No, a thermocouple makes use of a phenomenon originally discovered by a fellow called "Seebeck" in the 1800's. Seebeck discovered that if certain pairs of different metals are connected together, a small voltage will be produced at the junction where they're joined.http://www.nphheaters.com/technical/thermo_letter.htm
And, what's perhaps more important, is that the voltage generated at that junction will increases as the temperature of that junction goes up, and it'll go up very linearly with the temperature. That is, a graph of the voltage generated by that junction versus temperature would be a straight line.
The thermocouple itself consists of a metal conductor in the middle surrounded by an electrical insulating material and then there is a copper sheath on the outside. The metal conductor in the middle is connected to the copper sheath only at the end of the thermocouple (which sits in the pilot light flame). Because of the Seebeck effect, a small voltage will be generated at the end of the thermocouple, and that voltage causes current to flow through the metal inner conductor, through the coil of an electromagnet and then back through the exterior copper sheath of the thermocouple. And, it's that electromagnet that holds a magnetized steel plunger up against spring pressure so that gas continues to flow to the pilot light.
The kind of thermocouples used on boilers and furnaces only produce a few thousandths of a volt. Those gas valves will require 24 volts AC to energize the main electromagnet that opens the valve that allows gas to flow to the burner trays.
Until recently, gas fired water heaters didn't have electric power supplied to them. So, gas water heaters have to use the voltage generated by a thermocouple sitting in the pilot light flame to power the electromagnet that holds the pilot light valve open AND the electromagnet that holds the main burner valve open. To provide additional power, hot water heaters use something called a "thermoPILE" instead. A thermopile is nothing more than a whole bunch (dozens, I think) of thermocouples all connected in "series" (pronounced "end-to-end"). Just in the same way that you can connect eight 1.2 volt batteries end-to-end to make a 9.6 volt battery pack for a cordless drill, you can connect multiple thermocouples in series to generate a higher voltage from the heat of the pilot light. Typically, the thermopile in a hot water heater generates almost one volt. (I think it's 0.96 volts, actually.) And, it's that one volt that's used to power both electromagnets in the gas valve on a gas fired hot water heater.
And, that brings us to trouble shooting problems.
If the pilot light stays on, it means the thermocouple or thermopile is good. If the pilot light won't stay on, connect a new thermocouple or thermopile to the gas valve and have a helper heat it with a butane lighter. If the gas valve works fine when the spare thermocouple is being heated by the lighter, you need to replace the old thermocouple or thermopile.
Also, since there's negligible voltage drop along the metal inner conductor or external copper sheath of a thermocouple, you can use a longer thermocouple or thermopile to replace a short one.
Heating equipment simply uses thermocouples because they generate a voltage. You can also make use of the fact that the thermocouple's voltage increases linearly with temperature to make an electric thermometer, and thermocouples are also used for that in monitoring equiment.
Thermocouples gradually generate less and less voltage as they get older. In the field of metallurgy, there's a process known as "annealing". Just in the same way that white styrofoam appears to be made of small "beads", all metals consists of "crystals" where all the atoms within each crystal are arranged in a certain pattern, and that pattern is oriented in a particular direction. In neighboring crystals in the same metal, the atoms will be in the same pattern, but will be oriented in some other random direction. This is exactly the same as the change in the direction the streets run at from place to place when small towns grow together to form one big city. Anyhow, "annealing" is when you heat a metal up (or heat it and cool it in cycles). While the metal is hot, impurities in the metal will gradually congregate at the interfaces between the crystals in the metal, and some crystals will gradually grow in size at the expense of their neighbors. As the impurities in the metal separate out at the crystal bounaries, and you get fewer and fewer boundaries as the crystals grow, the internal electrical resistance within the
thermocouple goes up, and it's voltage output goes down. So, thermocouples don't fail like light bulbs that work perfectly until they don't work at all. Rather, they fail like most things which gradually perform less and less well.
And I think that's all I know about thermocouples.
(Newer electronic stoves use something called a "thermistor" (which changes it's resistance linearly with temperature) to control the heating element in the oven. Thermistors vary their resistance with temperature more linearly than thermocouples vary their voltage with temperature.