Thomson's Experiment

#### Thomson's Experiment

• One of the Scientists that worked with Cathode Ray Tubes was an experimentalist named J. J. Thomson.
• He was the one who discovered that the Cathode Rays were deflected by electric fields.
• After that experiment, he worked on another experiment with Cathode Ray Tubes.
• The aim of the experiment was to measure the Charge to Mass ratio of the Cathode Rays.
• It was set up as follows:
• A CRT was built with two parallel electric plates in one axis
• A magnetic field was produced by current carrying coils in the other axis (perpendicular to the electric plates).
• The Cathode Rays were passed through two thin collimator slits to produce a thin beam rather than a Ray.
• This was aimed at a fluorescent screen to produce a dot.
• The experiment was as follows:
• Stage 1
• The electron gun was initially fired without any electrical input (no E or B).
• This position of the dot was noted on the screen.
• Then both fields were turned on.
• The strengths of the fields were then altered so that it passed through undeflected.
• This means that the dot was at its initial position without the fields on.
• This was to equate the Magnetic force and the Electrical force.
(1)
$$F_{magnetic} = F_{electric}$$
(2)
$$qvB = qE$$
(3)
\begin{align} v = \frac{E}{B} \end{align}
• Stage 2
• The Electric field was then turned off.
• This meant that there was now a deflection, but it was only due to the magnetic field.
• Thompson then measured the radius of this deflection.
(4)
$$F_{magnetic} = F_{centripetal}$$
(5)
\begin{align} qvB = \frac{mv^2}{r} \end{align}
(6)
\begin{align} \frac{q}{m} = \frac{v}{Br} \end{align}

Substitute (3) in

(7)
\begin{align} \frac{q}{m} = \frac{E}{B^2r} \end{align}

And so the charge to mass ratio could be found.

#### Implications

• Thomson found that the ratio of the Cathode Ray particles was 1800 times greater than that of a Hydrogen ion
• He then decided that its mass was 1800 times smaller rather than having a charge 1800 times greater through a series of other experiments.
• Which meant that it was a sub-atomic particle
• Since the ray was the same regardless of the materials used for the Cathode, he proved that these Cathode Ray particles were a constituent of every atom.
• Later, these would be known as electrons.
page revision: 4, last edited: 28 Oct 2011 12:07