August 3rd, 1942, Washington DC. The United States Treasury Building. Lieutenant Colonel Kenneth Nicholls walks into the office of Under Secretary of the Treasury Daniel Bell with a request that sounds absolutely insane. He needs to borrow metal, a lot of metal. Not for coins, not for reserves, for something he can’t fully explain because it’s classified at the highest level. Belle asks the standard question.

How much do you need? Nicholls replies, 6,000 tons. There’s a pause. Belle’s face changes. Then he asks, “How many Troy ounces is that?” Nicholls doesn’t know. Neither does Belle. Growing impatient, Nicholls responds, “I don’t know how many troy ounces we need, but I know I need 6,000 tons. That is a definite quantity.

What difference does it make how we express the quantity?” Belle leans forward, indignant. Now, young man, you may think of silver in tons, but the treasury will always think of silver in troy ounces. What Nicholls couldn’t tell Belle was the truth. He needed this silver to help build a machine that would separate uranium atoms.

A machine that would help build the world’s first atomic bomb. And before this conversation ended, the number wouldn’t be 6,000 tons. It would be $14,700 tons of silver, worth over $300 million in 1942, equivalent to nearly $6 billion today, borrowed from the American people’s reserves to build magnets the size of houses.

This is the story of the Calotron, the massive, desperate, brilliant solution to the Manhattan project’s most impossible problem. How do you separate uranium 235 from uranium 238 when they’re almost chemically identical? Do you understand why nickels needed silver? We need to understand the challenge facing the Manhattan project in 1942.

Scientists knew that uranium 235 was the key to an atomic bomb. When the neutron hits a uranium 235 nucleus, it splits apart, releasing enormous energy and more neutrons, which split more atoms, creating a chain reaction. This was the principle behind the atomic bomb. But there was a catastrophic problem. Natural uranium contains only 0.

72% uranium 235. The rest, 99.28% 28% is uranium 238, which won’t sustain a chain reaction. To build a bomb, you need uranium that’s at least 90% pure uranium 235. You need to separate two isotopes that are chemically identical. They have the same number of protons, the same number of electrons, the same chemical properties.

The only difference, three neutrons. Uranium 238 has 146 neutrons. Uranium 235 has 143 neutrons. That tiny difference in atomic mass is the only thing separating a useless element from the most powerful explosive material on Earth. How do you exploit that three neutron difference on an industrial scale? Multiple approaches were being explored.

Gaseous diffusion, thermal diffusion, centrifuges, all had problems. All faced enormous technical hurdles. None were guaranteed to work in time to affect the outcome of World War II. Then came Ernest Lawrence. Lawrence was a physicist at the University of California, Berkeley, and the inventor of the cyclron, a circular particle accelerator.

In 1941, British physicist Mark Olant visited Lawrence and told him about Britain’s atomic research. The British Mod Committee had determined that an atomic bomb was possible, but they needed enriched uranium, and they needed it fast. Lawrence had an idea that sounded almost absurd. Electromagnetic separation. Use magnetism to separate uranium isotopes.

The principle was simple in theory. Ionize uranium. Give it an electric charge. Fire it through a powerful magnetic field. The magnetic field will deflect the ions based on their mass. Heavier uranium 238 ions will curve less. Lighter uranium 235 ions will curve more. Catch them in different collectors.

It was essentially a mass spectrometer scaled up to industrial proportions. He converted his old 37in cyclron into the first prototype. They called it the Calatron, California University cyclron. On December 2nd, 1941, just 5 days before Pearl Harbor, the Calatron was switched on for the first time. It worked. A uranium beam intensity of 5 micro ampers reached the collector.

Lawrence’s hunch was confirmed. the process could work. By January 1942, they produced 18 micrograms of uranium enriched to 25%, 10 times more than any previous method had achieved. But there was a staggering difference between proving a principle in a laboratory and enriching enough uranium 235 to build a bomb. The Manhattan project needed kg of weaponsgrade uranium.

To get there, they needed massive calutrons, hundreds of them. And that required magnets, enormous magnets with coils wound from highly conductive metal. By mid 1942, the Manhattan Project’s chief engineer, Colonel James Marshall, and his deputy Kenneth Nichols, faced a brutal reality. To build the electromagnetic separation plant, cenamed Y12 at Oakidge, Tennessee, they would need 5,000 short tons of copper for the magnet windings.

Copper was the best conductor. Copper was essential. But copper was in desperately short supply. World War II had created an insatiable demand for copper. electrical wiring for ships, airplanes, tanks, communications equipment, ammunition casings. Copper was everywhere in the war effort. Every industry wanted it.

Every military branch needed it. Copper was rationed, controlled, fought over. The Manhattan project couldn’t get 5,000 tons of copper. It was impossible. So, Nicholls and Marshall started looking for alternatives. What else conducts electricity almost as well as copper? Silver. Silver is the most electrically conductive element on Earth, even better than copper. An 11 to 10 ratio.

