A humidifier is an appliance that increases humidity (moisture) in museums, libraries, office’s or entire process / production area’s.

The most common portable humidifier, an "evaporative  humidifier", which consists of just a few basic parts: a reservoir, wick, rotating wheel, hygrostat and fan.

The wick is typically a foam pad mounted on a wheel which is rotated by a small motor, hot air is drawn into the humidifier at one end and is forced to leave through the sides of the rotating foam on the wheel. When the hygrostat calls for humidity, the motor is turned on causing the wheel to rotate slowly through the pan of water and preventing the foam pad from drying out.

This type of humidifier if used for long periods will require to be cleaned thoroughly regularly(monthly) using sterilizing tablets or fluid to prevent bacteria.  

Other popular humidifiers are;

Warm Mist Humidifier - which boils water, releasing steam and moisture into the air. Warm mist humidifiers are more commonly used in health related circumstances or alternatively vocal entertainment environments rather than cool mist types of humidifiers because steam is sterile and free from minerals. Warm mist humidifers are more expensive to use as they require more electricity to boil the water.

Ultrasonic Humidifier - A humidifier with vibrating diaphragm which operates at an ultrasonic frequency to generate water droplets which leave the humidifier in a cool mist vapor.