The Patch Clamping Technique is an in vitro, electrophysiological technique which allows you to study either a single ion channel in a cell or the all of the ion channels in the cell membrane, depending on the configuration of the cell. This techniques allows you to measure the currents flowing through one ion channel or the sum of all the currents flowing through all the ion channels in a cell membrane. The technique involves using a blunt glass pipette, 1-2 microns in width and trapping a patch of the cell membrane with the glass pipette. The ion channel associated with that patch of membrane is now also trapped. Inside the glass pipette there is an ionic solution and the cell is surrounded by a bath solution. There is a strong seal between the glass pipette and the patch of membrane, a Giga Ohm seal (10^9 Ohms) which allows the patch of membrane to be isolated from the rest of the cell membrane.
Alberts B. et al (2008), Molecular Biology of the Cell, Fifth Edition page 681, Garland Science, Taylor and Francis Group, LLC, New York, USA