Mike Magut, age 14, of Trumbull, Conn., for his question:

WHAT EXACTLY IS THE KELVIN SCALE?

A comfortable room temperature may be stated as 68 degrees F, 20 degrees C or 293 degrees K. The startlingly different figures were not invented to confuse us, at least not on purpose. They are based on three different scales used to measure temperature. The K scale is the Kelvin scale, also called the A or absolute zero scale.

Before the 1700s, there was no precise way to measure temperature. Whether Monday was warmer than Tuesday was a matter of personal opinion based on guesswork. Then Gabriel Fahrenheit invented a workable thermometer, plus a temperature scale with equal degrees.

For no known reason, the temperature of freezing water was set at 32 degrees and the temperature of boiling water at 212 degrees. On the famous old Fahrenheit scale, the difference between the freezing and boiling water came to 180 degrees.

A century or so later, Anders Celsius of Sweden simplified the older scale. He set the freezing point of water at 0 degrees and the boiling point at 100 degrees, separating these key temperatures by a neat 100 degrees. For a time, this was called the Centrigrade scale, but in 1949 it was renamed the Celsius scale to honor its inventor.

So matters stood until the 19th century, when William Thomson of Belfast delved into an assortment of fascinating topics, including zero temperatures, math, compasses, depth sounding, water pressure and tide predictions. In 1886, after helping to lay the famous transatlantic cable, he was knighted and became Lord Kelvin.

In his work, this brilliant busy man needed a scale to measure temperatures down to absolute zero  which is as cold as anything can get. He used degrees equal to Celsius degrees  but on the Kelvin scale, 0 degrees is 273 degrees colder than 0 degrees C; 0 degrees K is close to absolute zero, and sometimes the K scale is called the A scale.

On the C and F scales, temperatures below the freezing point of water have a minus sign. minus sign. The K scale has no minus signs because nothing can get colder than its zero. On this scale the freezing point of water is 273 degrees  which is the same temperature as 0 degrees C and 32 degrees F. The boiling point of water is 100 degrees C and 373 degrees Kelvin, or Absolute.