For another device like a smartphone to use any of the Bluetooth "services" which the @boardname@ has, it must first be [paired with the @boardname@](/reference/bluetooth/bluetooth-pairing). Once paired, the other device may connect to the @boardname@ and exchange data relating to many of the @boardname@'s features.
The Bluetooth magnetometer service allows another device such as a smartphone to wirelessly receive data from the @boardname@'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 @boardname@ 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.
For more advanced information on the @boardname@ Bluetooth magnetometer service including information on using a smartphone, see the [Lancaster University @boardname@ runtime technical documentation](http://lancaster-university.github.io/microbit-docs/ble/magnetometer-service/)
