P*t$&"; Cathode Ray"" ^ Oscilloscope c
By W. E
THEORY OF THE TUBE
IN view of the fact that many prospective users of cathode ray oscilloscopes are still unfamiliar with the construction and operation of the cathode ray tube itself, it is proposed to deal with this subject at the outset.
There is no doubt that a full understanding of the design of tubes and their associated circuits is of utmost importance to the user of an oscilloscope, enabling him to realise fully the capabilities and limitations of the instrument.
In considering the design and operation of the cathode ray tube, we can regard it as a development of the ordinary thermionic diode. In Fig. 1 (a) we have an evacuated container with a heated filament or cathode emitting a stream of electrons. The electron is, of course, a minute negative charge of electricity. Normally, a heated filament only emits a few electrons, which pass back to it in the absence of anything tending to remove them. If we insert another electrode, and maintain it at a ''positive potential with respect to the cathode (filament), then the negative electrons, obeying the ordinary laws of attraction and repulsion,
MILLER, M.A. (Cantab.), M.Brit.l.R.E. jTßltf r of "The Wireless d VMW^UVh ' „Q HttO-TEXHM«**CHA31 $0f7 ode, an"^/
Fig. 1—Three stages in the development of a simple cathode ray tube.
flow round the external circuit to the cathode, forming the anode current. The number of electrons reaching the anode depends, up to a point, on the positive potential to which the" anode is raised, and in addition, the higher the potential, the greater the acceleration of the electrons, and the greater their speed on reaching the anode. So far we have the ordinary diode valve.
Now consider Fig. where we have the same diode, excepflHh| the anode is pierced with a small ^J^in its centre. Again we have the electrons reaching the anode, but this time some of them will pass through the hole in the anode. Of these, the slower ones will only just pass through, and will be attracted back to the anode. Those with the greatest speed, withstanding the attraction of the anode, will travel in a straight beam to the wall of the evacuated bulb. Between these two extremes will be electrons of intermediate speeds which will be bent from the straight team to a greater or lesser extent, partly by the attraction of the anode, and partly by mutual repulsion. Their paths are indicated roughly by the dotted lines, and it will be seen that the