S5-INVENTION OF THERMOMETER

 

By PRAGYANSHARMA POLAVARAPU (Vrs.Somanchi)

Scientists created  thermometer to measure “level of heat” in degrees of temperature to acceptable accuracy. Invention of the thermometer is a very great event in physics as it helped in finding temperatures of substances and for  comparing  temperatures of substances .

In a long solid glass rod (of a few millimeters radius and roughly a feet long) a very narrow , perfectly straight and very smooth capillary is  created  by machine process along its entire length .The glass tube with the central capillary path  is fused to a tiny pea-size mercury reservoir at bottom in such a way that there is continuous path from the mercury bulb into the capillary tube for mercury to move during heating or cooling of the bulb. The top of the capillary tube is sealed.

 Mercury is a non-sticking “liquid metal” and conducts  heat very quickly and proved very suited  for use in  a thermometer. Manufacture of mercury thermometer is quite a difficult  technical process. The thermometer has to be  calibrated i.e. checked for accurate results and printed(etched) with a measuring scale on  the long capillary  before it is made ready for use .

 Even a little heat, even the warmth in the palm of hand, is sufficient to make the hair- thin  bright mercury line (mercury column) to expand  into the upper capillary portion. Since mercury does not stick to  the sides of the capillary , even small quantities of heat can make the  mercury column in the capillary rise  quickly as a  bright  shining line. When  the top  end of the long capillary is sealed,we have a fine thermometer which can be used for measuring temperature.

 The  thermometer was invented after several trials by the German  scientist and glass technician Fahrenheit ( 1686 – 1736).

CALIBRATION OF THERMOMETER

Let us study how a  newly manufactured  thermometer tube is calibrated for use.If the thermometer bulb is kept in a bowl of melting ice the mercury column  in the capillary comes down for a few moments and then  entirely stops moving and going down further ! Scientists could not first explain why the mercury column stops .Now we know that the heat supplied to melting ice is used to  reduce the attraction between the H2O molecules in solid state till all the H2O molecules change to  the liquid type loose bonding.

In another experiment the mercury bulb at bottom of thermometer is kept  immersed in pure boiling water. Since  thermometer is made of “hard glass” it does not break when immersed in boiling  water. Surprisingly the mercury level in capillary  tube again  stops rising  when the  thermometer  bulb is immersed in pure boiling water. Here the heat supplied is used to remove entirely  the mutual attraction between H2O molecules  till all the molecules change to gaseous state  and follow the freedom of the  gaseous state.

This  stopping of mercury levels when the thermometer bulb is kept in melting ice or boiling water was a great help for scientists. It helped scientists to create a temperature-measuring scale . First it was Fahrenheit scale of measurement. Now scientists use the centigrade scale of measurement universally in all scientific experiments because it is based on metric system.

Now about calibration of the new thermometer. The two “fixed  points”  –the melting point of ice and boiling point of water—are  permanently marked(etched) on the thermometer tube. The distance in between is divided into hundred equal divisions and these  marks also are etched on the thermometer .Each division is  called one degree of temperature in the centigrade scale.

Suppose if we want to know temperature of a hot liquid we keep the bulb of thermometer immersed in the liquid for a few moments. The mercury level in capillary rises to a certain level and stops there to indicate heat level of that  substance . If the reading stops at  70 degrees point we recognize  that the temperature of the liquid is 70 degree centigrade!

The invention of thermometer was a wonderful facility