The electron was the first subatomic particle to be discovered, and Sir Joseph John Thomson, OM FRS (18 December 1856–30 August 1940), a British physicist and Nobel Laureate in Physics, is credited with its discovery.
Thomson demonstrated in 1897 that the hitherto unidentified negatively charged particles that made up cathode rays (now known as electrons) had bodies far smaller than atoms and a very high charge-to-mass ratio. As part of his investigation into the make-up of canal rays (positive ions), Thomson is also credited with discovering the first evidence for isotopes of a stable (non-radioactive) element in 1913. Together with Francis William Aston, he performed the first mass spectrometric studies to ascertain the characteristics of positively charged particles, which sparked the creation of the mass spectrograph.
Thomson's research on the conduction of electricity in gases earned him the 1906 Nobel Prize in Physics. Thomson was also a teacher, and nine of his pupils, along with his son and other less senior colleagues, went on to win Nobel Prizes. A comparable list of top-performing students can only be found in Arnold Sommerfeld's mentorship history.
Personal And Academic Lives
On December 18, 1856, Joseph John Thomson was born in Cheetham Hill, Manchester, Lancashire, England. Emma Swindells, his mother, was from a local textile family. His great-grandfather established an antiquarian bookshop, which Thomson's father, Joseph James Thomson, maintained. Frederick Vernon Thomson, his younger brother, was two years his junior. Although reticent, J. J. Thomson was a devoted Anglican.
He received his early schooling in little private schools, where he excelled in science and showed a keen interest in the subject. At the remarkably young age of 14, Thomson was accepted to Owens College in Manchester (now University of Manchester), where he was influenced by physics professor Balfour Stewart, who got him interested in physical study. After starting his contact electrification experiments, Thomson quickly produced his first academic publication. When his father passed away in 1873, his parents' plans to enroll him as an apprentice engineer at the locomotive manufacturer Sharp, Stewart & Co were abandoned.
In 1876, he transferred to Trinity College in Cambridge. He earned his Bachelor of Arts in mathematics in 1880 (second in the tripos and second in the Smith's Prize). In 1881, he submitted an application to Trinity College and was accepted. In 1883, he earned a Master of Arts degree (along with the Adams Prize).
Discovery of the Electron
Many scientists, including William Prout and Norman Lockyer, had proposed that atoms were constructed from a more fundamental unit, but they thought that this unit would be the same size as the hydrogen atom, which is the smallest atom. The electron, a subatomic particle currently known as the electron, was first proposed by Thomson in 1897 as one of the fundamental components of the atom that was more than 1,000 times smaller than an atom. Thomson learned about this while researching the characteristics of cathode rays. Following his finding that cathode rays, also known as Lenard rays at the time, could travel through air far farther than an atom-sized particle would have anticipated, Thomson presented his recommendation on April 30, 1897.
By measuring the heat produced when the cathode rays hit a thermal junction and comparing it to the magnetic deflection of the rays, he was able to estimate the mass of the cathode rays. His research indicated that cathode rays had a mass that was constant regardless of the sort of atom they originated from and that they were nearly 1,000 times lighter than hydrogen atoms. He came to the conclusion that the rays were made up of extremely light, negatively charged particles, which served as an all-purpose atom's basic building block. Although he referred to the particles as "corpuscles," other researchers preferred the moniker "electrons," which George Johnstone Stoney had first proposed in 1891, before Thomson made his actual discovery.
Isotopes and Mass Spectrometry
Thomson and his research assistant F. W. Aston channeled a stream of neon ions through a magnetic and an electric field in 1912 as part of their investigation into the makeup of the streams of positively charged particles at the time known as canal rays. They then measured the deflection by positioning a photographic plate in its path. They noticed two spots of light on the photographic plate (see figure on the right), which suggested two distinct parabolas of deflection, and came to the conclusion that neon is made up of two isotopes, or atoms with two different atomic masses. Frederick Soddy had earlier hypothesized the existence of isotopes to explain the disintegration of some radioactive elements; this was the first evidence for isotopes of a stable element.
Experiments with Cathode Rays
Earlier, according to Thomson, scientists questioned whether cathode rays were entirely material and marked the pathways of matter particles charged with negative electricity, or whether they were immaterial like light ("some process in the aether"). Although the particle hypothesis was sufficiently certain for Thomson to verify, the aetherial hypothesis was hazy.
Magnetic Deflection
Cathode ray magnetic deflection was the subject of Thomson's initial research. The apparatus's side tube on the left created cathode rays, which traveled through the anode and into the main bell jar before being redirected by a magnet. The fluorescence on a square screen in the container helped Thomson identify their path. He discovered that regardless of the anode and gas's materials, the rays' deflection was the same, indicating that the rays were of the same form no matter where they came from.
While proponents of the aetherial hypothesis acknowledged that negatively charged particles might be created in Crookes tubes, they thought they were just a byproduct and that the cathode rays themselves were unimportant. In order to determine whether he could actually separate the charge from the rays, Thomson set off on his investigation.
Thomson built a Crookes tube with an electrometer positioned off to the side, away from the cathode rays. Thomson was able to determine the ray's path by studying the phosphorescent patch it produced when it collided with the tube's surface. Thomson noticed that the electrometer only showed a charge when he used a magnet to divert the cathode ray toward it. He came to the conclusion that the negative charge and the rays were identical.