I wonder if this is one of the reasons why the phone system standardized on 48 volts. With DC, you need to get above about 60 volts to have any chance of lethality. If someone were to grab the barbed wire, they'd get a jolt, but it wouldn't kill them.

No. These early phone systems were "Magneto", not the modern "Central battery" (CB).

Each subscriber had their own batteries (No. 6 cells), so there was no DC on the line.

There was Ring Current of course, but that has always been well above 50 Volts.

When I was in kindergarten in the late '50s, I pranked my class with electricity. I had a big fat electrolytic capacitor that I charged up before bringing it to school, and a screwdriver.

I don't remember the exact voltage I charged the cap up to, but it was definitely only a few volts.

So for show and tell, I explained how electricity worked, and then I grabbed one of the cap's terminals in each hand and started twitching and shaking like I was being electrocuted! Finally I somehow managed to break free, took a deep breath, and told my classmates "don't be as foolish as me."

To protect them from the danger, I then took the screwdriver blade and shorted it across the capacitor's terminals, with a big spark and a loud bang! Clearly, this cap had enough power to kill you.

I told my classmates, "Now it's safe. I discharged it. You can touch it now."

And they did. Nobody was harmed.

School was great back in those days. When I got to third grade and I wanted to etch a printed circuit board, I told my teacher I needed a tank of nitric acid, and she got it for me! But that is a story for another day.

This is a great story.

How were you able to learn about electricity at such a young age?

I grew up on all-in-one kits from Radio Shack and was heavily influenced by my grandfather (lots of HeathKit builds, ham radio, and model railroads).

Oh, I loved those Radio Shack all-in-one kits too - the spring connectors made it so easy to wire stuff up. Also did a bunch of Heathkits a few years later - color TV for the family, ham radio gear for myself, etc.

I was trying to remember my first experience with electricity and electronics, and it finally came back to me. Whenever our TV or radio went on the fritz (which happened quite regularly), I got to pull out all the tubes, and my dad would take me to the local convenience store where they had a tube tester. I would put each tube in the tester and learn what to set the dials to and what to look for on the meters and figure out which tube was bad. It was a great adventure!

73 de Mike WJ6V

Yes, it is technically true that it is the current that disrupts the heart rhythm. Theoretically, a 9v battery can supply enough current to cause a potentially fatal heart arrhythmia, if it's shorted through a person's blood. But it's also being pedantic.

Consider Ohm's Law: `I = V/R`. Why is it so frequently printed in this way, rather than the simpler `IR = V`? If we don't care about simplicity, why not `V = I/R`? What is special about `I = V/R`?

Resistance--be it your work load or your human fleshy bits--is usually roughly considered a constant--or at least a known--for a given application. And most power sources that you'll encounter in the wild are voltage-controlled and able to supply practically as much current as you want (aka "more than enough to kill you"). So Voltage is the independent variable, leaving Current to be the dependent variable.

While it is true that "it's the current that gets ya", it's the voltage over which we have control, so we tend to focus on that instead.

I always learned Ohm's law as, "V = I*R". You know, over the handful of classes that included it at some point. When you say 'Ohm's law,' my brain says 'vee-equals-aye-arr.' So...I did check wikipedia first to make sure I wasn't having a stroke, but I think 'V=I/R' is a typo.

Anyways, that does seem fairly self-explanatory. More volts means more current with the same resistance. More resistance means less current with the same voltage.

> Why is it so frequently printed in this way

FWIW, I was always taught Ohm's law as V=IR (GCSE and A-level Physics/A-Level Electronics/BA Computer Science). I don't recall ever seeing it printed as I=V/R except where a calculation called for that form.

In German, Ohm's Law is jokingly called 'Geller Triangle' (after Israeli illusionist Uri Geller).

http://www.mec-castrop-rauxel.de/bilder/gru04_02.png

By covering the magnitude you're looking for, you get the formula to calculate it.

> If we don't care about simplicity, why not `V = I/R`?

You should rewrite this, as the equation is wrong.

Yes, but current is dependent upon voltage and resistance.

You can have current without one or the other.

It's like saying that oxygen is the cause of a big fire, not the fuel.

I was always taught...

"It's the volts wot jolts, its the mills wot kills."

Yup, the resistance of your skin drops in the rain or heavy perspiration. While it's a nice mitigating factor it's better to just not mess around with anything that can deliver high voltage or high current.

There's a 1999 Darwin Award for this, even. A 9v multimeter killed someone because he broke his skin with the probes and lost the benefit of that resistance. That's pretty much a worst-case scenario, but it's still a vivid reminder that "low voltage" is not "safe".

Hmm. For whatever reason this did not show up when I posted my own comment... I'm impressed one can run Ethernet over barbed wire.

Well ethernet didn't start over twisted pair. It started over thick coax. Something that is not that far from setting up a couple of radios.

Now you are more likely to get the minimum speed this way (1MB?) than the GB stuff some of us have come accustomed too.