Close your eyes and picture the city of Paris. Now imagine the city without its most famous landmark: the Eiffel Tower.

The unthinkable almost happened.

When French engineer Gustave Eiffel built this tower for the Paris World’s Fair of 1889, it created a sensation. The iron structure contrasted sharply with the historic stone buildings of Paris. What’s more, at 300 meters (984 feet), it became the tallest structure in the world. It dwarfed the previous record holder — the 169.3-meter (555-foot) Washington Monument in the U.S. capital.

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Eiffel’s four-legged iron archway was supposed to last only 20 years. That’s when Eiffel’s permit to operate the building would expire and the city could choose to tear it down.

Erected for the 1889 Paris World’s Fair, shown here, this iron archway was not expected to last more than 20 years. Lib. of Congress’ Tissandier Coll. / LC-USZ62-24999

And it initially seemed the building indeed was in peril. Three hundred prominent artists and writers publicly expressed their hatred for Eiffel’s iron giant. In a petition published in the French newspaper Le Temps just as construction was beginning, the group referred to the Tower as a “giddy ridiculous tower dominating Paris like a gigantic black smokestack.”

A French novelist of the time, Charles-Marie-Georges Huysmans, declared that “it is hard to imagine” that people will allow such a building to stay.

Yet from the beginning, Eiffel had a strategy to save his building. If the Tower was linked to important research, he reasoned, no one would dare take it down. So he would make it a grand laboratory for science.

Areas of research would include weather and the brand-new fields of powered flight and radio communications. “It will be an observatory and a laboratory such as science has never had at its disposal,” Eiffel bragged in 1889.

And his strategy worked. This year marks the iconic structure’s 125th birthday. Over the years, research conducted there has brought dramatic and unexpected payoffs. During World War I, for instance, the French army used the Tower as a giant ear to intercept radio messages. It even led to the arrest of one of the war’s most famous and notorious spies.

Gustave Eiffel was an engineer. His vision was to make his Parisian masterpiece too valuable to dismantle — by making it a laboratory for science. Lib. of Congress’ Bain Coll. / LC-DIG-ggbain-32749

Not a moment to lose

Yet the Tower’s studies would go beyond Eiffel’s wish to preserve his building, says Bertrand Lemoine. He directs research at the French National Center for Scientific Research in Paris. In 1893, not long after the Tower’s completion, Eiffel resigned from his engineering firm. He now had the time — and money — to explore his keen interest in the natural world.

And he wasted no time.

Scientific research began just one day after the Tower opened to the public on May 6, 1889. Eiffel installed a weather station on the Tower’s third (and highest) floor. He connected instruments by wire to the French weather bureau in Paris. With these, he measured wind speed and air pressure.

In fact, one of the more striking instruments installed on the Tower from its earliest days was a giant manometer. It’s a device that measures the pressure of gases or liquids. A manometer consists of a U-shaped tube containing mercury or another liquid at the bottom. One end of the ‘U” is open to the air, the other is sealed off. The difference in height of the liquid in the two parts of the U is a measure of the pressure of the air (or liquid) bearing down on the open end.

By 1900, manometers were common. But the Tower’s enormous one stretched from its summit to its base. The length of the tube enabled scientists to measure pressures 400 times greater than that at sea level. Until now, no one had been able to measure pressures this high.

A wind laboratory

In 1904, Eiffel dropped a cylinder down a cable (shown here) for a series of experiments to measure wind resistance. Scientific American, March 19, 1904

French scientists already had succeeded in measuring temperatures to an accuracy of one hundredth of a degree Celsius. But no one had tried to put those recordings in any kind of meaningful chart or graph. Eiffel was the first, notes Joseph Harriss, author of The Tallest Tower (Unlimited Publishing, 2008). From 1903 though 1912, Eiffel used his own money to publish charts and weather maps. These helped the French Weather Bureau adopt a more scientific approach to weather measurements, Harriss explains.

The Tower also played a pivotal role in the emerging field of aerodynamics. That’s the study of how air moves around objects. Eiffel had first seriously considered the effects of wind as he began designing his building. He feared that a strong air current might topple the Tower. But he also was interested in aviation. In 1903, the Wright brothers piloted the first motorized airplane. That same year, Eiffel began studying the motion of objects racing down a cable from the Tower’s second floor.

