Lately we had a lot of very nice QSO’s on local repeaters about the good old days in ham radio. The subjects varied between talking about our failures, our first working designs to – believe it or not – having sex on “In-A-Gadda-Da-Vida” by Iron Butterfly.
But let’s not talk about sex here, because that activity came around the corner much later in life. Let’s talk about the Guild Of Battery Lickers, a guild all participants admitted to belong to. Fact: when we were around 10 years old or so, we were as poor as hell. We didn’t have voltmeters. We didn’t have anything, for that matter. Parts were scavenged from old discarded radios which we picked up from the side of the street, just before the garbage trucks drove into the neighborhood.
Many of our early designs required batteries. The only way we knew to figure out if a 9 Volt or 4.5 Volt battery still had some life in them was putting the terminals to our tongue. In time our tongues became almost as accurate as a voltmeter.
As it turns out, there’s science behind this principle. SparkFun Electronics explains:
While it is impossible to know the exact resistance between two points on your body (blood, bone, muscle tissue, and skin all have different resistances), we can generally assume that dry skin has a resistance of 100kΩ. This will change based on perspiration, hair, etc. For the sake of an example, let’s take a 9V battery and touch both terminals to our skin. What happens? Not a lot. With some Ohm’s law
magic math, we can calculate the approximate current flowing between the terminals:
Now, take that same 9V and lick the terminals. Seriously. It’s safe (mostly). This time, assuming the 9V has some charge, you should have felt a rather unappetizing tingle course through your tongue. Why did you feel it this time? Well, saliva is much better conductor than skin. We can approximate the resistance of a wet (with saliva) human tongue at about 7kΩ. More math gives us:
That’s about 1.3mA of current flowing between the battery’s terminals, which is a good deal more than the current flowing during the dry skin test (0.09mA).
Your Body’s Response
As it turns out, your body has a pretty strong reaction to electric current. Even tiny amounts of current can be felt and be potentially dangerous.
|1 – 5 mA||Tingling sensation|
|5 – 10 mA||Pain|
|10 – 20 mA||Involuntary muscle contractions|
|20 – 100 mA||Paralysis, heart stoppage|
Probable effects of current (DC) flowing through a human body