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NSF Press Release


NSF PR 03-37 - April 1, 2003

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Liberty Bell Passes Stress Test

ARLINGTON, Va.—How do you move a wounded, 2,080-pound patriot? Very carefully. Recently, under the watchful eyes of curators, conservators, surveyors, and engineers, a team of riggers deftly lifted the fragile Liberty Bell off of the supports on which it has been resting for a quarter century and confirmed the Bell can be safely moved into its new home this fall.

Recognizable across the globe with its famous crack, the 250-year-old bell is remarkably frail for its size. The metal is far from pure. Rather, it is a mixture of various metals, voids and contaminants, intermingled "like a piece of fruitcake," said Karie Diethorn, Supervisory Museum Curator at Independence National Historical Park in Philadelphia. The park is operated by the National Park Service.

Of particular concern is a hairline fracture that extends from the main crack to the rear of the Bell—if the hairline fracture splits, so does the icon.

"In terms of assessing its fragility, we really don't know, so our plan is to subject it to the least amount of stress possible," said Diethorn.

The Liberty Bell has traveled several times, even across the country, before settling in the 1976 Bicentennial facility where it now rests. The latest move will bring the Bell closer to Independence Hall, its original home.

Although the Bell will move only 200 yards into the new museum, curators are taking every precaution to assure its safety. Steve Arms, president of MicroStrain, Inc. tracked movements of the metal along the crack using tiny, wireless sensors he developed as part of the NSF Small Business Innovation Research program. The devices are extremely sensitive, able to detect motion as small as 1/100th the width of a human hair.

The researchers used custom attachments to place two metal sensors, originally developed for the semiconductor industry, on the metal around the Bell's main crack. One sensor monitored stresses that could widen the crack, another monitored stresses from shearing motions. Stresses along the main crack will warn of dangers that could spread to the hairline fracture. The team also hung a third sensor inside the Bell to monitor potentially jarring rocking motions.

The Bell surface is extremely sensitive to both scratching and chemical damage, so the team could not use glues or directly touch metal to metal. Ultimately, Andrew Lins, chief conservator of the Philadelphia Museum of Art, worked with MicroStrain's Steven Mundell to carefully clamp the devices to the Bell, inserting a special paper where the sensors would otherwise touch, and potentially mar, the surface.

Fortunately, Arms's sensors detected no motions greater than several millionths of a meter, tiny movements that do not seem to stress the Bell.

"We can use the data that we collected to create upper and lower limits for vibration, and sound an alarm during the move to warn the riggers if the limits are exceeded," said Arms. "That will allow us to move the bell to the new location and basically mimic the conditions that we got here today, which we feel are safe." he added.

Because of the nature of the move, all of the technology had to be small and wireless. Despite these restrictions, the devices are capable of continuously streaming enormous amounts of data to a laptop that Arms monitored at the site. However, the proprietary software is web compatible, so all of the monitoring could theoretically have been done from anywhere in the world.

MicroStrain's work with the Liberty Bell, and all of the stress sensors and equipment, were provided pro bono. "I just thought this would be a fun project to take on, and a challenging one," said Arms, "we wanted to try to protect a national treasure."

The Liberty Bell was forged in 1752 at Whitechapel Bell Foundry in England — the same foundry that forged Big Ben (the 13-ton and, ironically, cracked bell within the Great Clock of Westminster) and the bells of Washington National Cathedral.

According to historians, the Pennsylvania Assembly probably ordered the Bell in 1751 to commemorate the 50th Anniversary of Pennsylvania's Charter of Privileges, religious and political freedoms that the state had enjoyed since its founding. The Assembly asked for the biblical inscription: Proclaim Liberty throughout all the Land Unto all the Inhabitants thereof - Leviticus 25:10

The Bell was to hang in the clock tower of the Pennsylvania State House. The State House was later renamed Independence Hall, and the Bell — once known as the State House bell —was renamed "Liberty Bell" by abolitionists who adopted it as their symbol in the 1800s.

The Bell cracked soon after its arrival in Philadelphia and was recast (from the original metal) by local craftsmen John Pass and John Stow in 1753. Even that casting had problems, and the Bell that now rests in the display hall is the third casting.

Over the next century of continual use, a crack had begun to form that had to be filed down to prevent a jarring noise when the Bell was struck (the filing marks are still apparent today). In February, 1846, the Bell was repaired and rung in commemoration of George Washington's birthday. The repair is visible today as a wide jagged crack spanned in two places by rivets. While it once rang the pitch of E-flat, the Bell has not pealed since 1846.

