I was once approached by a gentleman who was about to invest in a "black box" that was purported to produce more power out than what was put in. He had taken the device to a local university for evaluation, knowing it would be rare to find a device that actually pulled off this magic.
What he showed me was a box with metering on the input and output, an AC plug, and an incandescent light bulb for a load. The kids (hopefully freshmen and not graduates) had measured currents and voltages and found their measurements substantiated the claims, but no one would put their name on a document stating this.
Having taught instrumentation and calibration for 10 years, I knew most all of the pitfalls one could run into while measuring power. First thing I did was drag out and dust off an old oscilloscope and look at the waveforms. The input, of course, was a good, clean sine wave, but the output was a ragged, distorted 60Hz mess.
I asked the potential investor to find out what meters were used to make the measurements. He made a call and gave me the brand and model. It was a typical shop-grade VOM that I was very familiar with. The meter's AC circuit is a peak detector diode circuit and the scale is calibrated to read RMS (.707xpeak).
I then grabbed a handy, true RMS meter and duplicated the measurements made previously at the university. Wow, what a difference! Instead of 20 percent more power out than in, it was almost the opposite. My test convinced the investor, so no money changed hands. I'm sure he is happy today that he didn't fall into the trap.
This entry was submitted by John Gray and edited by Rob Spiegel.
Tell us your experiences with Monkey-designed products. Send stories to Rob Spiegel for Made by Monkeys.
I have been asking the following question on the final exam of our Instrumentation and Measurement course since 2001. After going through a semester of various transducers, models, and calibration routines for all kinds of instruments I always get a couple of students that would still part with their money because of how much profit there could be made if it actually worked (or if they could convince others that it worked). The problem I am having is that as our smartphones continue to improve, I feel like Professor Harold Hill in the Music Man -- somebody may actually invent one...
"Soon after returning home for break, a good friend of yours from high school tells you that they plan to invest in a new invention they saw on the Internet. They invite you to get in on the action. The new invention is a small hand-held device about the size of a smartphone called the "Diecorder". The Diecorder is a new aid for people on a diet. You simply aim the device at a plate of food and it automatically scans it for the number of calories, grams of fat, and names and amounts of all of the vitamins contained in the food. You can get in on the ground floor of this invention for only 2000 bucks. Assuming that you have $2000 to invest, do you think this is a wise investment? Please state why or why not."
When your lab is only one scope, one meter, and one homebrew function generator and power supply, the scope is stage center. But this one wasn't working right.
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