What device detects differences in current between two conductors and shuts off the circuit if necessary?

The device that detects differences in current between two conductors and can shut off the circuit if necessary is a Ground Fault Circuit Interrupter (GFCI). This safety device is designed to protect against electric shock by monitoring the electrical current flowing through a circuit. If it detects a discrepancy, which indicates that current is leaking through an unintended path—such as a person—it interrupts the circuit almost instantaneously, usually within milliseconds.

This functionality makes GFCIs particularly useful in areas where moisture is present, such as kitchens and bathrooms, where the risk of electric shock is higher. By shutting off the power at the first sign of a ground fault, GFCIs help prevent serious injuries and fatalities due to electrical accidents.

In contrast, other options do not serve the same purpose. A fuse protects a circuit by breaking the connection when a specific current limit is exceeded, but it does not actively detect ground faults. A surge protector protects devices from voltage spikes but does not monitor current flow between conductors. A transformer changes the voltage of electrical power and does not have any protective monitoring or shut-off capabilities related to ground faults. Thus, the GFCI stands out as the correct answer due to its unique ability to detect and respond to current imbalances effectively.