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His company, MicroStrain Inc., had placed five sensors on the famed Liberty Bell in Philadelphia. The bell's caretakers at the National Park Service wanted to move the historic treasure, and were using Arms' devices to measure strain on the bell's crack. One wrong reading, and movers might put too much stress on the bell and split it apart.

Observers from the National Park Service, National Science Foundation, city of Philadelphia and the media looked on. Arms and a co-worker used a laptop computer to monitor vibrations recorded by the sensors as the bell was lifted off its stanchions and moved 10 feet.

Pressure, indeed.

Karie Diethorn, chief curator of Independence National Historical Park, which houses the bell, admits she felt a small pang of uncertainty as the bell was hoisted into the air: "I can't really say I was nervous. . It was more a question of wondering what the bell's response would be."

The bell responded well. MicroStrain recorded only 1 micron of shearing motion from the cracked sides of the bell rubbing against each other, negligible for a 2,000-pound bell with bronze walls 2 inches thick. Arms said simply, "It went very well."

The March 13 move was merely a trial run. Next up is an ambitious plan to move the bell 200 yards down the street to a new museum and display center that is scheduled to open Oct. 9. Arms will be on hand again with his displacement sensors to ensure the bell's safety. Once it lands in its new home, MicroStrain will leave an accelerometer inside the bell permanently to measure how much it's jarred by passing pedestrian and auto traffic.

Arms said he was approached by the Philadelphia Museum of Art, which owns the Liberty Bell. (Diethorn and the National Park Service only maintain it for public viewing.) Museum officials knew they would need some type of motion sensor for the delicate observation and found MicroStrain, nestled five hours north in Williston, Vt., through a Web search.

Arms, MicroStrain's president and founder, immediately volunteered to do the work for free. "It sounded like a fun project," he said - and extensive publicity around the project doesn't hurt either.

MicroStrain put five sensors on the bell. Two displacement sensors measured strain along the crack itself: one to measure horizontal motion if the crack grew wider, and one to measure shear if the sides starting pulling against each other. Three accelerometers tracked the bell's physical motion as it was moved.

"We're using that move event to start a process of data collection," Diethorn said.

During the official move in October, the 200-yard trip will take five hours, and Diethorn hopes to make a public spectacle of the event as the bell moves into its new home. The bell has been moved several times, most recently in 1976. This is the first time sensors have been used to monitor its safety.

Arms founded the company in 1986 while he was a graduate student studying biomechanical injuries. He made his own sensors to track the strain on people suffering from torn anterior cruciate ligaments - the excruciating knee injury that can sideline skiers or football players for months. Doctors took note of the equipment. "They asked where I got the sensors, I told them I made them, and they asked if I could make some for them too," Arms said.

MicroStrain's business falls along three lines: displacement sensors, like those used to measure strain on the bell; orientation sensors, which Arms originally intended for paralysis victims learning to move again; and wireless sensor networks to measure strain on large structures such as bridges or dams.

The most promising product line so far, Arms said, is the wireless sensor systems. Many customers are departments of transportation; another customer uses the sensors to make equipment that mounts car doors precisely on the auto chassis. "We're finding really fast growth here," Arms said.

Arms said MicroStrain today has revenues of several million dollars annually and has been profitable since he started the company. Today it has 18 employees. (The company has never accepted venture capital.) Revenues grew 18 percent in 2002, and he expects them to jump 50 percent in 2003 - in fact, he wonders whether he should hire more people to accelerate the company's growth even more.

"I worry whether I'm holding the company back," Arms admits. "I want to grow the company faster, but not too fast."

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