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Brief History of Thermometers


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To appreciate the history of thermometers, we must begin with thermoscopes. Discussion of thermometers on wikis often starts with Philo Mechanicus, a Greek engineer who noted the expansion and contraction of air as the temperature rose and fell. In the 1500s, European scientists took this observation and created thermoscopes from it. These relatively simple devices were glass tubes with trapped air that were submerged in water. The water level would rise and fall as the air expanded and contracted, and this early invention was the first step in how the thermometer invented.

Water thermoscope

A thermoscope, however, was of limited use since it could only demonstrate that the temperature was changing but not to what degree. In the late 1500s, Galileo designed a water thermoscope that was rudimentary but the first of its kind to allow variations to be measured. It was a significant advancement, but while often called the Galileo thermometer, it was still technically a thermoscope.

Galileo thermometer

What Is a Thermometer?

It was not until 1612 that an Italian inventor, Santorio Santorio, added a numerical scale to a thermoscope, and this is technically the first thermometer—albeit crude. In 1654, Ferdinand II, the Grand Duke of Tuscany, invented the first thermometer that was liquid—alcohol in this case—in a glass, but his approach was still not accurate, and there was no standard to the scale.


The first true modern thermometer – which means it used mercury and has a standardized scale—was developed by Daniel Gabriel Fahrenheit, a German physicist, in 1724. He designed the Fahrenheit Scale to record temperature changes accurately, and that scale remains in use in the United States, its territories and some other areas, such as the Bahamas, Belize and Cayman Islands.


The Importance of Galileo

Galileo was the first scientist to experiment with substances other than air when it comes to contraction and expansion. While it may seem like a simple and obvious idea now, it was anything but back then. No one had yet conceived of it, and that Galileo did set the foundation for much of what would come later.


Other Key Advancements

Another crucial innovation came from Christian Huygens in 1665. He conceived of the notion of using the boiling point and the melting point of water, and these are fundamental to the temperature scales we now use. Carlo Renaldini in 1694 took this concept a step further by conceiving both the melting point and the boiling point each as a distinct fixed point along what is a universal scale.


Galileo had often used alcohol and created the first alcohol thermoscope. Fahrenheit found the mercury expanded much more significantly to heat than alcohol did and began manufacturing thermostats that were filled with mercury and sealed. It was Sir Isaac Newton who presented the ideas upon which Fahrenheit devised a universal scale using the idea from Huygens and Renaldini.


Medical Thermometers

Medical thermometers are an essential aspect of the history of thermometers in general and predate Fahrenheit. Santorio created the first, but it was impractical in many ways. The first practical medical thermometer was created by Sir Thomas Allbutt, an English physician. His invention was only 6 inches long and could accurately read a temperature in just five minutes. Although a flight surgeon, Theodore Hannes Benzinger, invented the first ear thermometer during World War II, it was Dr. Jacob Fraden at Advanced Monitors Corporation, its CEO, who created in 1984 what is now the most widely used of ear thermometers, but that required digital thermometer technology.


What Is a Digital Thermometer?

Like their predecessors, digital thermometers are temperature-sensing devices that are easily portable and have permanent probes. The differences, however, are that the sensors are digital, and these devices usually have a convenient digital display. The way a digital thermometer works depends entirely on the type of sensor employed. Sensor types include resistance temperature detector or RTD, thermocouple and thermistor. Each type has advantages and disadvantages.

Is a Digital Hygrometer the Same Thing?

No. Digital hygrometers, such as ThermoPro digital hygrometers, which are among the most popular on the market are used to measure the humidity—and in particular relative humidity—of air. One must appreciate how the thermometer was invented to appreciate how the hygrometer was made, and they are often closely associated because of their combined use in meteorology and other sciences.

Thermopro Hygrometers

How Do RTD, Thermocouple, and Thermistor Differ?

RTDs are made up of wire windings that exhibit responses to temperature changes. The hotter they are, the higher their electrical resistance, and that can be measured accurately. Thermocouples comprise a different pairing of metal wires that create a thermoelectric voltage. As the temperature changes, there is a near-immediate change in voltage, which can be measured. Thermistor elements are semiconductor devices that are highly sensitive due to the relationship between temperature and their electrical resistance. There are two main types: negative temperature coefficient or NTC devices and positive temperature coefficient or PTC devices.


How Do Meat Thermometers Work?

Meat thermometers, such as those made by ThermoPro, are bimetallic thermometers. They are used to measure the internal cooking temperature of meat whether it be beef, poultry or whatever else. These thermometers have two parts: a temperature rod and a display. These can be either conventional, such as a dial-based readout on the top of a rad, or digital, which is often an LCD readout that can either be attached directly to the rod or separate and connected to the rod via wiring or similar.

ThermoPro Food Thermometer

The thermometer rod contains two distinct metals that have been bonded together. Note that this is similar to the thermocouples mentioned earlier. One of the chosen metals expands at a lower temperature than the other, and the twisting of these metals will affect the dial or can be read by the device and translated into a specific temperature to be displayed via the readout.

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