1. Field of the Invention
The invention relates to a syringe comprising a cylinder, a piston rod and a piston disposed for displacement within the cylinder by displacement of the piston rod, whereby when the piston rod is displaced in one direction a medium is drawn into the interior of the cylinder while when the rod is displaced in the opposite direction the medium is expelled from the cylinder.
2. Description of the Prior Art
Such syringes are used, for example, for forcing media through filters. In this case, a filter assembly disposed in a filter holder may be connected to the outlet of the syringe. Such arrangements are used, for example, for the cleaning or sterile filtration of small quantities of liquid. In order, then, to propel the liquid, for example in a quantity of 1 to 50 ml, through the filter assembly which is connected to the syringe, a high pressure is required which has to be generated by hand. In the case of known syringes, the liquid or other medium to be filtered is forced through the filter assembly by an inward movement of the piston rod and thus an inward movement of the piston disposed in the cylinder. Under such circumstances, the hand must apply considerable force. In the case of very dense filter materials or where several filter materials are connected in series, particularly diaphragm filters, filtration in this manner is impossible.