German pathologist Richard Altmann is generally credited with first discovering mitochondria-organelles that provide cells with the majority of their chemical energy-for postulating that they were fundamental units of cell activity. Numerous scientists continued to make large strides in their understanding of mitochondria throughout the twentieth century.

TESLA COIL (1891).

The Czech inventor Nikola Tesla invented the Tesla coil, a high-frequency transformer that creates extremely large amounts of voltage, and which was used commercially for lighting and in radio transmission.

RADIO (1896).

While Italian engineer Guglielmo Marconi is traditionally credited with the invention of modern radio by using radio waves to create a system of wireless telegraphy (he received a patent for the creation in 1896), the contributions of Nikola Tesla, Karl Ferdinand Braun, and Heinrich Hertz, among others, were essential to the final design.

RADIOACTIVITY (1896).

Expounding on the closely preceding discoveries of German physicist Wilhelm Roentgen and French physicist Henri Becquerel, Polish chemist Marie Curie and her husband, Pierre Curie, formed a theory of radioactivity, which describes the spontaneous disintegration of atomic nuclei.

ELECTRON (1897).

British physicist J. J. Thomson, aided by Irish physicist John Townsend and British physicist H. A. Wilson, discovered the electron while experimenting with cathode rays, proposing that they were composed of negatively charged particles smaller than atoms, which he called corpuscles, later renamed electrons.

BLOOD TYPES (1901).

In 1901, Austrian physician Karl Landsteiner published results of studies in which he argued that four distinct blood types existed-distinguished by the presence of particular antibodies and antigens-and that blood transfusion between two individuals could be successful only if they shared the same blood type.

AIR CONDITIONING (1902).

In an attempt to solve a printing factory's difficulties with the effects of fluctuating temperatures and humidity on paper, Willis Haviland Carrier conceived of a way to reverse the process of heating to create cold air, thus controlling the amount of moisture in the air. Carrier would go on to start a company dedicated to air conditioning, and it was not long before models were made available for domestic use.

STRATOSPHERE (1902).

German meteorologist Richard a.s.smann and French meteorologist Leon Teisserenc de Bort are both credited with the discovery of the stratosphere, the second layer of the earth's atmosphere, in 1902.

ENGINE-POWERED AIRPLANE (1903).

Inspired by the efforts of German aerial engineer Otto Lilienthal, the Wright brothers experimented with the flying patterns of kites and eventually developed the first engine-powered airplane, which succeeded in performing sustained flight in 1903.

VITAMINS (1905).

While people knew for centuries that eating certain foods could prevent disease, the English doctor William Fletcher discovered in 1905 that unpolished white rice was instrumental in creating immunity to beriberi disease, while other kinds of rice were not, leading him to believe there were nutrients in the unpolished rice whose absence in a person's diet would increase their susceptibility to disease.

HORMONES (1905).

Confirming earlier scientific work on internal secretions in the human body, English physiologists Ernest Henry Starling and William Maddock Bayliss showed that a chemical agent released in one part of the body could affect the functioning of another part of the body, via the bloodstream. The discovery of hormones would later lead to the invention of both oral contraceptives and insulin.

Ma.s.s-ENERGY EQUIVALENCE (1905).

Theoretical physicist Albert Einstein stated in a paper published in 1905 that the ma.s.s of a body is equivalent to its energy content, expressed in the famous equation E=mc2, or energy equals ma.s.s times the speed of light squared.

SPECIAL RELATIVITY (1905).

The theory of special relativity-developed by Einstein in 1905-concerns the motion and behavior of particles moving at close to the speed of light, and is based on two postulates: that the speed of light is the same, regardless of the speed of the observer, and that the laws of physics are consistent when observed from any inertial, or nonaccelerating, frame of reference.

EARTH'S CORE (1906) Irish seismologist Richard Oldham deduced that the earth's core was made of less dense, more liquid material than the rock surrounding it by studying why earthquake waves moved slower through the earth's core than through the mantle.

NEUROTRANSMITTERS (1906).

Spanish physician Santiago Ramon y Cajal revolutionized the theories of the structure of the nervous system in the early twentieth century, aided by methods developed by Italian physicist Camillo Golgi. Cajal's theory that the nervous system was composed of billions of tiny nerve centers-to become known as neurons-led the discovery of neurotransmitters, chemicals that relay messages across synapses.

WASHING MACHINE (1908).

American engineer Alva John Fisher pioneered the first electric washing machine by attaching a motor to the traditional model of a hand-cranked washer. The Chicago-based Hurley Machine Company introduced the product in 1908.

GENES ON CHROMOSOMES (1910).

American embryologist Thomas Hunt Morgan's experiments with genetic mutations and the fruit fly Drosophila melanogaster Drosophila melanogaster led him and his team of students at Columbia University to discover how heredity was in part governed by genes transported by chromosomes. led him and his team of students at Columbia University to discover how heredity was in part governed by genes transported by chromosomes.

SUPERCONDUCTIVITY (1911).

