Vacuum bottle is an insulating container that keeps liquids hot or cold for many hours. It is also called a Dewar flask or a Thermos, which is the trademark for one particular brand. Vacuum bottles vary widely in size, ranging in capacity from 2 ounces (59 milliliters) to 15 gallons (57 liters). They are commonly used to carry coffee, juice, milk, or soup. Some types of vacuum bottles are used in industrial, medical, and scientific work to store chemicals and drugs, to transport tissues and organs, and to preserve blood plasma.
James Dewar, a Scottish chemist, invented the vacuum bottle in 1892. He developed it for storing liquefied gases. Although his flask was designed to prevent the entry of heat from outside the container, it worked equally well in keeping liquids hot by reducing the loss of heat from the inside.
The modern vacuum bottle has the same basic design as Dewar’s flask. It blocks the three processes through which heat is transferred—conduction, convection, and radiation (see Heat (How heat travels) ). A typical vacuum bottle has an inner container that consists of two glass or stainless steel bottles, one within the other. Glass and stainless steel do not transmit heat well, and so they reduce heat transfer by conduction. The inner and outer bottles are sealed together at their lips. Most of the air between the bottles is removed to create a partial vacuum. This vacuum hinders heat transfer by convection because it has few air molecules to carry heat between the bottles. The facing surfaces of glass bottles are coated with a silver solution. They act like mirrors and reflect much of the heat coming from inside or outside the container. In doing so, they prevent heat transfer by radiation. In stainless steel vacuum bottles, a layer of copper foil between the inner and outer bottles serves the same purpose.
Other features of vacuum bottles help minimize both the loss or entry of heat. Most vacuum bottles have a small mouth, which reduces heat exchange. The bottles are closed with a stopper made of cork, plastic, or some other material that conducts heat poorly. The inner container of a vacuum bottle is encased in metal or plastic. A rubber collar around the mouth holds the inner container in place, and a rubber cushion at the base serves as a shock absorber.
