Today In Science History – August 24 – Louis Essen and the Atomic Clock

Louis Essen
Louis Essen (1908-1997) Invented the first atomic clock. Credit: National Physical Laboratory, United Kingdom

August 24 marks the passing of Louis Essen. Essen was the English physicist who is known for his precision measuring time and the speed of light.

Essen always tried to measure the passage of time as accurately and precisely as possible. His college work centered around trying to find a method of using tuning forks and quartz crystals to use as frequency adjustors. He eventually developed the quartz ring clock in 1938.

The quartz ring clock consists of a ring of quartz that is electrically stimulated to produce regular oscillations. These oscillations would then be used to correct the phase of the main clock’s frequency and improving its accuracy. The device was so accurate, it could measure the minute variations of the Earth’s rotational speed. It also became the standard time keeping instrument in astronomical observatories around the world.

During World War II, like many English physicists, Essen worked on radar projects. One important physical constant of interest to radar technicians and physicists is the speed of light. Essen and A.C. Gordon Smith developed a device called a cavity resonance wavemeter. This device measures the energy of radio waves at different frequencies. They used this device to calculate the speed of light with an accuracy better than anything ever achieved before.

He returned to the study of timekeeping after the war. He was interested in using the frequencies of atomic spectra to standardize a clock. He began to collaborate with Jack Perry and in 1955, they built the world’s first atomic clock. Their clock was accurate enough to only lose one second of time every 2,000 years.

Atomic clocks are based around quartz crystals. Quartz crystals are special because they can be made to vibrate by applying an electric field to them, just like his quartz ring clock. This effect is known as piezoelectricity. Usually, an oscillating crystal driven clock is tuned to resonate at their resonant frequency and never calibrated again. The clock is accurate enough for the lifetime of the clock as long as it has power. An atomic clock’s crystal is constantly being checked and calibrated. Essen’s clock used the outer electrons of a cesium atom to tune the crystal’s frequency. A stable cesium atom has 55 electrons and the outermost two electrons have a slight energy difference between them. That difference is caused by their magnetic spin values which have different magnetic properties from each other. The energy difference is equivalent to a frequency of 9,192,631,770 cycles per second.

Part of the clock is an oven that evaporates cesium atoms from a small sample. Each evaporated atom contains an outer electron in either of the two energy states. The higher energy atoms are separated magnetically from the lower energy state atoms. The clock’s crystal is tuned as close as possible to the frequency difference between these two energy states. The crystal’s oscillation is used to drive radio waves at the lower energy state cesium atoms. These radio waves excite the lower energy state electrons into the higher energy state. The excited atoms are collected magnetically and counted by a detector. If the counted value changes, the crystal’s frequency is changed until the counter reads the expected value again. This constant tuning gives the atomic clock its accuracy.

Atomic clocks don’t have to use caesium for their reference frequency. In fact, atomic clocks have been made from hydrogen, rubidium, and ammonia. Cesium is used because it was used in the first clock that worked. Atomic clock technology has progressed to an accuracy of one-second loss in six million years.

Essen’s atomic clock success led him to champion the movement to use caesium oscillation frequencies for defining the length of a second. The old method of defining a second was based on the Earth’s motion around the sun. Essen’s clock showed this method was not very accurate and difficult to establish.

Time caught up with him in 1997. The man who newspapers called “Time Lord” or “Old Father Time” passed away on August 24.

Notable Science Events for August 24

2006 – The International Astronomical Union demotes Pluto to “Dwarf Planet” status.

Pluto New Horizons
Pluto as seen by New Horizons probe.

The International Astronomical Union (IAU) passed a resolution to define a planet by three criteria.

  1. A planet must orbit the Sun
  2. A planet must have enough mass to round itself by its own gravitational field.
  3. A planet must have cleared the space along its own orbit.

Pluto fails the third criteria. Pluto’s mass accounts for less than 7% of the total mass found in its orbit. Since it did not satisfy all three conditions, the IAU removed Pluto’s planet status and classed it as a “dwarf planet”.

1997 – Louis Essen died.

1918 – Ray McIntire was born.

McIntire was an American chemical engineer who invented foam polystyrene, commonly known as Styrofoam. He was searching for a substance to replace rubber as a flexible electrical insulator from polystyrene. Polystyrene is a good insulator but is too brittle. He thought he could soften it up by adding isobutylene under pressure, but what he got was bubbles of polystyrene that formed the foam Dow Chemical trademarked as Styrofoam.

1899 – Albert Claude was born.

Claude was a Belgian cytologist who shares the 1974 Nobel Prize in Medicine with George Palade and Christian de Duve for their discoveries concerning cell structure and function. He created the practice of cell fractionation where a cell is broken up and its components are separated in a high-speed centrifuge. This research is one of the foundations of modern cell biology.

1888 – Rudolf Clausius died.

Rudolf Clausius (1822 - 1888)
Rudolf Clausius (1822 – 1888)

Clausius was a German physicist and pioneer of thermodynamics. He introduced the concept of entropy and stated the second law of thermodynamics for the first time. He also introduced the concept of mean free path to gas kinetic theory to account for translational, rotational and vibrational movements of gas molecules. Together with Émile Clapeyron, he mathematically demonstrated the phase transition between two states of matter, which is the Clausius Clapeyron equation.

1832 – Nicolas Léonard Sadi Carnot died.

Nicolas Léonard Sadi Carnot
Nicolas Léonard Sadi Carnot (1796 – 1832)

Carnot was a French physicist who developed a theorem to describe the maximum possible efficiency of a steam or heat engine. Early studies in heat centered around the idea of caloric, a weightless gas believed to flow from hot objects to cold objects. Carnot postulated engines transferred caloric instead of absorbing caloric. He believed the most efficient engine was limited by the difference between the high and low operating temperatures. This operating cycle became known as the Carnot cycle.

The Carnot cycle is one of the principles that led to the development of the second law of thermodynamics. His research would help others to move away from the concept of caloric, and introduce the ideas of heat, entropy, and enthalpy.

Carnot died during a cholera outbreak at the age of 36. Unfortunately, belongings of a cholera victim are often disposed along with the victim. Much of his research was destroyed and lost.

79 – Gaius Plinius Secundus or Pliny the Elder died.

Pliny the Elder
Pliny the Elder or Gaius Plinius Secundus (23 – 79 AD)

Pliny was the Roman author of the Naturalis Historia or Natural History in 37 volumes. This was the encyclopedic collection of scientific knowledge of the time and the main authority for science into the Middle Ages. He was extremely opinionated about what went into his book and cared little for accuracy. He accepted the sphere as the shape of the Earth and rejected theories that later proved to be true like Pythea’s theory that the Moon causes tides. He was killed while witnessing the eruption of Mt. Vesuvius that buried Pompeii.