11 parts silver could replace 10 parts copper. To replace 5,000 tons of copper, they’d need approximately 5,500 tons of silver. But where do you find 5,500 tons of silver in the middle of a world war? The United States Treasury. The Treasury held massive reserves of silver bullion at the West Point Bullion Depository in New York.

Tons upon tons of silver bars, each weighing 1,000 troy ounces, about 31 kg. This silver backed the nation’s currency, represented the wealth of the American people, and was guarded more carefully than gold. Nicholls was authorized to approach the Treasury and request a loan. On August 3rd, 1942, Nicholls walked into Under Secretary Daniel Bell’s office.

He explained carefully, without revealing classified details, that the War Department needed to borrow silver. A lot of silver for a project of the highest national importance. Belle asked how much. Nicholls said 6,000 tons. Belle, stunned, asked how many troy ounces that was. Nicholls didn’t know. The two men sat there unable to convert tons to troy ounces, the absurdity of the moment hanging in the air.

Then Bell, recovering his composure, delivered his famous line. Young man, you may think of silver in tons, but the treasury will always think of silver in troy ounces. Bel agreed to the loan, but he insisted on strict accountability. The silver would be weighed to the troy ounce. Monthly reports were required. Guards would accompany every shipment.

At the end of the war, every ounce would be returned. But the final number wasn’t 6,000 tons. As the Manhattan project’s plans expanded, so did the need for silver. Eventually, 14,700 short tons, 13,300 metric tons, 430 million troy ounces of silver, were loaned from the treasury. At 1942 prices, this silver was worth over $300 million.

In today’s dollars, nearly six billion. It remains one of the largest financial transactions in American history. The United States government loaned itself a fortune in silver to build a weapon it wasn’t sure would work. The silver arrived in 1,000 troy ounce bars, each one about 31 kg, gleaming and heavy. They were transported under armed guard by rail to the defense plant corporation in Carterate, New Jersey.

There the bars were melted down and cast into cylindrical billets. These billets were then shipped to Phelps Dodge in Bayway, New Jersey, where they were extruded, forced through dyes under enormous pressure into strips. Each strip was 625 in thick, 3 in wide, and 40 ft long. 258 car loads of silver strips were shipped under guard to Alice Chalmer’s in Milwaukee, Wisconsin.

there. Engineers wound the silver strips onto magnetic coils and sealed them into welded casings. Each coil was massive, weighing tons, designed to generate intense magnetic fields. Finally, the completed magnets traveled by unguarded flat cars to Oakidge, Tennessee. Why unguarded? Because the magnets no longer looked like silver.

They were sealed in casings painted anonymous. Nobody would know what they were. At Oak Ridge, construction of the Y12 electromagnetic plant was already underway. The site covered 825 acres in Bear Creek Valley. Chosen because the surrounding ridges might contain a major explosion or nuclear accident. The plant would eventually comprise nine major process buildings and 200 support structures covering nearly 80 acres of floor space.

The scale was staggering. The calutrons were arranged in racetracks so named because the magnets formed oval tracks when viewed from above. Each racetrack contained multiple calutron tanks arranged in a circular formation. Inside each tank, uranium tetrachloride was heated and ionized. The ions were accelerated through a vacuum chamber and deflected by 180° by the massive silver wound magnets.

The heavier uranium 238 ions curved less. The lighter uranium 235 ions curved more. They struck different collectors. The separated uranium was then chemically recovered and processed further. But turning this concept into reality, required solving countless engineering problems. The vacuum tanks, 14 tons each, kept creeping out of alignment because of the magnetic forces, sometimes shifting 3 in.

They had to be fastened more securely. Moisture inside the magnet coils caused short circuits and rust, forcing magnets to be torn down, cleaned, and rewound. debris. Scientists called it gunk and crud, condensed inside the vacuum chambers, blocking slits and causing ion beams to lose focus. The ion beams generated intense heat despite their low intensity.

Over many hours, they could melt collectors. Engineers added water cooling systems. Procedures were developed for cleaning the equipment without contaminating the silver. And then there was the silver itself. Special procedures were instituted for handling it. When workers drilled holes in silver components, they placed paper underneath to catch every filing.

At the end of each shift, the paper was collected and the silver recovered. When machinery was dismantled for cleaning, every surface was wiped down. Even the floorboards beneath the equipment were eventually ripped up and burned to recover microscopic amounts of silver. Kenneth Nichols had to provide monthly accounting reports to the Treasury documenting every Troy ounce.

The scrutiny was intense. The responsibility was enormous. The first alpha racetrack became operational in early 1944 after extensive debugging. By spring 1944, multiple alpha and beta racetracks were running. The beta racetracks took partially enriched uranium from the alpha racracks and enriched it further, reaching the 90% purity required for weapons grade.

One of the most remarkable aspects of the Y12 plant was its workforce. Tennessee Eastman, a chemical company, was hired to operate the facility. They recruited thousands of workers, many from the rural areas surrounding Oak Ridge. Most had only high school educations. Many were young women, some barely out of their teens. These women became known as the Calatron girls.

