Physio Paraffin Wax Bath - Bain Marie Concept is consists of wide, cuboids, metal container made of three or four basic parts: Upper lid with insulated handle, an outer container that holds puff insulation inside, followed by an inner (or upper), smaller container that fits inside the outer one and which holds the water (around 30 Ltrs.) to be heated, and finally a small tank fixed underneath. 
Under the outer container of the bain-marie (or built into its base) is a heat source. The inner container is immersed with water i.e. working fluid. The smaller container, filled with the substance (paraffin wax) to be heated, fits inside the outer container filled with the working fluid (usually water), and the whole is heated at, or below, the base, causing the temperature of the materials in both containers to rise as needed.