VACUUM FLASK SECRET

Wairarapa Times-Age, 31 July 1939, Page 2

VACUUM FLASK SECRET

CONSTRUCTION OF BOTTLE.

CONSERVING THE HEAT.

The majority of us have seen the inside of a vacuum flask, that handy bottle-shaped vessel which will keep hot things hot and cold things cold for a long lime. Have you ever wondered why the walls of the glass arc silvered like a mirror, and how the bottle manages to keep the heat or cold so well?

A part of the secret of the vacuum bottle is indicated by its name. Air that is free to move is an excellent conductor of heat. A vacuum is the absence of air. and, consequently, the absence of what would otherwise conduct or lead the heat away from the contents of the bottle. The vacuum bottle consists. of a double-wall glass container inside a metal or composition container which forms the outside. The air is pumped out from the space between the double glass walls, with the result that the contents of the bottle are surrounded by a protecting layer of vacuum.

Insulating material—that is material through which heat does not pass readily—is used to fill the space between the glass bottle and the outer casing as a further protection against the escape of heat. These two precautions would be sufficient to guard against the loss of heat through conduction. There is. however, another way heat may travel, even through a vacuum —that is by radiation. It is radiant heat which reaches us from the sun. for the sun’s warmth bridges a gap of millions of miles of frigid, airless space. We know it is not carried by conduction, for the space between the earth and the sun is not heated, whereas conduction involves the heating of the substance which conducts the heat.

To prevent the loss of heat from the bottle by radiation, the double glass walls are silvered, like a mirror. Just as a mirror prevents the passing of light through it and reflects the light back on itself, the silvered walls of the vacuum bottle reflect back the radiant energy or heat, preventing its escape.

This triple guard against the passage of heat works both ways. It will keep hot liquids hot by preventing the heat from escaping, and it will also keep cold liquids cold by preventing outside heat from reaching the liquid to warm it.