At the bottom of the Thames River last week, Navy divers dug up the sensors and cables that measure magnetic signatures of military ships and submarines.

Even though the Navy is replacing the 20-year-old underwater range with a new one, most people don't know the range exists. And even those who have heard about it sometimes don't understand what it really does.

A steel-hulled ship is surrounded by a magnetic field. As the ship moves through the water and traverses the Earth's natural magnetic fields between the North and South poles, the ship's magnetic field changes.

The Navy checks its vessels to make sure the magnetism present in the hulls will not trigger magnetic mines or make the ships easier to detect.

The only Magnetic Silencing Facility in the Northeast is operated by the Naval Submarine Base in Groton. When military ships pass over the sensors and cables at the center of the channel, the information is converted into a computer file and Preston E. Tone-Pah-Hote Jr., who oversees the range operations, interprets the results.

The information is relayed to Navy officials, who determine whether the magnetization needs to be reduced. The range is also used by Coast Guard vessels and NATO ships.

Removing or neutralizing a magnetic field is a process known as degaussing, and the local range is often called a "degaussing range." It is even described as such on a chart in Tone-Pah-Hote's office.

But, Tone-Pah-Hote said, it technically is not. He does not have the ability to help a ship calibrate the degaussing equipment it has on board. Coils are wound in specific locations within the hull and the electric current that flows through them can be adjusted to reduce the ship's effect on the Earth's magnetic field.

The range is a "check range" since it checks ships' magnetic characteristics. Because those measurements fall under the umbrella of degaussing, the name "degaussing range" seems to have stuck.

Surface ships in the Atlantic normally go through a similar range in Norfolk, Va., or one in Mayport, Fla. The USS Bainbridge (DDG 96) and other ships used the local range when the one in Norfolk was repaired from 2010 to earlier this year. A magnetic treatment facility in Kings Bay, Ga., can minimize the level of permanent magnetism.

Tone-Pah-Hote, who served 24 years in the submarine force, estimates he takes about 300 measurements a year - anytime a naval ship enters or leaves the harbor he checks it. His office on the water's edge near Fort Trumbull has a great view, but none of the quirky equipment one might expect when hearing the name "Magnetic Silencing Facility."

The work on the new range, which began Nov. 1, is expected to cost $3.5 million and be finished in August. Tone-Pah-Hote said he does not expect any channel closures while the work is being done.

j.mcdermott@theday.com