Edison’s Tasimeter

Thomas Edison, Infrared Pioneer

I ran across an unusual infrared light meter today in an article from Applied Optics, Vol. 18, No. 22, November 1979. It is interesting on several levels because it was not patented by Edison, who patented nearly everything he did, and was instead given freely to “science.”

The little-known episode deals with Edison’s participation as an IR astronomer in the Draper Eclipse Expedition to Wyoming in 1878. It is well-timed for our purpose, for it samples the young Edison at the peak of his creative life, in a brief interlude sandwiched between the phonograph (1877) and the incandescent lamp (1879). At the time he worked in the almost magical atmosphere of his first laboratory complex in Menlo Park with a hired staff of twelve loyal assistants: a Camelot where at thirty-one he was both King and Merlin.

. . .

The tasimeter had come to life in a typically Edisonian way: not through any determined effort to
discover what was eventually found nor through pure chance or serendipity. Rather he arrived at it through a chain of loosely linked discoveries that led him through several fields, which he was willing to follow hand over hand wherever it took him. He could not always afford this kind of free adventure or the diversion from other specific assignments, but when he found himself upon this course he was at his very best. It was a style of research that fitted Edison well because of his breadth and imagination, his freedom from disciplinary boundaries, and his dogged determination to track down
every detail in any of his experiments.

In 1873 Edison was working on schemes to speed up telegraphic transmission in the Atlantic cable. To
duplicate in his laboratory the electrical resistance of the 3000-mile cable, he had experimented with a variable resistor made of compressed powdered graphite and had noted the marked electrical sensitivity of powdered carbon to pressure. Four years later, while working on ways of improving Bell’s telephone, Edison recalled his Atlantic cable experience and adopted compressed carbon as a telephone transmitter material; thus was born his “carbon button” telephone. There was a problem, however. The hard-rubber telephone mouthpiece was then handheld; heat from the hand transmitted as pressure to the carbon button produced loud static on the telephone line. He first tried to avoid the problem by switching to a cast-iron mouthpiece; this only changed the character of the noise, producing creaky sounds that he attributed to (thermal) motions of iron molecules, which he called “molecular music.”

Edison realized he was being beaten in the telephone game by thermal expansion. Given a lemon he made lemonade. For here was a new device that transformed temperature to resistance: a sensitive heat detector with a readout in the realm of electrical measurement where he was master. It could be made to detect radiant heat by putting the carbon button in mechanical contact with an expandable rod on which heat was focused.He could make it supersensitive by placing the button in one leg of a bridge circuit. To be sure there were a few mechanical details to be worked out and materials to be tried, but the principles were easily in his grasp. This much he knew when he wrote Professor
Langley to offer a gift to science. Who cared that it was not at first appreciated? Another of nature’s wily signals—this time heat— had wandered into the jaws of one of his traps, and he had it by the hind leg.

To the final instrument Edison added a small conical horn to focus heat and a dial to record compression, presumably for calibration. For the expandable rod he chose vulcanite-the same material that had caused the original problems in the telephone transmitter. The finished instrument was made of machined brass and was small enough to be held in the hand.

Of course, it should also be known that Edison had almost no respect for either education or pure theoretical science in any form:

Edison’s feelings toward the scientists and formal education in general— is better preserved, in opinions he freely gave in later years:

I would’t give a penny for the ordinary college graduate, except those from institutes of technology . . . they aren’t filled up with Latin, philosophy, and all
that ninny stuff”

His especial disdain was directed at theoretical or mathematical physicists, who seemed to typify dreamy dilettantes in ivory towers. In Edison’s pragmatic mind there was no question as to the relative worth of theoretical vs practical science: “I can hire mathematicians at $15.00 a week, but they can’t hire me.”

Soon, I’ll have to get back to my “ninny stuff.”