They sat at control panels in shifts covering 24 hours a day, monitoring dials, adjusting knobs, keeping the uranium ion beams focused and stable. The work required intense concentration and discipline. The control panels had dozens of gauges and switches. A small fluctuation could ruin hours of work. But here’s the astonishing part.

The Calatron girls didn’t know what they were doing. For security reasons, they were told nothing about uranium, nothing about isotopes, nothing about atomic bombs. They were simply trained to keep the needles in the green zones. Dial A had to stay between these numbers. If dial B dropped, adjust knob C. Follow the procedures. Don’t ask questions.

They were, as Kenneth Nichols later put it, trained like soldiers not to reason why. and they were incredibly good at it. Scientists from UC Berkeley initially operated the Calatrons to remove bugs and establish operating procedures. Then they handed control to the Tennessee Eastman operators. Nichols began comparing production data and noticed something startling.

The young women operators were outproducing the PhD physicists. Nicholls mentioned this to Ernest Lawrence. Lawrence was skeptical. How could high school graduates outperform trained scientists? They agreed to a production race. Lawrence’s scientists versus the Hillbilly Girls. Lawrence lost. The reason was simple. The scientists couldn’t help themselves.

When a dial fluctuated, they had to investigate why. They’d spend time analyzing, hypothesizing, testing theories. The young women trained to follow procedures without questioning simply made the adjustment and moved on. They kept the machines running at peak efficiency. It was a profound lesson in the difference between understanding and execution.

The scientists understood the physics. The young women executed the process. Years later, after the war had ended and the atomic bombs had been dropped on Hiroshima and Nagasaki, many of the Calatron girls learned for the first time what they had been doing. They had been separating uranium 235. They had been helping to build the atomic bomb.

Some felt pride, some felt horror. All were astonished that the fate of the war had in some small way rested in their hands. By July 1945, the Y12 plant had produced enough enriched uranium 235 for one bomb. That uranium became the core of Little Boy, the gun type atomic bomb dropped on Hiroshima on August 6th, 1945. The bomb contained approximately 64 kg of uranium, enriched to about 80% uranium 235.

When detonated, it released energy equivalent to 15,000 tons of TNT, destroyed five square miles of the city, and killed an estimated 70,000 people instantly. 3 days later, a plutonium bomb called Fat Man was dropped on Nagasaki. Japan surrendered on August 15th, 1945. World War II was over. The Y12 plant had achieved its mission.

The Calatrons had worked. But now came the reckoning with the silver. After the war, the electromagnetic separation process was abandoned in favor of gaseous diffusion, which was more efficient for large-scale production. The calatrons were dismantled. The magnets were disassembled. The silver had to be returned.

Every coil was carefully unwound. Every component was disassembled and cleaned. The silver strips were recovered, weighed, and inventoried. Workers ripped up floorboards beneath the machinery and burned them to recover microscopic traces. Every effort was made to account for every Troy ounce. In the end, out of 430 million troy ounces loaned, only 155,645.

39 troy ounces were lost. less than 0.036%. The missing silver had been lost to the process, vaporized, chemically bonded, embedded in materials that couldn’t be recovered. The treasury was satisfied. The silver was returned. The debt was paid. The final pieces of silver weren’t replaced until May 1970 when the last 67 tons of silver were swapped for copper and returned to the treasury nearly 30 years after the initial loan.

The story of the Calotron and the silver loan is more than a wartime anecdote. It’s a testament to what can be achieved when engineers face impossible problems with limited resources. The electromagnetic separation principle developed for the calotron lives on. Mass spectrometers today use the same basic concept Lawrence pioneered.

Ionized particles, pass them through magnetic fields, separate them by mass. These instruments are now used in medical research, environmental monitoring, forensic analysis, and space exploration. The Y12 facility still exists in Oakidge, Tennessee. It’s now called the Y12 National Security Complex, operated by the Department of Energy’s National Nuclear Security Administration.

It no longer enriches uranium, but it remains responsible for maintaining and producing components for every nuclear weapon in the United States arsenal. The story also reveals the extraordinary lengths governments will go to secure technological advantage during wartime. Borrowing 14,700 tons of silver from the nation’s reserves was an act of desperation and faith.

Faith that the science would work, that the engineering would succeed, that the investment would pay off. And it did. But the story of the Calatron girls reminds us that history’s great achievements often rest on the shoulders of ordinary people doing extraordinary work without recognition. Those young women sitting at control panels keeping needles in the green zones played a role as critical as the scientists who understood the physics.

Execution matters as much as understanding. If you found this story fascinating, you’ll want to hear more engineering problems solved under pressure. Next time, we’ll explore how P engineers and Stalog Luft 3 built a hidden ventilation system for the great escape tunnel, moving 200 tons of sand without the guards noticing.

The problem solving was absolutely brilliant. Hit that subscribe button so you don’t miss it. Drop a comment if you have questions about the Calatron or suggestions for future topics. And if you’re interested in more stories where engineering changed the course of history, check out our video on Enigma Codeereakers, who built the first programmable computer to crack Nazi communications.

Thanks for watching. Keep solving problems.