Tiny Sensors To Monitor Liberty Bell During Move

Jonathan Haeber
National Geographic News

July 4 weekend, 2003

How do you move a delicate American icon like the Liberty Bell without turning its famous crack into an infamous one? That was the dilemma National Park Service curators faced when they contemplated plans to move the fragile chimer later this October.

Casting impurities make the Liberty Bell prone to cracking. The bell first cracked in 1753 and was later re-cast. After it cracked a second time in 1846, two rivets were bolted into the bell. Today, a hairline crack runs above one of those rivets. Park Service staff says that, with enough stress, the bell could crack in half.

Which is precisely the result they don't want when they move the bell 200 yards (183 meters) from its longtime home at the Bicentennial Pavilion in Philadelphia, Pennsylvania, to a new display space at the Liberty Bell Center.

But thanks to new technology originally developed for the semiconductor industry, the bell may never crack again.

High-Tech Protection

The complicated process of moving the bell will incorporate micro-sensor technology that has been under development for nearly 17 years. MicroStrain, the Williston, Vermont-based company that developed the sensors with help from a National Science Foundation grant, will offer their services to the National Park Service for free.

Three miniature sensors will be attached to the bell to monitor stress and movement as fine as one-hundredth the width of a human hair. One sensor will measure stress that could widen the crack. Another will measure lateral, or shearing, stress. A final sensor will be placed inside the bell to measure any hazardous jarring or rocking movements.

The sensors will enable the bell to be slowly rolled on a cart to its new location, said Lynn McMinn, a MicroStrain spokesperson. If any hazardous stress occurs, data sent from the wireless sensors will sound an alarm from a nearby laptop computer.

A key to the sensors' development was to make them small enough for the job, said McMinn.

Special paper will be placed between the bell and the sensors to prevent them from scratching or causing chemical damage to the chimer's delicate surface.

As added precautions, Park Service staff conducted x-ray and ultrasonic tests using equipment from a nearby Boeing facility to help identify impurities and density differences in the Liberty Bell. Many metals comprise the Bell, which is mostly copper and tin. But the Bell also contains trace amounts of lead, zinc, arsenic, gold, and silver. Such impurities, together with flaws in its casting, are what make the Bell so delicate and prone to cracking.

The Legacy of Liberty

While the Liberty Bell is a well-known American icon today, it was not always so. It began service as an ill-sounding, Pennsylvania State-house chimer that rang to the tune of E-flat.

Residents didn't like the tone of the first bell, according to Phil Sheridan, a National Park Service spokesperson in Philadelphia. Two local craftsmen John Snow and John Pass offered to recast the Bell. "The second one, people liked," said Sheridan.

The Bell was not widely known by its present moniker until abolitionists adopted it as a symbol of the country's fractured unity at the time. The name "Liberty Bell" first appeared in an anti-slavery poem entitled, "The Liberty Bell," in the early 19th Century.

Liberty's New Location

Since its inception, the Bell has moved many times—including a transcontinental journey by train to San Francisco with numerous wayside stops in 1915.

Park Service staff said the Bell outgrew its present home.

"The only thing you can do in the [old facility] is see the Bell," said Sheridan. "This new facility will allow visitors a chance to spend time—to learn about the Liberty Bell, about the making of the Bell, about the symbols not only of liberty but the lack of liberty.

"The Liberty Bell is not only a symbol of people's freedom, but people looking for rights and freedom… It has now become an international symbol of freedom. What started out as just a bell in the State House of Pennsylvania has become an icon."

If all proceeds as planned, the Bell will move in October to the Liberty Bell Center on Independence Mall in Pennsylvania, just blocks away from the Bicentennial Pavilion—intact.

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