Although the Braun tube contained all the essential elements of the modern CRT, these existed in a fairly primitive form and it lacked many of the refinements which are now considered as basic components.
In the years following Braun’s work, one of his assistants, Jonathan Zenneck, added a second aperture to the neck of Braun’s design to improve beam focus and also included circuitry which did away with the rotating mirror used in Braun’s early design, allowing waveforms to be displayed directly on the tube’s screen. An American researcher, Professor Harris Ryan, also demonstrated an oscillograph which had magnetic deflection coils mounted at right angles to each other, allowing him to display alternating current waveforms.
The work of Zenneck and Ryan, though important, produced what were, essentially, refinements to Braun’s original design and are not regarded as particularly significant advances. It wasn’t until the period between 1903 and 1905 that the first of these were to appear. It was during this time that the German physicist Arthur Wehnelt was responsible for two major developments in CRT technology, both involving the cathode itself.
Wehnelt was the first person to heat the cathode indirectly, rather than by applying a voltage to it directly. He also discovered that coating the cathode with oxides caused it to produce a much higher quantity of electrons at greatly reduced input voltages. Previous tubes had required currents of around 100,000 volts to operate, while Wehnelt’s tube needed just 100 volts. He then discovered that putting a cylinder with a small aperture at one end around the cathode, and applying a negative voltage to the cylinder, allowed the number of electrons entering the CRT to be controlled. The negative current repelled the electrons, sending them back towards the cathode, until, at sufficiently high voltages, the flow of electrons could be cut off entirely. This cylinder is present in all modern CRTs and is known as the control grid.
Source: The Cathode-Ray Tube, Peter A Keller