He sent objects of different shapes down the 115-meter (377-foot) cable. Wires linked these objects to recording devices. Those devices measured the speed of the objects and the pressure of air along the direction of travel. Some of the objects Eiffel studied moved as fast as 144 kilometers (89 miles) per hour. That was speedier than early aircraft.

Scientific American reported on one of these early experiments in its March 19, 1904, issue. A heavy cylinder, capped by a cone, sped down the cable in just 5 seconds. Eiffel had installed a flat plate in front of the cylinder. So during the object’s descent (see photo), the wind’s pressure thrust that plate backward. This provided a new way of measuring the resistance that air exerts on a moving object.

Conducting hundreds of such experiments, Eiffel confirmed that this resistance increases in proportion to the square of the object’s surface. So doubling the size of the surface would quadruple the wind resistance. This finding would prove an important guide in designing the shape of airplane wings.

Here is the air inlet for the tunnel used to make wind resistance measurements on airplane wings. Scientific American/ May 28, 1910

In 1909, Eiffel built a wind tunnel at the bottom of the Tower. It’s a large tube through which a strong fan pushes air. Air flowing around stationary objects placed in the tunnel would mimic effects during flight. This allowed Eiffel to test several models of airplane wings and propellers.

The findings provided new insight into how airplane wings get their lift. When nearby residents complained about the noise, Eiffel constructed a larger and more powerful wind tunnel in Auteuil, a few kilometers away. That research center — the Eiffel Aerodynamics Laboratory — still stands. Today, however, engineers use it to test the wind resistance of cars, not planes.

Saved by radio

Despite these successes, it was another area of research — radio — that ensured Eiffel’s Tower would not be torn down.

In late 1898, Eiffel invited inventor Eugène Ducretet (DU-kreh-TAY) to carry out experiments from the Tower’s third floor. Ducretet was interested in making practical use of radio waves. This electromagnetic radiation is generated, just as visible light is, by accelerating electrically charged particles.

In the 1890s, the main way that people communicated over long distances was by using a telegraph. This device conveyed messages, using a special code, across an electric wire. Ducretet became the first person in France to transmit telegraph messages without the wires. Radio waves carried the messages.

Inside the wireless telegraph station of the Eiffel Tower in 1905. Scientific American/ Feb. 2, 1905

His first wireless transmission took place on Nov. 5, 1898. He sent it from the third floor of the Tower to the historic Panthéon (PAN-thay-ohn), a burial place for famous citizens of Paris that was 4 kilometers (2.5 miles) away. One year later, wireless messages were sent for the first time from France to Great Britain across the English Channel.

In 1903, still worried that his building might be dismantled, Eiffel got a clever idea. He asked the French military to conduct its own research on radio communications at the Tower. He even paid the army’s costs.

French army captain Gustave Ferrié (FAIR-ee-AY) worked from a wooden shack at the base of the Tower’s southern pillar. From there, he made radio contact with forts around Paris. By 1908, the Tower was broadcasting wireless telegraph signals to ships and military installations as far away as Berlin in Germany, Casablanca in Morocco, and even North America.

Convinced of the importance of radio communications, the army set up a permanent radio station at the Tower. In 1910, the city of Paris renewed the structure’s permit for another 70 years. The Tower was now saved and set to become the symbol of Paris. Within a few years, radio science at the Tower would alter the course of history.

It would start that same year, in 1910. That’s when the Tower’s radio station became part of an international time organization. Within two years, it broadcast time signals twice a day that were accurate to within a fraction of a second. These and similar broadcasts from other stations in America, Great Britain and elsewhere changed everyday life. Now people anywhere could compare the times on their wristwatches with that of a distant, highly accurate timekeeper.

When the clock (left on wall) hit midnight (and again 2 and 4 minutes later), it sent signals of the time out by the Morse key on a telegraph machine. In 1910, it was not yet able to do this wirelessly. Scientific American/ June 18, 1910

That was a huge achievement during an era when different cities — and certainly different countries — did not always synchronize their clocks. Understandably, this created confusion in railroad schedules and other time-sensitive information.

The time broadcasts also made it possible for ship engineers to determine their position at sea by accurately calculating their east-west position on Earth’s surface, also known as longitude.

