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Current is a net flow of electrons.
Current is measured in Amps.
At a junction, the amount of current entering is the same as the amount leaving, just like counting cars at a road junction.

One Amp is 6.2x10¹⁸ electrons passing a point per second - lots!

Connect an ammeter in series with the circuit being measured.

An ideal ammeter has zero resistance and the voltage across it is also zero.

In a conductor, a current is a net flow of electrons. For a current to flow, there must be ...

1. A complete circuit. In this circuit, current can NOT flow.
   
   
    
2. A potential difference (some volts) attracting or repelling the electrons.

Kirchoff's Current Law

At a junction in a circuit, the number of electrons entering the junction is equal to the number leaving. The rule is the same for cars at road junctions.

In this circuit, all the current flows through A1. None flows through the volt meter. If A1 reads 10 amps and A2 reads 4 amps, the other 6 amps must be flowing through A3.

AC and DC

• DC: Batteries provide a steady direct current (DC).
• AC: The mains power supply uses alternating current (AC) because, unlike DC, the voltage can be stepped up or down using a transformer.
  This makes the high voltage national distribution grid possible and it's easy to design power supplies for applications needing different low voltages.

Brain Teaser

Here is a brain teaser.
Assume all the components are ideal and work out the current in each ammeter.
The current through each resistor is one amp.
Or build it in the Lushprojects Simulator

Conventional Current and Electron Flow

Thanks xkcd

Back in history, it was hypothesised that electric currents flowed a bit like water. They guessed which direction it was flowing and, like toast falling jam side down, they got it wrong. When electrons were discovered they were flowing the opposite way. Now almost all the arrows on circuit diagrams point the wrong way and it's too inconvenient to change the symbols or rewrite all the text books to fix the problem. We are stuck with it. Americans tend to use conventional current. The rest of the world tends to use electron flow. Water flows down hill. Conventional current flows down the page on most circuit diagrams. Electrons float upwards towards the positive line at the top of the diagrams. The arrows in the diode and transistor symbols point the way the electrons DON'T flow.