Final answer:
A negative value on an ammeter usually indicates reverse current flow, either due to reversed leads or a discharging electrical system in the aircraft.
Step-by-step explanation:
If the ammeter in an aircraft shows a negative value, this typically indicates that the current is flowing in the opposite direction to what was expected or intended. An ammeter should be placed in series with the circuit it is measuring, ensuring that it does not affect the current flow significantly.
A negative reading can arise in a couple of scenarios. It may suggest that there has been an error in how the ammeter was connected, such as reversing the leads. This could cause the ammeter to register current flowing in the reverse direction. Alternatively, it could mean that the electrical system is discharging, such as a situation where the battery is being drained instead of charged.
The fact that the ammeter reads a negative value could also relate to the direction of motion or a difference in potential that causes current to move in a direction opposite to the positive standard flow from the ground to the cloud (considering conventional electric current direction).