Mercury Step-Down Voltage Converter.
This converter allows a low power use 110-120V device from the USA to be powered from a 230-240V AC UK mains socket.
You can use of US electrical equipment with either 2 or 3 pin plugs with this converter. The transformer is not designed for constant use. The maximum rating is 100VA (which is about 85 Watts), so it cannot be used with very power hungry devices. In particular do not use it with anything that produces heat such as hair dryers or hair straighteners.
Great for light power US devices such as radios, DVD players etc.