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Measuring Energy

In Chapter Four of the "Meet Brother Gregory" story, Brother Matthew invents a new unit for the measurement of heat; the horko, which is the Czech word for 'heat'. He was too late. The English already had a unit they called the 'British Thermal Unit', or BTU based on their own 'foot - pound' units of measurement. But in Europe, in all those countries overrun by the French Emperor Napoleon Bonaparte, a new system of measurement was gaining a foothold, particularly in science. This was the 'metric' system, which was based on a unit of measurement one forty-millionth of the earth's circumference.

The metric system, which has standards for measuring distance, volume, heat, and temperature, but, curiously not time, is built up in units of ten. Latin prefixes are used for fractions and Greek prefixes are used for multiples of ten, such that ten meters becomes one dekameter (a Greek prefix), whereas a tenth of a meter is a decimeter (a Latin prefix).

To measure heat transactions, the metric physicists needed a metric unit. They chose a unit based on the amount of heat (or energy) needed to raise the temperature of one gram of water from 14.5 to 15.5 degrees centigrade. Technically this standard should be called the gram-calorie but most people just call it a calorie, a word derived from the Latin word for 'heat'.

Apart from the obvious use of metric units (the gram and the degree Celsius) this definition of a standard 'calorie' contains two other parts that need explanation; why water and why 14.5 to 15.5 degrees?

Experience and experiments quickly showed that all substances need different amounts of heat to raise their temperatures by one degree Celsius. Stand by a blacksmith as he or she pounds on a red hot bar of iron, the sparks will fly. Those shards of metal are very, very hot, (their temperature is high). But if one of them lands you your bare skin, apart from the shock, it is unlikely that you will be burned. On the other hand, let a drop of hot water fall on your skin at a temperature much below that of the hot iron, and you will probably be scalded. Despite the difference in temperature, water is holding much more heat than the iron.

It takes one calorie of heat to raise the temperature of one gram of water by one degree, but the same calorie of heat will raise the temperature of one gram of aluminum 4.5 degrees, one gram of iron by 9 degrees, one gram of copper by 11 degrees, one gram of silver 18 degrees and one gram of lead by 33 degrees. This clearly shows the distinction between heat and temperature.

In setting the 'calorie standard' therefore it is important to define the substance to which the heat is being added. It also gives us another property of substances called the specific heat, i.e. the amount of heat (or energy) needed to raise the temperature of one gram of that substance by one degree Celsius.

Even water is not consistent in this regard. The amount of heat (or energy) needed to raise the temperature of water by one degree, varies according to where on the Celsius temperature scale you start. One gram of water at 1 degree Celsius needs more energy to raise its temperature to 2 degrees Celsius than it takes to raise the temperature of one gram of water at 92 degrees Celsius to 93 degrees Celsius. The standard unit - the calorie - must, therefore, also include in its definition, the range of temperature used.

Now the definition is complete.

Science@a Distance
© 2001, Professor John Blamire