A lot of very hard work is going on at the National Portrait Gallery in Washington, D.C.

A muscled guy in an undershirt tightens a big bolt with his wrench; a farm worker bends almost in half, filling his sack with cotton; Rosie the Riveter rolls up her sleeve to tackle her factory job. They’re all part of an exhibition called "The Sweat Of Their Face: Portraying American Workers."

But not all the laborers are big and burly.

A forlorn young girl — she can’t be more than 11 or 12 — stands at a long row of spools of thread mounted on a big piece of complicated machinery. Photographer Lewis Wickes Hine wrote her name and height on the back of the picture: "Sadie Pfeifer, 48 inches high, has worked half a year." She had a job in 1908 at the Lancaster Cotton Mills in South Carolina. You can almost hear the noise, feel the heat.

"And there she is, this little girl, alone, facing an enormous machine," says Dorothy Moss, who curated this show with David C. Ward. She says Hine was a crusader, and his cause was to abolish child labor. "He would often disguise himself as a Bible vendor or newspaper deliverer, other professions, to get into these mills."

Hine put himself at risk to take these pictures, and, with the camera as witness, reforms and regulations were enacted. This exhibition showcases centuries of American workers. They are, as Moss says,"the people who were building this country, who may be on the sidelines, who…