In 1911, Dutch physicist Heike Kamerlingh Onnes tested the behavior and properties of metals such as lead, tin, and mercury when placed at liquid helium temperatures, and discovered that they lost all resistance when cooled to cryogenic levels. This quality became known as superconductivity.

COSMIC RAYS (1913).

The discovery of cosmic rays-particles that bombard earth from beyond its atmosphere-was the culmination of the work of a number of scientists in the early twentieth century, although the German physicist Werner Kolhorster did receive a n.o.bel Prize for his work and research in the nascent field. However, Kolhorster's experiments leaned heavily on earlier discoveries by Victor Hess and Theodor Wulf.

ELECTRON'S ROLE IN CHEMICAL BONDING (1913) Danish physicist Niels Bohr proposed his model of the electron (loosely based on British chemist Ernest Rutherford's model) in 1913, postulating that electrons travel in patterned orbits around the nucleus of an atom, and further theorized that the chemical makeup of an element is derived from the number of electrons in the atom's...o...b..t. Bohr's discovery revealed the electron's fundamental role in chemical bonding.

CONTINENTAL DRIFT (1915).

In 1915, German meteorologist and geologist Alfred Wegener published a book in which he argued that all the continents of the earth had once been part of one ma.s.sive landma.s.s called Pangea, which had slowly split apart over time. Wegener's ideas were initially rejected, but became universally accepted by the 1960s.

MOVING a.s.sEMBLY LINE (1913).

Heralding the era of ma.s.s production, the Ford Motor Company inst.i.tuted a moving a.s.sembly line to construct cars under Ford's leadership in 1913, lowering the price of cars and quickening their production. The inspiration for the a.s.sembly line came from nineteenth-century midwestern meatpacking factories.

THEORY OF GENERAL RELATIVITY (1915).

Theoretical physicist Albert Einstein argued in 1915 that matter warps time and s.p.a.ce, allowing large ma.s.ses to bend light. One of the seminal aspects of this theory was Einstein's idea that the pull of gravity in one direction was equivalent to the force of acceleration in the opposite direction. Einstein's theory was proved in 1919 in a study of solar eclipses.

HELICOPTER (1920).

Many failed, but promising models of primitive helicopters preceded the type created by Argentinean inventor Raul Pateras Pescara. Pescara's helicopter was the first to achieve cyclic pitch, or control of the rotor blades, and he set the world record in 1924 for flying close to a half mile in a little over four minutes.

QUANTUM MECHANICS (1925).

The field of quantum mechanics, the physics of atomic and subatomic scales, can be loosely dated back to 1925, when Werner Heisenberg published his first paper on the topic, but was largely created as a result of efforts of a number of innovative thinkers, including Einstein, Bohr, Planck, and others, working from the 1900s to the 1930s.

LIQUID ENGINE ROCKET (1926).

American physicist Robert H. G.o.ddard overcame criticism of his belief in the future of rockets and helped pioneer the field in 1926, when he set off the first liquid-fueled rocket in a New England cabbage field.

UNCERTAINTY PRINCIPLE (1927).

First presented in a letter in 1927, German physicist Werner Heisenberg's uncertainty principle stated that the more precisely the position of a subatomic particle position was known, the less precisely one could know the particle's momentum. Interpreted in a number of ways, the most influential notion has been the idea that the act of observation changes the very object being observed.

TELEVISION (1927).

American inventor Philo Farnsworth filed a patent for the first complete electronic television in 1927, though technological developments (cathode ray tube, Audion vacuum tube) leading to this final stage were contributed by many engineers and inventors over the course of the previous century.

PENICILLIN (1928).

While healers dating back to ancient civilization realized that molds could be used to help cure infection, it was a famous mistake in Scottish biologist Alexander Fleming's laboratory that eventually brought penicillin to public attention as a miraculous antibiotic. The discovery occurred when a spore of Penicillium notatum Penicillium notatum floated into a petri dish containing mold, sparking Fleming's observation that the spore was inhibiting the growth of the bacteria. floated into a petri dish containing mold, sparking Fleming's observation that the spore was inhibiting the growth of the bacteria.

EXPANSION OF THE UNIVERSE (1929).

While working at an observatory in California, American astronomer Edwin Hubble determined that the universe was expanding while measuring the redshifts (shifts in the frequency of photons) of distant galaxies and discovering that they were moving away from each other at a rate constant to the distance between them.

JET ENGINE (1930).

The credit for the invention of the jet engine is shared between German engineer Hans von Ohain and RAF officer Frank Whittle, who both independently developed the engine model, propelled by ignited, compressed air and based on the principles of Newton's third law of physics.

NEUTRON (1932).

British physicist James Chadwick discovered the neutron-a subatomic particle with no electrical charge-in 1932, putting in place some of the first steps toward the development of the atomic bomb.

RADAR (1935).

Scottish meteorologist Robert Watson-Watt drew on previous research using radio waves to sense inclement weather and successfully employed a shortwave radar in 1935 to detect a bomber in the air, a discovery that would prove instrumental in Britain's defense during the Battle of Britain.

TAPE RECORDER (1935).