Liberty Bell Videos

The following videos require the RealPlayer Plug-in. Click on the real player icon to get your copy of the RealPlayer Plug-in.

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Segment 1
Attaching Sensors
Motion detectors attached to the Liberty Bell

Play Segment 1


Segment 2
Riggers Prepare for Lifting
Preparing to lift the Liberty Bell

Play Segment 2


Segment 3
Lifting the Liberty Bell
Lifting the Liberty Bell

Play Segment 3


Segment 4
Comments from Steve Arms, President, MicroStrain, Inc.
Steve Arms

Play Segment 4


Segment 5
Comments from Karie Diethorn, Supervisory Museum Curator at Independence National Historical Park in Philadelphia
Karie Diethorn

Play Segment 5



Public Relations, MicroStrain Inc: Lynne McMinn, (802) 326-4215, Public Affairs Officer, Independence National Historical Park: Phil Sheridan, (215) 597-0060;

Official National Park Service Web site on the Liberty Bell:

MicroStrain Web site:

Big Ben at Whitechapel Foundry Web site:

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Video Available Liberty Bell Videos

Liberty Bell in its current pavilion
The Liberty Bell in its current pavilion located on Market Street in Philadelphia, Pennsylvania. Independence Hall is visible in the background on the left.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 421KB)

view of the crack
A view of the famous crack as viewed from within the Liberty Bell. The crack was originally a hairline, and by the 1840s it was a threat to the bell. In an attempt to repair the crack, part of it was drilled out (creating the widened, visible crack familiar to visitors) and rivets were inserted to hold the two sides of the fissure together. Unfortunately, the "repair" also deadened the sound, and the Bell can no longer ring. Also visible within the Liberty Bell is the spider, a structural brace that helps the Bell support its own weight-much like a flying buttress helps distribute weight away from a cathedral roof. Without the spider, the fractured Bell might collapse on itself.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 209KB)

Andrew Lins and Steven Mundell
Andrew Lins (sitting), chief conservator of the Philadelphia Museum of Art and metals conservation consultant to the National Park Service on preserving the Liberty Bell, worked with MicroStrain's Steven Mundell to carefully clamp sensor devices to the icon.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 517KB)

a view of the sensors
A close-up view of the MicroStrain NANO-DVRT wireless sensors clamped to the Liberty Bell (the top and bottom sensors are the same design, but oriented perpendicular to each other). With even tiny motions of the crack-on the scale of millionths of a meter-the metal rod shifts its placement. The movement is detected by electrical coils, and that information is transmitted to a wireless receiver. The wires visible in this image are connecting the sensors to a wireless transmitter inside the Bell.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 684KB)

a close-up view of the spider
A close-up view of the spider, a structural brace that helps the Bell support its own weight. Also visible is the wireless transmitter that conveys the signals from the sensors on the crack to the computer that is recording the stresses (black box on top in blue tape) and a MicroStrain G-link sensor that detects rocking motion (black box near lip of Bell).
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 403KB)

Riggers position tackle
Riggers from the George Young Co. carefully position tackle that will be used to hoist the Liberty Bell.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 483KB)

The Liberty Bell hangs free
The Liberty Bell hangs after being hoisted out of its supports. Visible are the straps that hold the Bell, with protective padding separating the strap fabric and the Bell metal.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 480KB)

MicroStrain president Steve Arms
MicroStrain president Steve Arms monitors the incoming stream of sensor data as the Bell is lifted.
Photo Credit: Curt Suplee, National Science Foundation
Select image for larger version
(Size: 384KB)


Liberty Sensor in Action
Animation depicting the motion of the DVRT sensor core within the NANO-DVRT sensor. As the core moves further into the electrical coils within the block at the base of the diagram, the coils change the manner in which they conduct current. The current change is proportional to the motion, and the device can be sensitive to motions of less than one millionth of a meter.
Photo Credit: Stephen Pendo, MicroStrain, Inc.

initial measurements on the Liberty Bell
Steve Arms conducts initial measurements on the Liberty Bell.
Photo Credit: MicroStrain, Inc.
Select image for larger version
(Size: 42KB)

model of the Liberty Bell crack
A wooden model of the Liberty Bell crack which the researchers at MicroStrain used to design their custom sensor mounts.
Photo Credit: MicroStrain, Inc.
Select image for larger version
(Size: 49KB)

Larger versions (Total Size: 3,672KB) of all images from this document

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