# Why are we connecting voltmeter in series with power when we measure voltage across it?

I am a physics teacher and demonstrated in the class that the voltage across at wall socket is around 220V ( in Europe ) and my student asked why we are connecting a voltmeter in series with power outlet ( I stated to my students previously that if we want to measure voltage of the resistor, we make a parallel connection with it.) I explained them that we need to think the power source as a battery and the internal resistance of new opened battery is very small compared to the internal resistance of the voltmeter.

He came out with the second question that the voltage drop across the voltmeter is not the same as voltage in the power source because there is internal resistance of power source of which we don't know. He claimed that the voltage across the power outlets needs to change depending the distance of wall outlet to the power plant.

My question is: what is the internal resistance of the circuit coming from power plant to our home? Does it make too much difference?

• To potential answer writers: circuit questions and answers should include a diagram. To understand the output resistance of a power outlet, you have to remember that there's a transformer. Then again, volt meters have very high input resistance... – DanielSank Jan 21 '17 at 17:11

The power company generates "moderately high" voltage (e.g., 10000 V), and runs that power through a step-up transformer, which converts the power to very high voltage (e.g., 300,000 V). Since energy is conserved, when the voltage in the step-up transformer rises, the current drops. This means that the minimum current necessary to support all electrical loads is sent to neighborhoods, which minimizes the power lost to electrical resistance in the transmission lines, because those losses are seen to be $P = I^2 * R$. Thus, when transmission current is minimized, power transmission losses are also minimized.