I've used "touch lamps", where you turn it on and off by touching the lamp. As I understand it, this works by putting a small electrical voltage through the human body: touching the lamp completes the circuit, allowing electricity to flow from the lamp to the ground. Is this safe? What makes it safe? Is it because it is using very low voltage? Do they have special circuitry to protect people?
Your understanding of the way these lamps work is not quite correct. The lamps are basically capacitive touch sensors. Similar to a low, very low, resolution touch screen for a smart phone.
The idea is you're a capacitor. The lamp's surface is also a capacitor. The lamp knows how long it takes to charge itself. If you touch the lamp, you add to its capacitance and it takes longer to charge, the lamp detects this time difference and turns on.
It's perfectly safe. It's safe because the current is incredibly low. Voltage stings, current kills.
Don't you love lamp? I love lamp.
There are a couple of different techniques that can be used. One of them is the capacitive sensing technique as the touch screen on the iphone and other smart phones. Basically, the circuit in the device generates a high frequency (KHz range) signal that it applies to a capacitive sensing contact. When an electrically conductive object comes near the pad, some of the signal is capacitively coupled away. The sensing circuit detects this and uses the signal to turn the light on.
The second method is to use a very high gain amplifier (might just be a couple of transistors) connected to a sensing pad via a very large resistor (meg ohms). Your body acts like an antenna, picking up the 60 Hz line frequency in the wires in the wall. If you touch the sensing pad (or even just get close to it) a sensitive enough amplifier will be able to pick up this 60 Hz oscillation and then use that to turn on the lamp. Either way, we're talking very small signals here, on the order of milivolts and microamps.
Both of these methods are completely safe if implemented correctly.