For another device like a smartphone to use any of the Bluetooth "services" which the micro:bit has, it must first be [paired with the micro:bit](/reference/bluetooth/bluetooth-pairing). Once paired, the other device may connect to the micro:bit and exchange data relating to many of the micro:bit's features.
The Bluetooth magnetometer service allows another device such as a smartphone to wirelessly receive data from the micro:bit's magnetometer. The magnetometer measures the strength and direction of magnetic fields including the earth's and so it can be used as a digital compass and indicate the way the micro:bit is pointing relative to magnetic north.
Using the Bluetooth magnetometer service you could, for example, create a smartphone application which displays your direction of travel, updating it in real time.
No additional code is needed on the micro:bit to use the Bluetooth magnetometer service from another device.
For more advanced information on the micro:bit Bluetooth magnetometer service including information on using a smartphone, see the [Lancaster University micro:bit runtime technical documentation](http://lancaster-university.github.io/microbit-docs/ble/magnetometer-service/)
