Experimental thermionic electron tubes
Fig. 44.11 shows a thermionic tube designed to show the deflection of moving electrons in an electric field. Its electron gun consists of a cylindrical cathode with a hot metal filament inside to provide a supply of electrons by thermionic emission. The electrons are ac
celerated towards a cylindrical anode and emerge as a thin flat beam from a horizontal slit at the end. Their path is rendered visible when they cross a vertical fluorescent screen.
At the top and bottom of the screen are two horizontal metal plates with connections to the outside of the tube. If a potential difference is applied to these plates, the electric field set up between them deflects the electron beam towards the positive plate. This suggests that the electrons carry a negative charge.
Fig. 44.12 shows the thermionic equivalent of the Perrin tube (page 512). Its electron gun is similar to that in the tube just described except that its anode has a small hole instead of a slit. This provides a thin beam of electrons which can be deflected
by a magnetic field at right angles to their path so that they bend round and enter a collecting cylinder connected to a gold-leaf electroscope. The charge collected on the electroscope can be shown to be negative in the way described for the original Perrin tube (page 512).
Maltese cross tubes with an electron gun similar to that of the Perrin tube are also available.