General Cathode Ray Tube Results

There were a number of results gathered over the years by cathode ray tube researchers.

1) If an object is placed in the path of the cathode ray, a shadow of the object is cast on the glowing tube wall at the end. This showed that the cathode rays traveled in straight lines.

2) The cathode ray can push a small paddle wheel up an incline, against the force of gravity. This showed that the cathode ray carried energy and could do work.

3) The cathode ray is deflected from a straight line path by a magnetic field, suggesting that the two were related in some way. The discovery of this effect in 1855 predates by some ten years the unification of electricity and magnetism by James Clerk Maxwell.

4) Although there was some speculation that the cathode rays were negatively charged, it is not shown to be true by experiment until 1895, just two years before Thomson announces the electron.

5) J.J. Thomson is the first individual to succeed in deflecting the cathode ray with an electrical field. He did so in 1897. The cathode rays bend toward the positive pole, confirming that cathode rays is negatively charged.

What is the e/m ratio?

e/m ratio stands for charge-to-mass ratio of the electron.

The modern value for the charge on the electron (to four significant places) is 1.602 x 10-19 coulombs and the electrons mass is 9.109 x 10-31 kilograms.

Therefore, the modern value for the e/m ratio is 1.759 x 1011 C/kg. Usually, grams are used rather than kilograms giving a numerical value of 1.759 x 108. Often, books round off the 1.759 portion to 1.76.

However, there is one problem. Many textbooks and articles use the m/e ratio, that is the mass-to-charge ratio. Reversing the above figures and using grams rather than kilograms gives a value of 5.686 x 10-9 g/C.

The e/m ratio is important because that is as far as Thomson could get with his cathode ray tubes. Knowledge of the value of 'e' or of 'm' would be needed to get to the other once you knew e/m, which Thomson did know.

Elsewhere you will find discussion of how the value for 'e,' the charge on the electron was determined. (May 1996: This will be made a link when that section is written.)

Thomson's Calculations

For a fuller discussion of the below, please see "The Discovery of Subatomic Particles" by Steven Weinberg. It was published in 1983 by W.H. Freeman and Company. The ISBN is 0-7167-1488-4.

Thomson had developed formulas based on the deflection of the cathode ray by the electric field and by the magnetic field. Just below are GIFs of each formula.

By carrying out the experiments and measuring the proper values, he could calculate what the charge-to-mass (e/m) ratio was for the cathode ray. However, the two formulas above could not give either the charge or the mass by itself. A different experiment would have to be carried out.

Notice that both equations depend on knowing the velocity of the cathode ray. (Several years previous to 1897, Thomson had measured the cathode rays' velocity, but he grew to distrust the results.) However, both equations can be used as a ratio if the deflections by the two fields are made to be equal. Then, the mass, charge and both lengths cancel, leaving us with:

Since Thomson knew both the electrical and magnetic field strengths as well as the amount of deflection, he could easily solve for the velocity. That value was then inserted along with the other values into the deflection formulas shown above. An easy calculation gave the charge-to-mass ratio.