Electrical instruments in aircraft provide the pilot with information about the health and status of the electrical system. 

A voltmeter measures electrical potential difference (voltage), expressed in volts, between two points in a circuit. It is connected in parallel across the bus bar or battery terminals and indicates whether the electrical system is at the correct operating voltage. A voltmeter does not measure current flow. 

An ammeter, by contrast, measures current flow (amperes) and is connected in series in the circuit to indicate the rate of electron flow. An ammeter can be configured to show charging current into the battery, total alternator output, or load current depending on where it is placed in the circuit. 

In an aircraft electrical system a voltmeter is used to confirm that the bus voltage is within the correct operating range, typically around 27 to 28.5 volts in a 28V DC system. A low voltage reading may indicate a failing alternator, excessive load, or a discharging battery. The voltmeter and ammeter serve complementary diagnostic functions and together give the pilot a complete picture of electrical system status.

Indicate the flow of current from the battery to the alternator → INCORRECT. Current flow measurement is the function of an AMMETER, not a voltmeter.

Indicate the flow of current from the alternator → INCORRECT. Again, current flow is measured by an ammeter. An ammeter placed in the alternator output circuit can indicate the current delivered by the alternator to the bus and battery. 

Indicate the flow of current into the battery → INCORRECT. This describes the function of an ammeter configured to show battery charging current. Some aircraft use a zero-centre ammeter that shows positive deflection when the battery is charging and negative deflection when the battery is supplying load current.

Indicate the voltage in the electrical system→ CORRECT. A voltmeter measures and displays the electrical potential difference (voltage) present at the bus bar or across the battery terminals. It confirms whether the system is operating at the correct voltage and alerts the pilot to voltage anomalies such as alternator failure (voltage drop) or overvoltage conditions.