## In general, substances expand slightly when heated, and shrink when cooled

This fact gave mankind its first tool for measuring temperature accurately; that is, the mercury thermometer. Thermometer comes from the Greek words therm, "heat" and metron, "measure"; it is an instrument "to measure heat".

The mercury thermometer was invented in 1714 by the German physicist Gabriel Daniel Fahrenheit who filled a hollow bulb with mercury and allowed it to expand, when heated, up a very fine enclosed and evacuated tube. The amount by which the mercury thread crept up the tube was proportional to the temperature. The glass also expanded but not nearly as much.

Fahrenheit placed the mercury-filled bulb in a mixture of equal parts of salt and snow at the melting point and marked the height of the mercury column as 0. He next let it warm to the temperature of the human body and marked the new height as 100.

#### The Celsius scale in an inverted form (0 degrees up to 100 degrees) was devised eight years later in 1750 by Martin Strömer, a pupil of Anders Celius, and this system has since been used in almost all scientific work.

By drawing a hundred equal divisions between the two marks, he invented the Fahrenheit scale. On this scale the melting point of pure ice is 32 degrees and that of the boiling of pure water is 212 degrees.

The word degree comes from the Latin de-, "down" and gradus, "step". In marking off small divisions from 100 to 0, the temperature goes "down steps".

In 1742, the Swedish astronomer Anders Celsius suggested that the temperature of melting ice be set at 100 degrees and that of boiling water be at 0 degrees; however, the 0 and 100 were reversed later.

The hundred-degree interval gave the new scale the name centigrade from the Latin centum, "hundred" and gradus, "step". It is the scale of "a hundred steps" from melting to boiling. It is also called the Celsius scale after the inventor.

The Fahrenheit scale is commonly in use among the general public in England and America, but it is the Celsius scale that is used in other civilized nations and among scientists everywhere, including those in England and America.

—From Words of Science by Isaac Asimov;
Houghton Mifflin Company; Boston, Massachusetts; 1959, page 47.

### The key words shown in bold above are defined in the section below.

evacuated
Emptied or removed the contents of.

Gabriel Daniel Fahrenheit filled a hollow bulb with mercury and allowed it to expand, when heated, up a very fine enclosed and evacuated tube.

interval
A space between two objects, points, or units.

The hundred-degree interval gave the new scale the name centigrade from the Latin centum, "hundred" and gradus, "step".

proportional
Properly related in size, degree, or other measurable characteristics

The amount by which the mercury thread crept up the tube was proportional to the temperature.

reversed
Turned backward or opposite in position, direction, or order.

In 1742, the Swedish astronomer Anders Celsius suggested that the temperature of melting ice be set at 100 degrees and that of boiling water be at 0 degrees; however, the 0 and 100 were reversed later.

thermometer
An instrument for measuring temperature, especially one having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol, that expands and rises in the tube as the temperature increases.

In general, substances expand slightly when heated, and shrink when cooled. This fact gave mankind its first tool for measuring temperature accurately; that is, the mercury thermometer. Thermometer comes from the Greek words therm, "heat" and metron, "measure"; so, it is an instrument "to measure heat".

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