You’ve probably seen one of those hollow circular devices that enclose transmission lines and are connected by smaller wires to an electrical measuring device, particularly an ammeter. This device, known as a current transformer or current sensor, protects ammeters from power overload by reducing dangerous amounts of current flowing through a power line to more tolerable levels as it enters the measuring device.
Among other things, a current transformer can step-down electrical current to hundreds of times lower than the original amounts. In physics, it is proven that the passing of an electric current through a wire creates a magnetic force around that wire. This magnetic force allows the production of a secondary current in the current transformer that is much lower than that of the wire. Conversely, magnetic force may also induce an electric current just as how generators produce electricity.
The device through which the much lower current flows (i.e. the current transformer) consists of a hollow core and coil of wire called the secondary winding. The number of loops the coiled wire makes on the core of the current affects the amount of current passing through the secondary winding. Therefore, fewer loops mean lower electric currents. This is how a current transformer protects an ammeter from receiving too much electric current.