Tape recorders began appearing in the early 1930s, led by German technology companies. German-born engineer Semi Joseph Begun developed the first consumer tape recorder, a "Sound Mirror," in 1935 by employing his research on magnetic recording, using a specially coated paper and plastic.

NYLON (1937).

While heading the research department at DuPont, American chemist Wallace Carothers developed nylon-"the miracle fiber," a man-made synthetic rubber-in part to create an alternative to silk, which at that time was difficult to obtain because of shaky trade relations with j.a.pan.

ECOSYSTEM (1935).

First coined in 1935, "ecosystem" came to be fully defined in 1935 by British chemist Arthur Tansley as a natural system in which all physical and organic elements coexist and function as a more or less complete unit.

KREBS CYCLE (1937).

The Krebs cycle, the chemical mechanism by which a cell's respiratory system functions, was formulated by German biochemist Hans Krebs in 1937, building on extensive advances by multiple scientists over the preceding decade in understanding the way cells convert nutrients into energy.

ATOMIC REACTOR (1938).

Italian physicist Enrico Fermi and Hungarian physicist Leo Szilard created the first nuclear reactor in the 1930s, based on studies conducted by Fermi and his colleagues on beta decay and the theory of neutrons.

COMPUTER (1944).

German engineer Konrad Zuse is credited by many with inventing the first fully functioning modern computer, based on a binary system, in 1944. However, Charles Babbage, Alan Turing, and John Vincent Ansoff can all also be credited with inventing various forms of computers.

DNA AS GENETIC MATERIAL (1944).

The idea that DNA carries genetic material was initially established by the famous Avery-MacLeod-McCarthy experiment in 1944, which demonstrated that, since DNA could cause the transformation of bacteria, it could be seen to play a major role in hereditary transfer and the pa.s.sing of genes from one generation to the next.

MICROWAVE OVEN (1946).

Percy Spencer, an American engineer, discovered the possibility of creating a microwave oven somewhat by accident when he noticed a candy bar melting while building a magnetron for Raytheon during an experiment with electromagnetic radiation.

TRANSISTOR (1947).

Enlisted to improve upon the vacuum tube, experimental theoretician Bill Shockley and physicists Walter Brattain and John Bardeen experimented with semiconductors at Bell Labs, eventually producing a reliable transistor that could amplify and switch electronic signals.

RADIOCARBON DATING (1949).

While at the University of Chicago, American physicist Willard Libby worked with his colleagues to develop radiocarbon dating, a method of determining the age of organic substances by how much carbon-14 is present in the material, revolutionizing the field of archeology.

ARTIFICIAL PACEMAKER (1950).

Although a few primitive versions of artificial heart pacemakers had been designed before 1950, Canadian engineer John Hopps is generally credited with the invention of the device, which uses electrical impulses to regulate and simulate the normal beating patterns of the heart. Internal pacemakers would not be developed until 1958.

ORAL CONTRACEPTIVE (1951).

A group of loosely connected scientists, most prominently Harvard professor John Rock, developed the birth control pill in the early 1950s, funded in part by the American birth control advocate Margaret Sanger. Leading a research group at the pharmaceutical company Syntex, American chemist Carl Djera.s.si worked on developing a steroid hormone, cortisone, which eventually led to the synthesis of norethindrone, a progestogen, which became a fundamental part of the first successful oral contraceptive.

EARLY LIFE SIMULATED (1953).

In an effort to understand the conditions governing early life on earth, American chemist Stanley Miller and American physical chemist Harold Urey created a closed system, including the elements they believed were present in earth's early atmosphere such as hydrogen, methane, and water. Miller and Urey discovered that amino acids could be easily produced under such conditions.

DOUBLE HELIX (1953).

Drawing on previous studies of nucleotides in DNA, American molecular biologist James D. Watson and British molecular biologist Francis Crick experimented with models of different combinations of nucleotides using paper and wire, and eventually settled upon the intertwined, dual, nucleotide strands that we now recognize as the double helix.

VCR (1956).

The invention of the VCR, or video ca.s.sette recorder, is generally attributed to the American engineer Charles Paulson Ginsburg, who developed the device while at the Ampex Corporation by applying high-frequency signals onto magnetic tape.

LASER (1958).

While at Bell Labs, American physicists Arthur L. Schawlow and Charles H. Townes began intensive investigation of infrared or visible radiations, initially developing what they called a maser, which would later evolve into "light amplification by stimulated emission of radiation," or a laser.

GPS (1958).

GPS, or Global Positioning System, a navigational system that uses satellites as reference points to calculate geographical positions, was developed by the American engineer Ivan Getting and his team at the Raytheon Corporation, at the behest of the U.S. Department of Defense, after the initial foundational work of Guier and Weiffenbach tracking the orbit of Sputnik Sputnik in 1957. in 1957.

COSMIC MICROWAVE BACKGROUND RADIATION (1965).

While working with receiver systems at Bell Labs, American astronomers Arno Penzias and Robert Woodrow Wilson were confounded with a sound they could not identify, which they ultimately realized was cosmic microwave background radiation, a remaining radio trace of the Big Bang.

PULSARS (1967).