The Electric Drill
Hand-held electric drills are ubiquitous. They usually look like a pistol, with a trigger-like switch. They are also used for driving screws and are often provided with a hammer action which makes them capable of being used as masonry drills. In fact, screw guns or electric screwdrivers are generally suitably modified drills. These drills typically employ a universal motor with brushes. The original designs featured a single forward speed with a simple on-off action of the trigger; they could operate equally well on AC or DC power. Modern variable speed drills contain solid state phase control circuits that limit their use to AC power only. As a tradeoff, the electronics now give them variable speed, reversibility and torque control.