How could a time signal determine longitude? The Earth is 360 degrees around. It rotates from east to west at a rate of 15 degrees per hour. That means each 15 degrees of longitude is equal to a time difference of one hour. To find out how far east or west a ship was from home, a sailor would compare the local time with the time signal being broadcast at the same moment from back home. Such radio signals were beamed from a series of tall structures, including the Eiffel Tower.

Gathering military intelligence

By September 1914, just weeks into World War I, it looked like the German army would overrun France. German battalions were approaching the outskirts of Paris. The French army ordered explosives to be laid at the base of the Eiffel Tower. The military would rather destroy it than let it fall into enemy hands.

Then, engineers at the Tower intercepted a radio message from German General Georg von der Marwitz. He was commanding a unit advancing on Paris. He had run out of feed for his horses, the message said, and would have to delay his arrival. Taking advantage of the delay, the French army used every taxi in Paris to carry some 5,000 troops to the town of Marne, about 166 kilometers (103 miles) away. That’s where many of the German troops were stationed.

The French battled the Germans there, and won. Ever after, it was known as the Miracle of the Marne. And although the war wore on for another four years, Paris was never invaded.

World War I soldier guards the Eiffel Tower’s wireless station in 1914 or 1915. Lib. of Congress’ Bain Coll. / LC-DIG-ggbain- 17412

In late 1916, engineers at the Tower’s listening post intercepted another message. This one had been sent from Germany to Spain, a country that had not entered the war. The message referred to an agent known as “Operative H-21.” The French realized that this was the code name for the Dutch exotic dancer born Margaretha Geertruida Zelle. Today she’s remembered as the beautiful spy Mata Hari. That message helped lead to her arrest.

From then on, broadcasting became the Eiffel Tower’s main contribution to science and technology. In 1921, the Tower’s radio station transmitted the first music programs in France. Fourteen years later, a transmitter on the Tower beamed France’s first television signals from a studio nearby. In 1957, satellite dishes installed atop the Eiffel Tower increased the building’s height to 320.75 meters (1,052 feet). Today, some 100 antennas decorate the Tower’s top, which extends to 324 meters (1,062 feet).

Even though the Tower is no longer a site of active research, the structure itself owes much to science. Eiffel did not have a mathematical formula to guide him in building a tower that could withstand the winds and support its 10,000-metric-ton weight. But the man succeeded by drawing diagrams of the forces that would impact the building. He also used previously collected information about the effects of wind together with his own experience in building large railroad bridges and other structures, including the interior of the Statue of Liberty.

According to a study recently commissioned by the company that now operates the Eiffel Tower, the building is indeed sturdy. Its analysis concluded that neither extreme temperatures, nor fierce winds, nor massive snowfalls should prevent the tower from lasting another 200 to 300 years.

Power words

accelerate To change the rate of speed or the direction of something over time.

aerodynamics The study of the motion of air and its interaction with solid objects, such as airplane wings.

air pressure The force exerted by the weight of air molecules.

electric charge The physical property responsible for electric force; it can be negative or positive. An electron, for instance, is a negatively charged particle and the carrier of electricity within solids.

electromagnetic radiation Energy that travels as a wave, including forms of light. Electromagnetic radiation is typically classified by its wavelength. The spectrum of electromagnetic radiation ranges from radio waves to gamma rays. It also includes microwaves and visible light.

engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.

exponential curve A type of upward sloping curve.

lift An upward force on an object. It may occur when an object (such as a balloon) is filled with a gas that weighs less than air; it can also result when a low-pressure area occurs above an object (such as an airplane wing).

longitude The distance (measured in angular degrees) from an imaginary line — called the prime meridian — that would run across Earth’s surface from the North Pole to the South Pole, along the way passing through Greenwich, England.

manometer A device that measures pressure by examining the levels of liquid, often mercury, inside a U-shaped tube.

telegraph A device used to transmit electrical signals from place to place that originally used wires.

radio waves A type of radiation, generated just like the rainbow of colors that make up visible light, by the acceleration of a charged particles. Radio waves have much longer wavelengths than visible light and can’t be detected by the human eye.

wind tunnel A tube-shaped facility used to study the effects of air moving past solid objects, which often are scale models of real-size items such as airplanes and rockets. The objects typically are covered with sensors that measure aerodynamic forces like lift and drag. Also, sometimes engineers inject tiny streams of smoke into the wind tunnel so that airflow past the object is made visible.