Background
             In 1962 a new design of dimmer was publicly demonstrated in London for the first time. Known as the SCR or Thyristor, the dimmer used two solid-statesemiconductor devices to chop each half cycle of the mains sine wave such that the current to the lamp was continuously variable. The technique was not new, as it was the principle behind the so-called 'electronic' dimmers of the previous decade which used thyratron valves. Experimental dimmers had been made previously using thyristors, but the breakthrough came when the thyristor became commercially available in a range of power ratings suitable for lighting applications.
            
             Thyristors are devices which switch on very quickly when triggered. By using a thyristor in a lamp circuit as a gate to block alternating current from the mains supply to the lamp, and then opening the gate at a prede-termined point in the mains half-cycle, a proportion of the full mains supply only is passed to the lamp. This process of blocking and releasing current happens each half cycle of the mains supply (in other words, 100 times a second), and the rapidly switching mains supply is averaged out by the thermal response of the lamp's filament to appear a smooth and constant dimmed level to the eye. By reducing the 'blocking' period and thus extending the 'on' period, the lamp receives more current per cycle and the
            
             illumination appears brighter to our eyes. If the blocking period is longer the lamp is dimmed, and if the thyristor is not triggered, no current at all passes to the lamp.The thyristor dimmer was launched into a market which, for many years, had relied on resistor dimmers, auto transformers, magnetic dimmers or 'electronic' valve dimmers, and the benefits were immediately appreciated. Compared to the technologies it replaced, the thyristor dimmer was smaller, lighter, faster and reliable, more energy-efficient, was not affected by temperature changes, was more suited to remote control...