When the mercury rolls to one end of the tube, the mercury conducts current between the leads and the switch is closes. When the tube is tilted the other way, the contacts are not connected. Burgular Alarm Switch. If you are trying to make the switches in your installation unobtrusive, then burgalar alarms systems may offer just what you need. Radio Shack carries serveral intersting burgalar alarm switches including one that is thrown when a beam of light is broken and another that is thrown when a magnet comes near.
Resistance is the measure of how easy it is for current to flow through a circuit.
Suppose you want to know how hard someone is pressing down with their thumb on a picture on the wall. You would find something to convert the pressure from the thumb into an electrical resitance, a force sensative resistor. You would put the force sensative resistor under the picture on the wall and send a current to the resistor and depending on how much current flows through, you know how hard the person was pressing. After you decide on your variable resistor you will want to install it into a analog input circuit with your microcontroller . You would then program your microcontroller to watch the variable resistor and perhaps program your authoring software to react to the microcontroller using serial communication.