The Crookes Tube and Related Experiments

By the middle of the 1800's the combined contributions of Franklin, Galvani, Volta, Faraday, and, of course, many others, had placed the science of electricity on a solid footing. Oersted in 1819 had noticed that a current in a wire would cause a compass needle to deflect, thus laying the groundwork for the unification of electricity and magnetism by Ampere, Maxwell and others.

Sparks had been a source of interest in electrical studies for some time and several workers had measured the voltages necessary to create a spark in air. In 1855, Geissler invented the mercury vacuum pump thus making it possible to produce good quality vacuums in laboratory equipment. Crookes (1832 - 1919) built a simple apparatus in about 1875 to study the effect of gas pressure on the voltage necessary to cause a spark. A biography of Crookes can be found by clicking here. The apparatus is shown below.

Crookes was able to examine the voltage necessary to get a spark (which could easily be detected by the use of an amp meter in the system). He found that the voltage decreased as the pressure decreased, reaching some minimum when the tube was fully evacuated. Later workers discovered that it was actually possible to see the beam by slightly modifying Crookes design. As shown below.

In this variation, the anode is shaped as a ring so that some of the electrons traveling from the cathode pass through the ring and strike the screen. This design was particularly effective as it provided an easy, visual way to experiment with the cathode rays.

Closely related to these experiments was the discovery of "canal rays" using the apparatus below.

This apparatus is the forerunner of modern mass spectrometry.

The astute student will realize that one can create this apparatus by simply reversing the leads of the previous experiment. If there was a small amount of residual gas in the tube, electrons moving from the cathode to the anode would strike gas molecules creating positively charged ions. These ions would then stream toward the cathode, pass through the ring, and strike the screen.

In 1987, Joseph John (always called J.J.) Thomson (1856 - 1940) modified the Crookes tube to include both charged plates and a magnet. The plates and magnet were at right angles to one another. The apparatus is illustrated below.

By measuring the deflection of the beam of electrons Thomson was able to determine the charge to mass ratio of the electron. Thomson's Nobel address has the occasional equation, but it is very readable by a modern student. You may notice that he still refers to electrons as particles of negative electricity (or corpuscles) even though the name electrons had been proposed by G. Johnstone Stoney in 1894. Further, toward the end of his address he describes having measured both hydrogen and the electron and has determined that the electron is 1/1700 the mass of the hydrogen.

We must now leave (temporarily) this thread of science and visit briefly with Prof. Wilhelm Roentgen in his laboratory containing a Crookes tube on steroids.