Having taught Map/Compass & Survival for over 15 years...the 9V battery and steel wool demo is one that always wakes up the sleepers in the students. I always show this trick to my classes. The finer steel wool works best, as you are causing a short circuit via the steel wool, and the higher the resistance the better...ie. fewer, thinner strands of steel wool. STeel burns, of course, which is how it is cut...get it hot and supply O2. so, touch a few strands of the steel wool to the 9v battery terminals, and when it glows, blow on it. Be sure to have it where you an drop it safely...as you will SOON.
I grew up in the carbon zinc era where batteries had suffciently high internal resistance and low current capability not to be much of a fire threat. I had to adapt to the new cell chemistry that could be quite dangerous if mistreated. As for 9V transistor radio batteries, they too started off as carbon zinc piles and only later on became alkaline power houses.
About the only batteries with any clout in the carbon zinc days were the very large and heavy ignition batteries that looked like extremely overgrown D cells on steroids but with screw terminals on top. We'd use a couple of those or maybe a hotshot battery, another oversize carbon zinc style, for starting model airplane engines (to heat the glow plugs). To get an idea of the size, picture a cylindrical D cell battery the size of an 18 ounce beverage bottle.
While a 9 VDC battery isn't much of a biological hazard, especially when routing electrons through your tongue, the risk of death only occurs if suffcient current can flow across your chest cavity inhibiting normal heart electrical activity. It only takes around 100 milliamperes to block the heart from beating. More dangerous is 60 Hz AC as it'll induce fibrillation, the heart fluttering rather than pumping.
Back to the good old days of carbon zinc, the only batteries that could deliver sufficent current through your skin resistance to maybe stop your heart were the B batteries for portable vacuum tube radios. Yes, radios at one time used these odd glass tubes filled with nothing to act as amplifiers and diodes. But to get electrons to traverse the nothingness required significant voltage. B batteries might have contained a suffcient number of cells stacked in series to produce over 90 VDC per battery. Put two or three in series and you got plate voltages in the range of 180 to 270 VDC. Ouch!
About the only household electronic appliance these days with vacuum tube technology is the microwave oven. Do not play with its innards if all you know about are IC's and transistors! The magnetron tube has a lethal high voltage power supply to make it do its power RF oscillation at 2.4 gigahertz.
Yes, I've spent a whole lifetime with masking tape over the 9v batteries in the junk drawer. It was always just something you do when you put a 9v away, like licking the ends before poping them into a toy robot or train.
Tim is certainly correct about the fire starting kit used by some scouts. Fine steel wool and even a weak battery would be good for a few lights. The trick was to pack the battery in a plastic baggy with the dry tinder, and have the steel wool in a different plastic baggy. As with most fire-starting mixes, the ingredients must be kept apart until time to start the fire. As far as I can tell, though, it is not an officially sanctioned method of starting a campfire.
I have wondered about doing that same thing but using a 120 volt cord and a lot more steel wool, but I have not experimented with it. It might approach the "exploding bridgewire" method used in many military systems, but it would probably be less predictable.
I have not done that with a battery in the pocket, because I read about somebody else doing it and I decided to avoid that problem by putting masking tape over the battery terminals. That took care of that.
But I had a dumb co-worker who put about fifty dollars worth of coin cells in a drawer with nothing to prevent short circuits, and who was offended when I was critical of doing something that ruined all of those batteries. They were not the rechargable ones, they were the expensive long life ones. MY assertion was that it was much smarter to leave them in the original packing material until time to install them. His STUPID defense was that it was not as neat.
I grew up doing it all the time. The 9v battery was the standard for transistor radios. They were portable, they were filled with Beatle music, and they ran through batteries quickly. So it was a regular activity, licking the batteries bumping around in the junk drawer to see which of the weak batteries still has some juice.
I've done that a time or two. But it's probably not the best way to test a battery. I don't know of any side effects. But better to be safe than sorry. I don't know of any side effects....side effects...side effects... But better to be safe than sorry. Kids don't try it at home.
As a kid, I had read about a fire starter kit comprising of a 9V and steel wool that you can use to start a fire for a campout. Being a good Cub Scout, I decided to put a battery and some steel wool in my Dad's bag for our upcoming campout. Fortunately for us, the bag caught fire outside of our house and did not require the fire department. I obviously missed that you must put the steel wool in a baggy before storing by the battery.
Festo's BionicKangaroo combines pneumatic and electrical drive technology, plus very precise controls and condition monitoring. Like a real kangaroo, the BionicKangaroo robot harvests the kinetic energy of each takeoff and immediately uses it to power the next jump.
Design News and Digi-Key presents: Creating & Testing Your First RTOS Application Using MQX, a crash course that will look at defining a project, selecting a target processor, blocking code, defining tasks, completing code, and debugging.
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.