The unit consists of a three concentric pipes of a very small thickness as compared to diameter. They are arranged concentrically over two supports and closed with the help of two disces. The annual in between the cylinders are filled completely by two different insulating materials. Thermocouples at proper position are used to measure the temperature. Inside the inner pipe a nicrome wire heater is placed. Heat input to the heater is given through a variac and measured by a voltmeter and ammeter.