SIPHON GAUGE, in pneumatics, is a tube of glass bent so as to form two branches equal and parallel to one another, and each from 6 to 8 inches in length ; the tube is hermetically closed at one end and left open at the other. One of the branches is filled with mercury; and, both of them being in vertical positions, with the closed and open ends upwards, they are, by means of a brass stem terminating in a screw, affixed generally to the under surface of the table carrying time plate of an air-pump. The siphon is contained in a cylindrical glass vessel, a little exceeding it in length, which is closed at the lower and open at the upper extremity ; and the open end of the cylinder is screwed to the table of the air-pump immediately about the orifice of a brass tube which passes through the pump-plate 'and opens into the receiver placed upon the plate, so that there is a free communication between the air in the cylinder, in the open leg of the siphon, and in the receiver. This gauge has a scale of inches, decimally subdivided.
While the pressure of the air in the receiver and in the open branch of the gauge is more than a counterbalance to the weight of the column of mercury in the closed branch, the gauge presents no indications : from the time however that, by continuing the process of exhausting the receiver, the pressure of the air in the open branch becomes less than the weight of the column of mercury in the other, that column descends in the latter branch and rises in the former; and then the degree of rarefaction in the receiver is indicated by the difference between the heights of the columns of mercury in the two branches of the siphon. [Ara-Pune; Peaa-Gatmel