Suddenly, what is the voltage (volts) of the arrow part (the switch is part away) on the photo?
The correct answer is "unknown" Since this part is not connected anywhere, I do not know is unstable voltage (volts). It could cause a malfunction. But, in the previous experiment remains in the display of the voltmeter "0V", was not unstable. This is because in voltmeter already has "scheme when 0V that is displayed ".

It called "scheme" that is the "pull-down resistor". And "resistance" is the difficulty of that electricity flows. It is called a "resistor" in order to give the resistance components of the circuit, referred to as "resistance". And say what happens if you put a pull-down resistor, the negative terminal of the battery will lead to a total current through the resistor. There is a feature called "voltage occurs when a current flows through" to the resistance. (This is the "Ohm's law." This is to learn in high school) If the voltage does not occur, since no current flows through the switch-off state. So, the ammeter is 0V is being transmitted.

I added a pull-down resistor.
Voltage does not become unstable even if the switch is turned off by now be reliably 0V.