An immersion heater comprises a resistance wire (1) embedded by casting inside a protective ceramic housing (2). Prior to casting, the surface of resistance wire (1) is provided with a layer of waterglass which, as it expands and/or evaporates in a later thermal treatment, forms between resistance wire (1) and ceramic material (2) a necessary clearance (3) which prevents the ceramic material from breaking as a result of thermal expansion of said resistance wire. <IMAGE>
