One of the most practical inventions of all time was the light bulb. The light bulb contains a filament made of tungsten wire which is twisted into a secure coil. This part of the light bulb allows the passage of electricity which in turn heats the coil to the point of being white hot. The filament has the capability of reaching temperatures of up to 2500 C (4500 F). Tungsten was the adopted material as it has an extremely high melting point. An inert gas is utilised in the surrounding bulb to prevent the combining of the metal with the oxygen in the air. This is necessary otherwise it would result in the filament burning out. The gas is usually something like argon, which is generally placed in the bulb under reduced pressure.
In the modern type of light bulb, each filament coil usually consists of even smaller coils. This means that the filament will be lengthy, but very thin. With this sort of arrangement the light output is dramatically increased.
Within a domestic property the lighting circuit is almost as extensive as the power circuit. Lights are wired in a looped fashion and control of individual lights is via a switch or, more often these days, by an electronic dimmer control.
The thyristor dimmer was developed in the early 1950's. It is an extension of the transistor, possessing four layers of crystal, rather than the transistor's three. As well as having the usual semiconductors to enable current to pass through in just one direction, it also has the additional feature of not letting any current pass unless a separate `biasing' voltage has been applied between two of the crystal layers.
To utilise this feature, the dimmer must contain two thyristors. These thyristors are wired in such a way that a current can be conducted in opposite directions. Within any half cycle of the alternating current, one of the thyristors will be passing the current and the other will not. While in the other half of the cycle, the roles are reversed. The position where the bias allows current to travel through the thyristor, can be varied from fully on to zero. The point at which the current waveform is chopped off determines the amount of power reaching the lamp and hence its brightness.
