In the Guernicke pump, the pump body is composed of two superimposed parts, separated by a metal diaphragm covered by a coat of oil and equipped with a valve that opens when the piston is raised and closes when it is lowered.
The upper pump body communicates directly with the outside; the lower pump body communicates, through a hole in the lower part of its wall, with an annular space surrounding it which, in turn, communicates with the recipient in which the vacuum is to be created.
The piston, with a gasket of very soft leather and covered with a coat of oil, on being raised closes the hole connecting the annular space and the lower pump body and compresses the air imprisoned in the latter. At a certain point, the air thus compressed raises the valve connecting the two pump bodies and, gurgling in the oil covering the valve, together with a bit of oil, passes into the upper pump body without leaving any clearance volume.
Raising of the piston is facilitated by the fact that a pipe connecting it with the annular space sucks in air under the piston, thus making the pressure on its two faces is the same.
When the piston is lowered, a spring closes the valve that connects the two pump bodies and a vacuum is formed in the space above the piston; but the air below, which arrives from the annular space through the pipe for that purpose, hinders its descent; to facilitate it, a valve situated at the base of the piston allows air to penetrate into the upper part as well until the hole between the pump body and the annular space is reopened.
The process is repeated and the pressure in the recipient decreases progressively, if necessary to the limit pressure, which is on the order of a thousandth of a millimetre of mercury.