Originally Posted by Morley
No, the "secret" is the water, it conducts electricity very well.
Actually, plain water is not a great conductor. It needs something in it, like salt, to make it very conductive. Tap water often has enough contaminants in it, or chlorine, to make it conductive enough to pass enough current at household voltage (120) to hurt or kill you. But plain tap water will not conduct enough current at 12 volts to hurt you, and if it would, the resistance of your body would stop it anyway.
I just went to the garage to put some numbers on this.
Two large copper lugs for surface area, attached to my meter, and dropped into a glass of tap water (Tennessee's finest) produced a current of 40 milliamps from a 12 volt car battery. That is 0.040 amps. With just the meter probes without the large copper lugs only produced 10 milliamps.
I then measured the resistance of the water, with the copper lugs fully submerged and about 1.5" apart, there was 300 ohms of resistance, certainly not highly conductive. With just the meter probes, it was 500 ohms.
I then checked my body resistance one wet hand to the other, and got 50,000 ohms resistance. This can vary over time, and person to person. It takes about 5 milliamps to kill you, and at 50,000 ohms would require 250 volts. That is why you rarely hear of anyone killed by 120 volts. It can happen, depends on the body's resistance at the time, and the voltage path from supply to ground. If it is in a path that can carry the current through the heart or brain area, then it can happen. Usually contact with 120 volts only produces a pronounced shock. 240 volts is far more dangerous, as that can easily produce the required approx. 0.005 amps current.
Anyway, it is very difficult to get any kind of feelable shock from a car battery, a deadly current from contact with one is absolutely impossible.