Early turret clocks used wrought-iron pendulum rods, which were forged bars of rectangular section. The rating nuts were wing nuts formed from a long thin rod wound into spirals. Wooden rods were in frequent use, as wood has a very small coefficient of thermal expansion and, provided it is properly sealed, suffers little from humidity. Pitch pine and hickory were often used, in more recent years mahogany. Compensated pendulum rods on turret clocks were introduced in the 19th century. They are nearly always a combination of zinc and steel and are tubular in construction. A steel rod hangs from the suspension; at its lower end a collar supports a zinc tube running upwards the appropriate length. From the top of this tube a further steel tube descends and, fitting snugly over the bottom collar, carries the pendulum bob. This system was first devised by Edward Troughton. Although turret-clock pendulums beating seconds are the most common, the location of turret clocks provides scope for longer rods. Rods beating 1 1/4 or 1 1/2 seconds are common and at one time rods of astonishing length were introduced. Many beat 2 seconds and thus have a theoretical length of over 13 ft. , while there are a few recorded beating 1 1/4, 1 1/2 and even 3 seconds. It was thought that these long pendulums exercised 'dominion over the clock', evening out irregularities caused by friction and other imperfections in the train. Recent studies have shown that this view was erroneous, for the performance of a clock depends on the drain of energy in the pendulum due to windage and friction in comparison with the total energy in the system.