A ceiling fan is a fan mounted on the ceiling of a room or space, usually electrically powered, that uses hub-mounted rotating blades to circulate air. They cool people effectively by increasing air speed. Fans do not reduce air temperature or relative humidity, unlike air-conditioning equipment but create a cooling effect by helping to evaporate sweat and increase heat exchange via convection. Fans may add a small amount of heat to the room mainly due to waste heat from the motor, though partially due to friction. Fans use significantly less power than air conditioning as cooling air is thermodynamically expensive. In the winter a ceiling fan can also be used to bring warm air, which naturally rises, back down to occupants. This can affect both thermostat readings and occupants' comfort, thereby improving climate control energy efficiency. Many ceiling fan units also double as light fixtures, eliminating the need for separate overhead lights in a room. History Punkah style ceiling fans are based on the earliest form of a fan, which was first invented in India around 500 BC. These were cut from an Indian palmyra leaf which forms its rather large blade, moving slowly in a pendular manner. Originally operated manually by a cord and nowadays powered electrically using a belt-driven system, these punkahs move air by going to and fro. In comparison to a rotating fan, it creates a gentle breeze rather than an airflow. The first rotary ceiling fans appeared in the early 1860s and 1870s in the United States. At that time, they were not powered by any form of electric motor. Instead, a stream of running water was used, in conjunction with a turbine, to drive a system of belts which would turn the blades of two-blade fan units. These systems could accommodate several fan units, and so became popular in stores, restaurants, and offices. Some of these systems survive today, and can be seen in parts of the southern United States where they originally proved useful. The electrically powered ceiling fan was invented in 1882 by Philip Diehl. He had engineered the electric motor used in the first electrically powered Singer sewing machines, and in 1882 he adapted that motor for use in a ceiling-mounted fan. Each fan had its own self-contained motor unit, with no need for belt drive. Almost immediately he faced fierce competition due to the commercial success of the ceiling fan.