Why does the reading across a real battery drop once the battery is part of a working circuit?

1. A real battery is built from materials that themselves resist the flow of charge. 2. That internal resistance behaves like a small resistor in series inside the battery. 3. Once current flows, some potential difference is used up across that internal resistance. 4. What remains at the terminals is therefore less than the emf, which is why the reading drops. 5. The emf itself is set by the chemical reaction and stays essentially constant, so the third option is wrong. _Source: Siyavula Physical Sciences Grade 12 (Everything Science, CC BY 4.0), Chapter 10: Electric circuits_