Bobbin heater is constructed using several refractory ceramic blocks assembled together to the required length. The Nichrome wire heating element is inserted into the ceramic blocks, with a terminal block at one end.
This bobbin assembly is then inserted into a pre-assembled special protection tube (Stainless Steel, Titanium etc), when used in liquid immersion and gas application. However, bobbin heaters can also be used in direct air-heating application without the protection tube.