Quick questions on Cathode rays and Thomson's e/m: HSC Physics Module 8

A cathode ray tube is a sealed glass tube containing two electrodes and a low-pressure gas. A high voltage applied between the cathode (negative electrode) and anode (positive electrode) produces a stream of "cathode rays" travelling from cathode to anode. The rays make the residual gas glow and produce fluorescence on a screen at the far end of the tube.

Several observations pointed to particles:

J. J. Thomson designed an apparatus to measure properties of the cathode rays themselves. Inside the tube he added two horizontal parallel plates creating a uniform vertical electric field $\vec{E}$, and a pair of coils producing a horizontal magnetic field $\vec{B}$ perpendicular to both $\vec{E}$ and to the beam direction.

The measured $q/m$ for cathode rays is about 1800 times larger than for the lightest known ions (hydrogen). Two interpretations were possible: cathode-ray particles either have much larger charge or much smaller mass than hydrogen ions. The same ratio was obtained from any cathode metal (aluminium, platinum, iron), so the particles were a universal constituent. Charge measurements (later refined by Millikan) confirmed the small-mass interpretation.

If atoms are electrically neutral and contain negatively charged electrons, they must also contain positive charge. With no clearer picture available, Thomson proposed that atoms consist of a diffuse positively charged sphere with electrons embedded in it like plums in a pudding (or raisins in a bun). The model:

A beam of electrons enters a 4.0 cm region between parallel plates that produce a uniform field of $1.0 \times 10^4$ V/m. The electrons enter with speed $1.0 \times 10^7$ m/s. Find the vertical deflection while between the plates. ($e/m = 1.76 \times 10^{11}$ C/kg.)

Thomson measured the ratio. The charge $e$ on its own required Millikan's oil-drop experiment.

X-rays are high-frequency electromagnetic waves, produced when cathode rays strike the anode. Cathode rays themselves are electrons.

The plum-pudding model is wrong, but it was a reasonable model in 1897 given the data, and it correctly predicted neutrality with electrons inside the atom. It was overturned by direct experimental evidence (Geiger-Marsden), not by armchair reasoning.