Which device does NOT make use of the magnetic effects of an electric current?

Analyze the question: We need the device that works WITHOUT using the magnetic effect of current. Every other device uses it.

Concept: An electric current has three main effects: heating, magnetic, and chemical. Each device below relies on one of them.

Going through the options:

• A. Hot-wire ammeter: A current flows through a thin wire and heats it. The wire expands and sags, and this movement turns the pointer. It uses the heating effect, not the magnetic effect. It can measure both a.c. and d.c. because heating doesn't depend on direction.
• B. Moving-coil ammeter: A coil carrying current sits in a magnetic field and experiences a torque (motor effect). This uses the magnetic effect.
• C. Moving-coil loudspeaker: The signal current flows in a coil placed in a magnet's field. The coil moves back and forth and vibrates the cone. This uses the magnetic effect.
• D. Moving-iron ammeter: The current magnetizes two iron pieces, and they repel each other. This uses the magnetic effect.
• E. Telephone earpiece: The current passes through an electromagnet, which pulls on a thin iron diaphragm, so the diaphragm vibrates. This uses the magnetic effect.

Answer: The device that does not use the magnetic effect is the hot-wire ammeter, which works by the heating effect.

Tip: Think "hot-wire = heat." Any device with "hot," "heater," "fuse," or "filament" in its working is a heating-effect device. Devices with coils, magnets, or iron are magnetic-effect devices.