Thermometer is a medical device that can be found in every home. Why is that? It measures body temperature safely and is small in size. The use is simple. After pressing the button, thermometer shows the measurement digits on the screen in a few seconds or minutes, depending on the device. The measurement results are given in standard temperature units such as Fahrenheit or Celsius. Knowing body temperature can tell whether a person is sick or not. Modern thermometers can be digital or infrared. You can see the last type everywhere due to the COVID-19 pandemic. However, thermometers were not always the same as they are now. Let's take a look at how the device has been developed.

The first attempts to develop a device for measuring temperature date back to ancient Greece. Philo of Byzantium (ca.280 BC – 220 BC) discovered that air expanded when heated. He attached a tube to a hollow sphere and extended it over a jug of water. When the device was in the sun, air expanded out of the sphere and into the water, creating bubbles. Various authors have credited the invention of the thermometer to Hero of Alexandria (10–70 AD). He described a demonstration in which a closed tube partially filled with air had its end in a container of water. The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

Such a mechanism was later used to show the hotness and coldness of the air. These were primitive glass devices filled with liquids that expanded when they were warm and contracted when they cooled. Alcohol and mercury were the liquids most commonly used. The devices were named thermoscopes. They were the earliest type of thermometer and only showed changes in temperature but not numerical values.
One of the first thermoscopes was developed by Italian inventor, Galeleo Galilei in 1593. He used water as the liquid and glass bulbs inside an open tube to measure the temperature. The glass bulbs rose and fell with the changes in temperature. In 1612, another Italian inventor, Santorio Santorio, used a numerical scale on the thermoscope but it was very rudimentary. In 1654, the first sealed glass tube was developed by Ferdinand II, the Grand Duke of Tuscany. It contained alcohol and had a numerical scale, but wasn’t very accurate.

In the first decades of the 18th century in the Netherlands, Daniel Gabriel Fahrenheit made two revolutionary breakthroughs in the history of thermometry. He invented the mercury-in-glass thermometer - first widely used, accurate, practical device - and Fahrenheit scale, first standardized temperature scale. He assigned the freezing point of water at 32 degrees, the boiling point of water as 212 degrees and the normal body temperature as 98.6 degrees.
Later on in that same century, Swedish inventor Anders Celsius developed a numerical scale, called the Celsius or Centigrade scale. This scale was based on a scale of zero to one hundred where the freezing point of water is zero, the boiling point of water is 100 degrees and normal body temperature is 37 degrees.
The first medical thermometer that produced a body temperature reading in five-twenty minutes was invented by Sir Thomas Allbutt in 1867. It was six inches long. For almost a hundred years thermometers were basically unchanged. They contained alcohol or mercury and were considered to be very accurate. A new breakthrough has occurred after World War II, and infrared technology began to be used in thermometers. Infrared thermometers are intended to measure body, air, liquids and objects temperature by infrared radiation emitted by them.

Over time, it became clear that mercury thermometers are not safe. Mercury is a good conductor of heat and reacts quickly to changes in ambient temperature, but if thermometer breaks, a person can be poisoned by mercury vapor. With the rapid development of electronics in XX century, mercury thermometers were replaced by digital ones. A medical digital thermometer has a metal tip that changes its electrical conductivity depending on the temperature of the person's body. Then these changes are recorded by the device, translating them into the usual degrees of Celsius or Fahrenheit.

Today's digital and infrared thermometers are the most modern and accurate, safest and fastest devices that could measyre bogy temperature. The future will show how the technology for measuring body temperature will develop further. 
Photo Credit: Wikimedia, Asesen. 

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