Final answer:
To keep the current reading close to 250 mA, a shunt resistance should be calculated and used with a galvanometer to convert it into an ammeter with the desired full-scale reading, while ensuring minimal resistance is added to the circuit.
Step-by-step explanation:
To keep the current sensor's reading as close to 250 mA as possible throughout an experiment, there are a few methods that can be used. When working with a galvanometer with a given sensitivity and full-scale reading, you must consider the additional components that can be used to expand its range. For instance, if you have a 25.0-Ω galvanometer with a 50.0-µA sensitivity, to allow it to be used as an ammeter with a 300-mA full-scale reading, you need to calculate the appropriate shunt resistance to place in parallel with it.
To calculate this shunt resistance, you would use the formula that relates the galvanometer resistance, its full-scale deflection current, and the desired full-scale current of the ammeter. Since a galvanometer and its shunt are in parallel, the voltage across them is the same and can be used to calculate the required shunt resistance.
It's important to remember that an ammeter has to be placed in series with the circuit element whose current you want to measure, and it must have a small resistance to minimize its impact on the circuit. Additionally, other factors such as resistance in the wires, connections, and temperature variations can affect the measurements. These need to be minimized or accounted for when calibrating the apparatus to ensure the current reading is maintained as close to the required 250 mA as possible.