Conceptual Physics (12th Edition)

Published by Addison-Wesley
ISBN 10: 0321909100
ISBN 13: 978-0-32190-910-7

Chapter 17 - Think and Explain: 67

Answer

Once the jar reaches the same temperature as that of the boiling water, no more heat enters it because it is in thermal equilibrium with its surroundings. (The water in the saucepan is in contact with the even hotter saucepan bottom, so heat keeps entering, which is why that water does boil.) This is how a "double boiler" works.
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