To keep fermenter at the correct temperature.
This item provides the most direct contact heat.
 
A fully waterproof thermostatically controlled glass tubed immersion heater, It is suitable for glass and plastic containers and volumes between 1and 5 gallons. The support bung is designed to fit standard demijohn neck holes of 1 1/8" diameter and the unit is rated at a powerful 75 watts. It is pre-set at the factory to maintain a temperature of between 22 -24C, but this can be varied using an external adjustment. The heater needs to immersed to a minimum depth of 19cm, above a level marked on the tube.