Because Paul Horowitz has always loved gadgets, and because he had always wondered how much arsenic was on the skin of unwashed apples, he didn't notice the two graduate students talking as he leaned over his newest creation, the proton microscope. They were impatient to try their own more traditional experiments--analyses of ancient pottery shards--and they had driven from MIT to Lincoln Laboratories in Lexington earlier that morning expecting that Horowitz would not be there, that his new gadget would be free. Horowitz, who had come on impulse from his home three minutes away, did not seem pressured by this. Nor did he seem aware of a rapidly approaching lunch date, or the morning appointment for which he was already late. A meticulously considerate man under most circumstances, Horowitz was grinning at a shriveling slice of apple skin while people were waiting for him, ten feet and ten miles away.

Horowitz was concentrating on a helium-filled acrylic box connected by a series of pipes to an enormous Van de Graaf generator. The generator was splitting hydrogen molecules to produce protons, which were accelerated down the pipes and into the box. There the stream smashed into the apple skin, turning the helium in its path an eerie purple. The apple skin was browning and curling up under the bombardment. "Wow," Horowitz said. "This baby is taking quite a beating."

Horowitz looked up at the counters and scopes that caught the radiation bouncing off the apple, measured pulses, and eventually spit out sets of numbers that revealed which heavy elements were present. He made some quick calculations, and detected iron and calcium--he expected them. He looked at a third set of numbers, and found that it was arsenic--about 30 parts per million.

His observers were shocked, but Horowitz showed no surprise. He didn't know how much arsenic was dangerous, but he had heard that many orchards used an insecticide of arsenic and oil that could easily be washed off. The results would be good cocktail party conversation but little more, and Horowitz was already losing interest. He had a new idea.

"I want blood," he said, and left the room in search of a donor and a knife.

The graduate students looked at each other and smiled, and suddenly they didn't seem as hurried. Blood was just as mundane as the other samples Horowitz had analyzed that morning--Gainesburger, vitamins, chicken bullion and maple syrup. But they had remembered how much they liked Horowitz and his bursts of enthusiasm that sometimes went beyond "real physics," and they were pleased that his microscope was working so well.

"Paul is good because he's really interested in his work," one said. "When most physicists design an experiment, they stick to the minimum. But Paul will come up with some idea, he'll play around, and sometimes something important will come of it."

"And besides," he added, "Paul is sort of a genius."

Horowitz doesn't like that word. He knows his intelligence--and particularly his ability with electronics--sets him apart and increases expectations of him, but these vocational hazards of brilliance don't bother him. Like most geniuses, he doesn't mind being one--he just doesn't like being called one. It embarasses him.

Nevertheless, people have been calling Horowitz a genius--a boy genius, to be exact--since he was seven, and gurgled over the air in Morse Code, the youngest ham radio operator in the country. The talk continued as Horowitz went through Summit High School in New Jersey and Harvard, building gadgets in his free time like refrigerators with no moving parts and advanced telescopes. And when he began dating a tall blond nursing student named Carol Grodzins a few years ago, colleagues often approached her at parties and whispered, as if sharing a delicate secret, "Paul is such a genius"

Some of the whispering has died away as Horowitz has settled down. He married Grodzins two years ago, and she says that since he got tenure this year, fewer people pull her aside to murmur their awe. Horowitz generates less commotion just because he will probably be around Harvard for a long time.

This makes Horowitz feel more comfortable in the Physics Department, which is filled with boy wonders now grown up or fizzled out. And although other faculty members describe him to outsiders as "exciting" and "brilliant," he particularly enjoys their weekly Monday luncheons, where the Physics professors sit and casually discuss their work with none of the pretensions of prodigies.

But there has been no refuge from the boyish image, because with the peach-skin complexion and endless energy, Horowitz seems younger at 32 than most of the undergraduates in his electronics course, Physics 123. While they slump unshaven and bleary-eyed on their lab stools on the second floor of the Science Center, Horowitz stalks the room in short, quick steps, like a freshly-scrubbed Boy Scout armed with a calculator instead of a pocket knife. He pushes aside his thick, perpetually mussed hair, and talks in bursts about electronic circuits and gadgets.

The words tumble out in a rush, full of hard advice in terms of "bucks" and hours, and mixed with engineering jokes about explosions and electrocutions. "That's a good way to get yourself blown up," Horowitz says as he modifies a circuit plan on the blackboard. His students, who had to fight their way into the course, laugh briefly but quickly return to their slouches and listen carefully, sifting through the secrets about studying stars, building stereos, and what Horowitz might have found on an apple skin the other day.

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