“Discovery of Radioactivity” 1962, Educational Foundation for Visual Aids, National Committe for Audio-Visual Aids in Education, Eastman color, 15 minutes. Features re-enactments of famous moments in the study of radioactivity.

“An historical survey of progressive developments leading to our present knowledge of radioactivity, this is the story of Wilhelm Roentgen and his discovery in 1895 that certain crystals become fluorescent when they are subjected to cathode tube rays. It includes Henri Becquerel’s discovery that uranium salts give off invisible rays, Marie Curie’s isolation of polonium and radium, and the work of Julius Elster and Hans Geitel, who measured the rays of an electroscope. Analyses by Ernest Rutherford revealed alpha, beta, and gamma rays. Further studies showed how a substance decays at a particular rate and that radioactive substances contain a mixture of different atomic elements. The discovery of radioactivity led to a new conception of the structure of the atom and of matter.”

0:25 In 1895, Wilhelm Röntgen’s experiments on cathode rays and crystal fluorescence led to the discovery of invisible rays capable of penetrating lightproof paper and causing fluorescence; he named these “X-rays.”

1:36 Röntgen soon found X-rays had unexpected characteristics, demonstrating their ability to image bone structures through tissue, and that metal resisted these rays.

2:35 In Paris, Henri Becquerel, inspired by Röntgen, investigated fluorescence in uranium salts. He discovered that uranium salts, even without exposure to sunlight (and thus not fluorescent), blackened photographic plates, proving the uranium itself emitted invisible rays.

4:06 Becquerel further discovered these rays could discharge a condenser by ionizing the air, demonstrating that certain minerals naturally emit invisible rays capable of discharging condensers and blackening photographic plates.

4:52 Marie Curie began a systematic search for other minerals emitting these rays, leading to her discovery that pitchblende discharged a condenser much faster than other minerals, suggesting it contained a more powerful radioactive element.

5:48 Marie and Pierre Curie then set out to isolate this substance from pitchblende in their shed laboratory. Through a process of pounding, dissolving in acid, and various isolation techniques, they eventually found two powerful radiating substances: polonium (named after Marie’s native Poland) and radium.

6:56 Only a minute quantity of radium was obtained from a ton of pitchblende, but its rays were millions of times more powerful than uranium’s; they called these “radioactive rays.”

7:22 Julius Elster and Hans Geitel measured these rays using an electroscope, demonstrating that radioactive gas (from thorium) caused the electroscope to discharge, and their graph showed that radioactivity gradually diminishes over time.

8:47 Ernest Rutherford further analyzed radioactivity, showing that a radioactive substance in a lead case emitted three distinct types of rays when exposed to a magnetic field.

9:36 He named these Alpha, Beta, and Gamma rays, concluding that Alpha rays were positively charged, Beta rays negatively charged, and Gamma rays were uncharged electromagnetic radiation.

10:09 It was discovered that radioactivity diminished in an unusual way, where a substance’s radioactive atoms decay by half over a specific time period called the “half-life.”

11:00 The half-life is the time taken for a substance to lose half its radioactivity, a concept illustrated by Elster and Geitel’s earlier graph, showing a consistent decay period for each half.

12:16 When a radioactive atom emits a ray, it transforms into a new element, which may then decay into yet another, forming a definite “decay series” (e.g., uranium-radium, actinium, thorium series).

12:53 The film provides an example of thorium decaying through a series of intermediate materials (mesothorium 1, mesothorium 2, etc.) eventually forming lead (thorium D), with these intermediate materials identified as isotopes of known substances.

14:04 The continuous decay and simultaneous appearance of various ray types meant the atom could no longer be considered stable and indivisible, but rather capable of decay, releasing energy as alpha, beta, and gamma rays.

14:44 The discovery of radioactivity profoundly changed the conception of atomic and matter structure.