In the earliest days of the medical profession, there was no device available to measure a patient's temperature. Patients were either too hot, normal, or too cold, depending upon the doctor's personal observation methods. It was not until nearly the seventeenth century that scientists devised an instrument that could detect the changes in air temperature, and many years later that a medical thermometer was constructed.
The first thermometers were created to measure the changes in atmospheric temperature. The most famous of these was invented by Galileo in 1592; called an air-thermoscope (or air thermometer). It consisted of a long glass tube with a wide bulb at one end. The tube was heated, causing the air within to expand and some to be expelled. While still warm, the open end of the tube was placed into a flask of water; as the tube cooled the warm air would contract, drawing water into the tube as its volume decreased. Once the tube-and-water system reached a steady state, any change in air temperature would cause the level of the water within the tube to rise or fall.
While the principles behind the air-thermoscope were scientifically valid, there were two major hindrances to its acceptance. First, the varying heights and widths of the tubes made it very difficult to graduate the device--that is, to establish a degree scale. The first scientist to do so was Italian doctor Santorio Santorre, who created a scaled thermometer in 1612. Second, it was soon discovered that the air-thermoscope was unreliable, giving widely varying readings for apparently identical temperatures. Scientists puzzled over this phenomenon until the 1660s, when it was realized that an open-ended system would react to air pressure as well as temperature (in other words, the air thermometer acted as a barometer as well). This realization came several years after the solution, for in 1654 Duke Ferdinand II of Tuscany had constructed a sealed liquid-in-glass thermometer that was not prey to the changes in air pressure.
With this as a starting point, European scientists began to perfect the design of the thermometer. One of the issues they strove to address was the need for an instrument that could travel by sea; ordinary liquid thermometers were rendered unreliable by the ships' rocking motions. In 1695 a French physicist, Guillaume Amontons, designed a thermometer using an unshaped tube filled with compressed air and capped with a layer of mercury. As the temperature increased the air would expand, causing the mercury level to rise; as the temperature decreased, the mercury would fall. Another Frenchman, René de Réaumur (1683-1757), sought to improve upon Amontons' design by replacing the air-and-mercury system with a mixture of alcohol and water. Réaumur's thermometer was remarkable in that he devised an 80-degree temperature scale based upon the freezing and boiling points of water--the same points that, years later, would become the basis for the more widely accepted scales of Celsius and Fahrenheit.
At the turn of the eighteenth century the most discouraging issue for scientists and instrument makers was the lack of a standard temperature scale. The level of glass-blowing technology was too poor to make thermometer tubes that were exactly alike, and so every scientist's temperature scale was different. All that changed in 1717 when a Dutch instrument maker, Daniel Fahrenheit, introduced a line of mercury-filled thermometers of nearly identical proportions. His use of mercury in very thin tubes allowed him to graduate the scale into many degrees; using the boiling and freezing points of water as fiducial points, Fahrenheit developed the first scale to be accepted as a worldwide standard, with water's boiling point at 212° and its freezing point at 62°.
The Fahrenheit scale enjoyed global popularity for many years, until the introduction of the hundred-degree scale by Anders Celsius in 1746. Several scientists had attempted to popularize a hundred point scale, but Celsius' was the first to also utilize water's freezing and boiling points as the zero and hundred-degree marks. Originally, Celsius placed the freezing point at 100° and the boiling point at 0°; this was reversed in 1747, at which time the centigrade (meaning "five hundred steps") scale began to rise in popularity. In 1946 the Celsius scale was adopted by most of the world as the official temperature scale.
Probably the most familiar thermometer is that found in a doctor's office, the clinical thermometer. It was invented in 1866 by Sir Thomas Clifford Allbutt, an English physician. The important features of this thermometer were that it was relatively short, usually no longer than six inches, and it responded quickly to the patient's temperature. Previous instruments required nearly twenty minutes to get an accurate reading, while Allbutt's thermometer could reach equilibrium in less than five minutes. This made it easier for doctors to follow the course of a fever, since temperatures could be taken more quickly and, therefore, more often.
Modern thermometers come in many different varieties. Mercury thermostats are used to regulate temperatures in ovens and household air systems. Digital thermometers have been designed to measure temperature changes using electronic equipment. New thermometers are being designed that can read a patient's temperature using infrared technology; these devices can determine a person's temperature in about one minute, and can take a reading from inside the ear, rather than the mouth.
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