An ammeter measures the amount of current in a circuit. That measure is in amperes. In the analog days of yore, ammeters could weigh almost a pound. But now, if you own an Apple Watch, you could have an ammeter on your writs. Here’s more from Hackaday:
Like many mobile devices, the [Apple Watch] comes with a magnetometer. This serves as an electronic compass, but it’s also as its name might suggest, an instrument for sensing magnetic fields in three axes. With a 3D printed bobbin that slides over the watch, and a few turns of wire, it can sense the magnetic field created by the current, and a measurement can be derived from it. The software on the watch is only a simple proof of concept as yet, but it applies some fairly understandable high-school physics to provide a useful if unexpected measure of current.