The adjustable weight at the lower end of a pendulum. The lenticular-shaped pendulum bob, often in cast iron for turret clocks, is probably the most common; it has the advantage of offering the lowest air resistance and therefore requires less energy from the train to maintain it in oscillation. The cylindrical pendulum bob found in many high-quality regulators and electrical impulse master clocks gives excellent results. The cylindrical shape has more air resistance, but this may be used to counteract changes in rate due to circular error. By the 19th century many turret-clock pendulums had become cast-iron cylinders with domed tops to prevent dust settling. Weights of several hundred pounds were not uncommon, as this was found to overcome the action of wind and weather on the hands. The bob of the Great Clock of Westminster ('Big Ben'), for example, weighs 700 lbs. Some 18th-century and earlier clocks have the major axis of a bun-shaped bob at right angles to the pendulum rod.