In the 1830s some attempts were made to build mechanical refrigerators that used the cooling effect of evaporation, but these were unsuccessful until the British physicist William Thomson (later Lord Kelvin) worked out the principle of the heat pump in 1851. A liquid is made to evaporate by lowering the ambient pressure and without heating it, so that it becomes extremely cold. The cold vapour is piped through a closed container, and the heat of whatever is in the container passes into the vapour, so the contents are cooled. After this the vapour is led out of the container and compressed to reliquefy it; this releases the heat that has gone into the vapour. The liquid is then vaporized, and thus cooled, again. The fluid travels in an endless cycle in a closed circuit of pipework, constantly taking heat out of the container and releasing it into the open air. All that is needed is some means of driving the fluid around the circuit—in most modern refrigerators this is an electric pump.