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A Tale of Two Lamps

Tue, 04/30/2013 - 9:34am
Karl Stephan, Consulting Engineer, Texas State University, San Marcos

A few months ago, my wife bought a lamp at an office-supply place to light up the table where my father-in-law plays dominoes with us. He’s 87 and his eyesight isn’t what it used to be, so having lots of light in the right place is important. The ceiling light doesn’t quite do the job.

I was impressed with her purchase. It’s an elegant-looking design that unfolds. When it’s folded it’s just an oblong white plastic pillar on a round stand, about ten inches tall. It unfolds like a book hinged at the top, and when you unfold it you find that the upper inside surface is lined with thirty white LEDs that turn on as the hinge opens up. You can set it at a convenient angle and it casts a smooth, even white light over several square feet.

It cost only twenty bucks or so, but it had a significant disadvantage in my view: it ran only on batteries, unless you bought a separate power supply (not provided). After finding that the batteries would last maybe four or five hours at the most, I did a quick calculation and concluded to buy a plug-in “wall wart” for the thing. I had to go to Radio Shack and assemble a custom one from their array of general-purpose power supplies, and spent about $25 on that. But once I got the lamp hooked up to the mains, it worked fine, and we have probably used it daily for the last six months or so. . .

Until one day last week, when I opened it up and it flashed for a second and went out. Well, I had to see what the problem was. Five minutes with a screwdriver in the garage revealed the cause. There are two small stranded wires leading from the base of the unit to the panel of LEDs in the upper part. One of the wires goes to a switch on the hinge that turns it off when you fold it up. The wire was soldered to the switch terminal and glued down to the case with some kind of silicone glue, so that the continual flexing at the hinge would not bend the wire where it goes into the solder joint at the stationary switch terminal. Experience has taught me that if stranded wire going to a solder joint bends back and forth, the strands will start to break after only a few bends. The glue holding down the wire had come loose, allowing the wire to move at the joint, and the wire eventually broke off.

I thought about fixing it, but in the meantime we needed something to light up the domino table. So I switched to a backup lamp.

The backup lamp has a bit of history attached to it. To the best of my knowledge, I bought it in the late 1960s at a Radio Shack in Fort Worth, Texas, back when there were only about ten Radio Shacks in the whole country. I guess you would call it a generic Tensor lamp. I find from a New York Times obituary that one Jay Monroe, a Cornell-educated engineer (like myself), liked to read in bed, but his wife objected to the large conventional lamp he used. So he took a twelve-volt automotive tail lamp, stuck it in a kitchen measuring cup for a reflector, and mounted it on an arm attached to a box containing a transformer that stepped the line voltage down to twelve volts and also served conveniently as a weight to keep the thing from tipping over. His invention, which he called the Tensor lamp, proved so popular that elite retail outfits such as Hammacher Schlemmer began to carry them, and the name “Tensor” briefly enjoyed so much popularity that any high-intensity low-voltage lamp was called a Tensor lamp, whether or not it was made under Mr. Monroe’s patent.

My Tensor-like lamp follows the same basic pattern: a round plastic base containing the transformer and a switch, a flexible spiral-steel “gooseneck” arm, and a conical black shade. During one of our several moves, it was packed so that something rubbed a hole in the shade in transit, but I patched it with some electrical tape. The thing is now about 45 years old, and still works fine.

Lamps evoke their eras. Kerosene lamps proclaim the 19th century just as clearly as incandescent lamps connote the 20th century. Many people of my grandparents’ generation could recall the day, usually sometime between 1900 and 1930, when their houses were first wired for electric lights. When the LED lamp was working, I would often meditate, during breaks in domino games, that it was the first of many more LED lamps we will use as incandescent lighting gradually becomes a thing of the past, just as kerosene lamps did for my grandparents.

I wonder if I should attach any significance to the fact that two lamps purchased 45 years apart for similar prices (in non-constant dollars: I think I paid something like $9.99 for the Tensor lamp) to fulfill similar purposes, had such radically different lifetimes. One is still working fine, having delivered many thousands of hours of service; the other worked for maybe a hundred hours and failed. The new lamp is more complex, true. But it doesn’t take a rocket scientist to figure out that silicone glue doesn’t stick to smooth plastic that well, and this kind of problem would have showed up in a series of reliability tests—say bending the hinge ten thousand times.

But except for extraordinary jobs like satellites, nobody does that kind of testing any more, especially not for inexpensive consumer products. The long-term-reliability test for most such items is called the marketplace, and the test engineers are the people who buy the product first. And face it—99 out of a hundred people who buy an LED lamp like this that breaks will simply throw it out and buy a new one.

I finally had some time a few days ago to fix the LED lamp. It was a tricky job involving a utility knife, a soldering gun, and some bathroom caulk that will stick to ceramic tiles, so I’m pretty sure it will stick to the plastic inside the lamp. It’s working again now, and we are back in 2013 rather than 1967 as far as lighting the domino table is concerned. In the bargain, I’ve learned something about how the consumer market for lamps has changed over the last four decades. And I’m not too happy about what I’ve learned.

Sources: The New York Times article “Jay Monroe, 80, Engineer Who Invented Tensor Lamp, Dies” appeared on July 2, 2007 and can be found at http://www.nytimes.com/2007/07/02/nyregion/02monroe.html.  The word “tensor,” by the way, would be familiar to a Cornell-educated engineer, who would know it as a type of mathematical object that transforms one vector into another vector.

This column originally appeared on the Engineering Ethics blog, you can find it by visiting http://engineeringethicsblog.blogspot.com/.

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