Anode voltage is the concept that makes vacuum tube technology work. It is what causes vacuum tubes to be able to perform their two main functions of amplification and rectification. Semiconductor technology is made possible because of this.
If a piece of metal is heated, it emits electrons, which carry a negative electrical charge. Charged particles are attracted to particles that carry an opposite charge and repel those with a similar charge. When a piece of metal emits electrons, it then carries a charge that is positive with respect to those electrons. This causes the electrons to return to the metal, because they are attracted to its opposite charge. This results in a cloud of electrons around the metal, which is known as a space charge.
A vacuum tube takes advantage of this effect, and it contains a metal plate called a cathode, which is heated. A second metal plate is added, called an anode, and when a positive charge is applied to the anode, it attracts the electrons emitted by the cathode, and current flows through the vacuum tube. This applied charge is called the anode voltage, and when it is positive, it causes current to flow faster and is called forward bias. When the anode voltage is negative, it opposes current flow and is called reverse bias. This last property, which allows current to flow in only one direction through the vacuum tube, is called rectification.
A tube with two plates is called a diode. Adding a third plate in the middle produces a triode and allows the tube to amplify an electrical signal. This third plate is called a control grid and is a mesh of wires that the electrons pass through on their way from the cathode to the anode. The grid is closer to the cathode, so any voltage applied to the grid magnifies the effects of creating or opposing current flow. So, small changes to the grid voltage create larger changes in the current flow across the tube.
A problem with this design is that, as the current is amplified across the tube, it causes changes in the anode voltage. This in turn affects the anode current and keeps the tube from amplifying at its full potential. A fourth element, called the screen grid, was added to minimize this effect.
The screen grid created a new problem, however — when the anode voltage would go lower than the screen grid voltage, electrons would flow from the anode to the screen grid. This caused a distorted output signal. The solution was to add another grid, called a suppressor grid. It is biased at the same voltage as the cathode and repels any emissions from the anode. This kind of five-element vacuum tube is called a pentode.
Transistors are three-element semiconductors that operate in a similar fashion as a triode, although the actual names "anode" and "cathode" are used only in certain types of transistor. The programmable unijunction transistor is one such example. Semiconductors serve the same functions of amplification and rectification, but their ability to do so in a much smaller package and with lower power requirements is what enables modern electronic and computer technologies.