Convicted spy Harry Gold was Philadelphia's Benedict Arnold

In addition to often packing 195 pudgy pounds on his rather unimpressive 5-foot-6-inch frame, he was known for a distinctive waddle due to bad feet, and was cursed with an obsessive-compulsive personality disorder that led to endless rewrites and list-making.

But the shy, unassuming South Philly chemist who seemed invisible to most people was probably the most infamous espionage agent in the city's long history.

None other than FBI chief J. Edgar Hoover called him a "master spy" and the centerpiece of the "Crime of the Century," but ordinary Philadelphians could pass him dozens of times on the street and never take notice.

Harry Gold was no James Bond in appearance, clearly incapable of causing women to swoon and men to gaze in admiration.

But he was possessed of strong convictions, dutifully completed all his assignments and was shrewd enough in his dealings that no one would have ever expected that this otherwise undistinguished little man was turning over the secrets of the atomic bomb to the Soviet Union.

Born in a suburb of Bern, Switzerland, in 1910, Heinrich Golodnitsky was brought to America by his parents a few years later, and the Gold family eventually settled in the old Jewish area of South Philadelphia.

His father worked as a low-wage cabinetmaker at the RCA Victor plant in Camden, while his mother taught local children Hebrew and Yiddish. A timid, sickly child, Harry had few friends, but developed a love for books and a hefty intellectual appetite, tackling Dickens, Milton, Browning and Shakespeare years before other kids would be encouraged to do so by their teachers.

Known for his compassion and helpfulness, Gold often tutored other children and would provide similar services to school colleagues and workmates, without a fee, throughout his life. Chemistry became his passion, and he eventually honed his scientific skills at the Pennsylvania Sugar Refinery on the Delaware River.

He went to Penn and was a moderately successful student, but had to drop out to help his family survive economically during the depths of the Depression.

When he lost his job at the sugar company, as so many others did during the 1930s, the family, now including younger brother Joe, braced for eviction.

After weeks of fruitlessly searching for work, Gold was able to nail down a decent-paying job as head chemist at a North Jersey soap firm.

There was one string attached, however. Tom Black, a Penn State grad and the man who helped him acquire the position, wanted Gold to join the Communist Party.

Gold resisted. He wasn't politically inclined and wanted no part of the communists. He considered most of the party members Black introduced him to as oddballs and "whacked-out Bohemians."

After he was rehired at the sugar refinery back in Philadelphia, he still had to fend off the recurring overtures, even though he felt a deep gratitude to Black for helping to sustain his family through the worst of the Depression.

After numerous unsuccessful attempts, Black tried

a different strategy. He asked Gold to help the struggling Russian people to get their economy moving. He wanted him to turn over company secrets from the refinery.

Surprisingly, Gold did it.

Though this initial step of industrial espionage seemed relatively minor - he was just trading in techniques for manufacturing shellac, lacquers and alcohol - it would lead to a 15-year career of passing on industrial and military secrets, then becoming a handler of other Americans supplying intelligence to the Soviets and eventually being assigned to work with the infamous German-born British physicist Klaus Fuchs, who was at the center of the conspiracy to give the Soviets the secret of the atomic bomb.

Incredibly, Gold carried out his many secret tasks - meeting Soviet agents in New York and Center City Philadelphia, traveling to Ohio, Tennessee and New Mexico to pick up valuable classified information, and spying on Leon Trotsky's supporters - without friends, family members or law-enforcement authorities ever noticing.

The FBI took an interest in Gold only after his most productive years as a spy - 1935 to 1945 - were long over.

Gold represents the best and the worst of 20th century FBI operations. During the mid-1930s, the bureau initiated only about 35 espionage investigations a year. While they vigorously pursued the likes of John Dillinger, Ma Barker, and Bonnie and Clyde, foreign agents like Gold were having a field day picking the fruits of the U.S. industrial, military and governmental establishment.

World War II would dramatically shift the bureau and Hoover's focus, and their search in 1950 for Fuchs' American contact after the Soviets shockingly detonated their first A-bomb in 1949 - the largest manhunt in their history - would be a shining example of extraordinary detective work.

Gold went on to admit his guilt, name names and ultimately serve half of his 30-year sentence. When he died in 1972, friends, neighbors and work colleagues were brokenhearted.

They knew him only as an almost-invisible man. They'd never be able to comprehend how such an inoffensive, considerate soul had been involved in such a heinous case of treason.

Allen M. Hornblum, a Philadelphian, is the author of the just-published "The Invisible Harry Gold."