Heated cathodes and emission
Many tubes heat a cathode to emit electrons, then use fields between electrodes to organize their motion. Richardson studied the relationship between temperature and thermionic emission. Heating and electrode operation require external energy, making the device more than a passive glass container through which current happens to pass.
References: [2]
Diodes and directional operation
Fleming’s two-electrode valve provided an asymmetric response useful for radio detection. The Science Museum’s early lamp examples are associated with his 1904 investigations. Their manufacture and later experimental use occurred at different times, so an older object date should not simply become the invention date of the valve.
References: [1]
Additional electrodes and signal control
Additional control electrodes allow an input signal to regulate a larger output change, enabling triode amplification. The added output energy comes from the power supply rather than multiplying the input’s energy. De Forest’s audion and subsequent improvements connected detector development with the emergence of useful electronic amplifiers.
From valves to solid-state devices
Size, power requirements and reliability encouraged searches for alternatives in communication and electronic systems. Bell Labs’ solid-state research produced the point-contact transistor, transferring signal control into semiconductors. Tubes and transistors can serve related circuit functions while differing in their electronic medium, fabrication and operating requirements.
References: [3]