands. By now Pierre had decided that the research in radioactivity was much more fruitful and interesting than his other research, so he joined Marie in a full-time effort. He would look at the physical properties while Marie would handle the chemistry. Their plan was to isolate radium from the pitchblende in which it was located.
First they needed a place to work. The university could offer very little space but finally allowed them to use a shed next to a courtyard. At one time, it had been used for dissection classes of medical students. However, the roof leaked, it was very drafty, and it had a dirt floor. The university had decided that the shed was unfit for cadavers and had moved them elsewhere. But it was all that the Curies could use, so they took it.
Their next step was long, tedious, and dangerous. We will take it up next time.
Modern Variations of Pierre’s Electrometer
- This electrometer was a simplified version of a detector for radioactive particles called the Geiger counter. Charged particles enter the tube, knocked electrons off atoms and thus created a current. Through a series of electronic pulses, the Geiger counter clicks when a particle passed through the tube. You will frequently see this counter in old sci-fi movies. (In a few months, we will meet Hans Geiger and see why he was motivated to invent such as device.)
- The smoke detector in your home works like the inverse of Curie’s electrometer. Inside the smoke detector is a very small radioactive source. The charged particles coming from the source knocks electrons off atoms. These electrons move in an air gap in a circuit. Thus, a current is present in the detector. When smoke enters, it absorbs the charged particles, and the current stops. When the current stops, the electronics in your smoke detector sets off the alarm.
All images via Wikimedia Commons.
Dean Zollman is university distinguished professor of physics at Kansas State University where he has been a faculty member for more than 40 years. During his career he has received four major awards — the American Association of Physics Teachers’ Oersted Medal (2014), the National Science Foundation Director’s Award for Distinguished Teacher Scholars (2004), the Carnegie Foundation for the Advancement of Teaching Doctoral University Professor of the Year (1996), and AAPT’s Robert A. Millikan Medal (1995). His present research concentrates on the teaching and learning of physics and on science teacher preparation.