Can an iPhone Detect EMF?

Partially, and it's worth being precise about which part.

What the iPhone actually has

Every iPhone contains a magnetometer — the same sensor that powers the Compass app and helps with orientation in Maps. A magnetometer measures the strength and direction of the magnetic field around the phone. That's a real, functioning sensor, not a simulated or estimated value.

"EMF" is a broader term than what a magnetometer measures. Electromagnetic fields include a wide range of frequencies — from the static magnetic field the compass reads, up through the radio frequencies used by Wi-Fi and cellular signals, up further still. A magnetometer covers one part of that range: the magnetic field, essentially static or slowly changing.

What a dedicated EMF meter does differently

Devices built specifically for paranormal investigation or electrical work — a K-II meter, a Mel meter, and similar tools — are often designed to detect a wider band of electromagnetic activity, sometimes including the AC fields given off by household wiring and electronics at 50–60 Hz, which a phone's magnetometer isn't built to isolate. These dedicated meters are purpose-built single-function tools. An iPhone's magnetometer is a general-purpose sensor built primarily for orientation, repurposed for a different use.

Neither is more "real" than the other. They're measuring different things, calibrated for different purposes.

What can move the reading

Because the magnetometer reads the field relative to the phone rather than the room, several things will change the value it reports:

• Nearby speakers, chargers, motors, and other electronics with their own magnetic signature
• Magnetic phone cases, mounts, or accessories
• Large metal objects or structures nearby
• Rotating or repositioning the phone itself, which changes its orientation relative to the field even if nothing in the room has changed

None of these are malfunctions. They're the sensor doing exactly what it's built to do — reporting the field as measured from wherever the phone currently is, oriented however it currently is.

What this means for an app that uses it

An app built around the iPhone's magnetometer, like Meridian Loom, is working with a real, functioning sensor — but a specific and limited one, not a general-purpose EMF detector. The honest framing is that it measures magnetic-field changes, full stop. What causes a given change — an appliance, a structural element, something else — isn't something the sensor alone can tell you.