The heater is fitted inside a glass tube (12) closed at the bottom and dipping into the water. The heating element is a PTC type (15) which is held centrally inside the tube by two contact strips (16,17) which are shaped to provide mechanical support from each side of the heater. The inside wall of the glass tube is coated with an electrical insulation layer which also has good thermal dispersing properties to assist in transmitting heat out of the tube. The contact strips are spaced from the inside wall of the tube by spacing inserts or by a sealing compound into which the outer surfaces of the contact strips are embedded. A typical coating for the inside of the tube is a silicate.
