"High tech", circa 1943.
Before those fancy-schmancy developments of "quantum mechanics" and "semiconductor technologies," there was the vacuum tube, poised to take over the world.
The development of the vacuum tube element is mired in concurrent discoveries, lawsuits, patent disputes, and all other manner of muddling he-did-she-did so it is hard to give an affirmative history of the device. For example, whose name do you recognize: Thomas Edison, or Frederick Gutherie? Because Gutherie gave an account of the mechanism of vacuum tube valves over a decade before Edison did, yet today that mechanism's colloquial name is the "Edison Effect."
The most basic of vacuum tube devices consists of a set of metal plates--one anode, one cathode attached to a heating filament--seperated from each other and enclosed in an evacuated glass bulb. Or, instead of a vacuum, an electrically inert gas was sometimes used. To work the tube, a circuit would be connected across the cathode and the anode, along with a source of electric charge, such as a battery. Then a second circuit, consisting only of another battery, would be attached to the terminals of the filament across the cathode. Once a positive potential was induced at the anode, a curious thing would happen: the filament would heat up rapidly and great amounts of thermal energy give the electrons in the filament the energy they need to reach the conduction band; the charge differences at the anode and the cathode would pull the electon away from the filament and onto the anode plate. This use of mostly thermal energy to provide charge was called "thermionic emission" (or, the "Edison Effect," if you prefer.) The general model equation for the emitted electrons was:
J = AGT2e-W/kTWhere J is the emission current density, T is the wire temperature in Kelvins (yes, it's in there twice) k is the Boltzmanm constant, W is the workfunction of the material, and AG is an odd constant largely determined experimentally from metal to metal.
Should we leave our vacuum tube at this simple two-plate configuration, then we have a diode; if the anode become negatively charged, as the negative cycle of an AC signal, it will repel emitted electrons and barely any charge flows in the circuit at all. However, some folks, such as American inventor Lee de Forest, realized that one could have a third plate; or, rather, some kind of mesh wall, between the cathode and the anode. Depending on the electric potential of the mesh, electron flow could either be repelled (mesh has negative potential) or hastened (positive potential). Thus there would be a control element in the vacuum tube; the most profitable consequence of this was that a signal, such as a radio signal, could be picked up by an antenna connected to the mesh; as the signal altered the potential of the mesh, the current through the tube responded; thus the weak signal could shape the power output of a battery to mirror itself. The signal would be amplified from its weaker version; this amplification development was almost immediately useful to a world that was beginning to reap the benefits of long-distance communication from radio, television, and telegraphs; not the least of which was that signals, dissipated and weak from long distance travel, could be reproduced at a much higher strength.
Perhaps the scientists of the time surveyed their achievements with the vacuum tube and decided they could not have too much of a good thing; or at least one could be forgiven for thinking that if she or he looked at the truly awe-inspiring line of vacuum-tube devices produced before the technology was rendered obsolete by semiconductor transistors. This line-up includes such truly ancient, decrepit technology as the klystron (a terribly powerful high-frequency signal amplifier), the eidophor (a device used to project film images onto theatre-sized screens), the series of multi-electrode devices (the tetrode, the pentrode, up to the nonode, with increasing numbers of meshes designed to improve the gain or reduce stray capacitance of the triode), and the Williams-Kilburn tube (the first Random Access Memory device, storing charge as information to be accessed by a secondary mechanical device).