Background of the Thomson e/m Experiment
Technology sometimes leads science in new directions and, more often in modern times, scientific discoveries lead to new technologies. In the mid-1800s, a powerful vacuum pump was invented by Heinrich Geissler. This new technology provided physicists with a tool to embark on a general inductive exploration of electricity and low-pressure gases. Between 1855 and 1875, several scientists—most notably Sir William Crookes—discovered and characterized some general empirical properties of cathode rays:
- Cathodes of different materials produce cathode rays with the same properties;
- Cathode rays travel in straight lines, perpendicular to the emitting surface, as long as no magnetic field is present;
- Cathode rays are deflected by an external magnetic field;
- Cathode rays can produce chemical reactions similar to the reactions of light on certain chemicals.
This is a classic example of inductive reasoning in which observations are collected and analyzed to produce a general result (See CRYSTAL-Alberta site). Empirical (observable) knowledge is never the end result in science. Almost immediately and, some would argue simultaneously, theoretical (non-observable) knowledge develops. In science, there is an intrinsic need to create ideas to explain and understand the empirical evidence.
According to J.J. Thomson, “the most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the aether [a hypothetical elastic fluid postulated to be the medium through which light propagates] … another view of these rays is that, so far from being wholly aetherial, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity” (Philosophical Magazine, 44, 293; 1897). Thomson goes on in the introduction of his scientific paper to note that the electrified-particle theory is the better theory for research because “it is definite and its consequences can be predicted”, whereas no predictions can be made using the aetherial theory. This illustrates an important requirement for scientific knowledge: it must be testable both logically and experimentally. If no predictions can be made, then a theory cannot be tested.
Prior to conducting his famous charge-to-mass experiment on cathode rays, Thomson conducted several other experiments to replicate work by other scientists and to counteract or disprove criticisms by the proponents of the aetherial theory. Replication by independent researchers is always necessary for scientific knowledge to become accepted by the scientific community. Even better, according to the philosopher Karl Popper, is attempting to falsify or disprove a scientific claim.
- Cathode rays are negatively charged particles.
Jean Perrin (1895) set up a cathode ray tube with a metal cylinder connected to an electroscope. He showed that the electroscope became negatively charged when the cylinder was bombarded with cathode rays. Supporters of the aetherial theory agreed that electric particles are emitted by the cathode. They disagreed that the effect on the electroscope was directly due to the negatively charged particles, but rather to effects of the medium (aether). Thomson repeated Perrin’s experiment with an improved design in which the cathode rays could only enter a hole in a metal cylinder (connected to an electroscope) when they were deflected at the correct angle by an external magnetic field. Thus he was able to show, with a higher degree of certainty, that cathode rays were directly the cause of the accumulated negative charge on the electroscope. Thomson’s work illustrates that critiquing experimental designs and repeating experiments with improved designs is an important part of scientific work.
- Cathode rays are deflected by an electric field.
If cathode rays are negatively charged particles, as Thomson and other English scientists believed, then they must be deflected by an electric field. On this point, all physicists would agree. The fact that no one had been able to experimentally demonstrate this deflection meant either the charged-particle theory was in doubt or the right experiment had not yet been done. Thomson had a hunch that previous experiments were unsuccessful because the tube was not sufficiently evacuated. He repeated the unsuccessful work of Heinrich Hertz and showed that cathode rays were deflected between charged parallel plates when the vacuum inside the cathode ray tube was greatly improved.
- Conductivity of a gas does not explain cathode rays.
One of the significant arguments against the charged-particle theory of cathode rays dealt with the conductivity of the gas inside the tube. According to some critics of the charged-particle theory, electrical discharges and gas conductivity account for any deflection of the cathode ray beam. Thomson studied the conductivity of gases at various pressures, particularly at very low pressures, and was able to show that all of the evidence is consistent with view that negatively charged particles travel along the cathode rays.
- Cathode rays are independent of the nature of the gas.
By photographing the deflection of cathode rays in a modified bell jar, Thomson was able to show that the path of the rays is independent of the type of gas present in the bell jar when the magnetic field and potential difference between the terminals were kept constant.
After performing these experiments, Thomson was satisfied with the charged-particle theory of cathode rays: “I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter. The question next arises, What are these particles? Are they atoms, or molecules, or matter in a still finer state of subdivision?” (Thomson, 1897). Thomson realized that to answer this question, some quantitative evidence was required and he had all of the experimental tools necessary to collect this evidence.