Evangelista Torricelli and the Torr

Evangelista Torricelli.

In Chapter 1 of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the units of pressure.

The SI unit of pressure is N m^–2 or pascal (Pa). The density is expressed in kg m^–3, so that ρg has units of N m^–3 and ρgz is in N m^–2. Pressures are often given as equivalent values of z in some substance, for example, in millimeters of mercury (torr) or centimeters of water.

Metric units are usually named after famous scientists. So who is Torr? Actually, the unit is a shortened version of the name of Italian physicist Evangelista Torricelli (1608–1647).

Torricelli was born in Rome. He eventually succeeded Galileo as the chair of mathematics at the University of Pisa. Torricelli’s work focused on the concept of pressure. Galileo suggested he study the rise and fall of water in a tube. Torricelli put water—and eventually mercury—in a tube closed at the bottom, turned the tube up-side-down, and let the liquid run out until it’s level stopped falling. There was an empty space at the upper closed end: one of the first vacuums ever created. German scientist Otto von Guericke subsequently used a pump to create much stronger vacuums, and brilliantly demonstrated the enormous pressure of air.

Torricelli’s work ultimately led to his invention of the barometer, which could be used to measure air pressure. Using a barometer similar to Torricelli’s design, French physicist Blaise Pascal measured the air pressure at different elevations (recording the pressure at the base and near the top of a mountain), proving that we live under a mountain of air. To honor these groundbreaking ideas, the two common units for pressure are the pascal and the torr. The barometer and the thermometer (a rudimentary version of which Galileo invented) paved the way for much future research. For instance, using the barometer the chemist Robert Boyle was able to relate volume and pressure in a gas: Boyle’s law.

Torricelli also worked on optics, including lens design. Another of his interests was mathematics, and his calculations of areas and volumes was a precursor to integral calculus. For instance, he discovered Torricelli’s trumpet, which is found by rotating the curve y = 1/x about the x axis. It has finite volume but infinite surface area. Unfortunately, his promising career was cut short. He died when only 39, probably of typhoid fever. 

Ultimately, Torricelli’s discovery led to measurement of a person’s blood pressure, one of the most common diagnostic procedures in medicine. When blood pressure is recorded as 126/64, both the systolic pressure (measured when the heart contracts, 126) and diastolic pressure (measured when the heart is relaxed, 64) are given in millimeters of mercury, which is equivalent to the torr. So, almost every medical exam honors the work of Evangelista Torricelli.

Barometers: The Surprising Science Story.

 https://www.youtube.com/watch?v=fXQVgCpHoRk