The inner container, filled with the substance to be heated, fits inside the outer container filled with the working fluid (often water, but alternatively steam or oil). The outer container is heated at or below the base, causing the temperature of the working fluid to rise and thus transferring heat to the inner container.
The inner container, holding the substance to be heated, sits inside the outer container filled with a working fluid (typically water, but sometimes steam or oil). The outer container is heated at or below its base, raising the temperature of the working fluid and subsequently transferring heat to the inner container.