Wednesday, July 21, 2010

Scales

In order for thermometers to give us a temperature reading, they must be marked with a standard temperature scale. How is this standard temperature scale derived?

Step 1: Choose an appropriate substance

Choosing a suitable thermometric substance and its physical property, e.g. a column of mercury. The volume of mercury is able to vary continously with changes in temperature.

Step 2: Choose two fixed points

Choose two standard degrees of hotness or coldness, which are easily obtainable and reproducible. We shall call them fixed points. it is common to choose the temperatures of pure melting ice and steam at one atmospheric pressure as fixed points. These two fixed points are called the lower and upper fixed points respectively. Record the values of the physical property of the substances at these two fixed points.

Step 3: Set up the scale

Divide the temperature range between the two fixed points into a fixed number of equal parts to obtain a scale. For example, on the Centigade (Celsius) scale, there are 100 equal parts or degrees between the lower and upper fixed points. by doing this, we assume that the physical property varies continously with temperature. This means that when temperature changes, the physical property must change evenly and continously.
The Centrigrade Scale

The centrigrade scale, or Celsius scale, is a common temperature scale. it is based on a simple experiment procedures that determine the two fixed points, called the ice point and steam point.

1. ice point (lower fixed point): This is the temperature of pure melting ice at one atmospheric pressure. It is assigned a value of 0 degrees celsius.
2. steam point (upper fixed point): This is the temperature of steam from water boiling at one atmospheric pressure. It is assigned a value of 100 degrees celsius.

To determine the ice point,

- Immerse the bulb at and the lower part of the stem of the thermometer into a funnel containing pure melting ice
-To ensure good conduct between the bulb and the ice, crushed ice should be used. The mercury level in the stem should be just above the surface of the ice.
- When the mercury level in the stem remains steady after some time, a mark is made at the point of the stem. This mark corresponds to the ice point and is assigned a value of 0 degrees celsius.


To determine the steam point,
- insert the thermometer into the apparatus. The bulb of the thermometer should be above the boiling water. The stem of the thermometer should protude above the top of the apparatus.
- the manometer is included to ensure that the pressure inside the apparatus is the same as the atmospheric pressure outside
- when the mercury level in the stem remains steady after some time, a mark is made at that point of the stem. This mark corresponds to that steam point and is assigned a value of 100 degrees celsius.


Note: FOR THE CENTRIGRADE SCALE, THE INTERVAL BETWEEN THE ICE POINT AND THE STEAM POINT IS DIVIDED BY 100 EQUAL PARTS. EACH MARK ON THE THERMOMETER IS A MEASURE OF 1 DEGREES CELSIUS.


Calculating temperature on a Centrigrade Scale

In a mercury thermometer the physical peroperty that changes continously with temperature is the volume of a fixed mass of mercury. We can measure th echanges in a volume of mercury, by measuringh the change of th elength of a mercury column or thread. This is only possible if the cross-sectional area of the column or thread in a thermometer is uniform. If we know the values l0 (height of mercury level at 0degrees celsius) and l100 (height of mercury level at 100 degrees celsius) of a mercury in glass thermometer, we can easily determine the temperature of any unknown body with that particular thermometer.
We must first find the height of the mercury level l0 when the thermometer is placed in temperature. Since the volume of mercury varies continously with temperature, we know that the height of mercury level is directly proportional to the temperature.
We can then find the value of unknown using the equation:
Unknown Degrees Celsius = l uknown- l 0/ l100 - l0 x 100

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