Fig. 42: If a glass tube, F, stopped at one end, 3 feet long or over, be filled with mercury and the open end immersed in a vessel of mercury, T, the column of metal in the tube will sink until it attains a height, M, of about 30 inches, varying according to the condition of the atmosphere.
The space between the mercury column and the top of the tube will be a fairly good vacuum. This fact was noted many years ago, and the gradual evolution of the mercurial air-pump based on this result can be followed in the articles on the mercurial air-pump by Silvanus P. Thompson, read before the Society of Arts, England, some years ago.
Geissler, the first manufacturer of the "Geissler" or vacuum tube for electrical research, seeing the inconvenience of the above-described operation and the meagre results obtained, invented the pump called by his name (Fig. 43).
F E is a stout glass tube some 3 feet long, having a bulb, B, at its upper extremity, and a rubber tube, S, attached to the curved end. A reservoir of mercury, R, connects with this rubber tube, and a special glass tap is fixed in the upper end of the glass tube at E, beyond which tap being the point of attachment for the object to be exhausted. The operation is as follows: On turning the tap a part of the way it allows a passage between the tube F E and the atmosphere. The reservoir R is then raised until the mercury flows into the bulb and up the tube to the tap. The tap is then turned a fraction, and the communication with the air is shut off and opened between the object to be exhausted and the tube F E. The reservoir is then lowered and the mercury falls, drawing down the air from the object into the tube. The tap is then turned as in the first place, and the reservoir R raised, when the air drawn into the tube is forced out by the rising column of metal. This operation being repeated many times, withdraws nearly all the air from the object—in fact, makes a fairly good vacuum. This pump has been much modified from the simple form described.
The form of pump most used in the United States lamp factories is based on the application of the piston-like action of a quantity of mercury dropping down a tube. This is known as the Sprengel pump, after the inventor.
Fig. 43. Fig. 44.
Fig. 44: F is a stout glass tube about 40 inches long by one-twelfth of an inch internal diameter, carrying the reservoir funnel R at the top, a piece of soft rubber tubing, S, nipped by a pinch-cock being interposed to admit of the regulation of the mercurial drops. The lower end of this "fall tube," as it is called, is immersed in mercury contained in a suitable vessel, V, a branch tube being blown or cemented into the fall tube to admit of the connection of the object to be exhausted at E. S is another piece of rubber tubing with a pinch-cock regulation. The point H is the normal barometric height of the mercury—about 30 inches. On attaching a bulb, for example, at E, and regulating the pinch-cock at the top of the fall tube F, a succession of drops of mercury falls down the tube, each drop acting as a piston to drive the air before it, sucking the same from the bulb, and forcing it down through the tube and vessel out into the atmosphere.
On its first being set into operation, the cushions of air between the drops silence their fall; but as a higher degree of rarefaction occurs, the air cushions become insufficient, and the drops fall with a sharp click on the top of the barometric column.