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.
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