Vacuum

Oil Pumps.

A great advance in cylinder pumps took place about the beginning of the present century, when the so-called oil pumps were placed on the market, although an oil pump was actually made by Robert Gill as early as 1874. The fundamental point about these pumps, of which the "Geryk" pump, in vented by Fleuss, will serve as an example, is that at every stroke all air is displaced from the dead space by oil. The piston-rod passes through a spring-controlled valve G seated in a partition halfway up the cylinder; both this valve and the piston D itself are covered by a layer of oil. When the piston is raised it lifts to wards the end of its stroke the valve G, and all the air and part of the oil on the piston pass through. The valve cannot close until the piston has descended a short distance, which ensures the layer of oil on top of the piston being maintained. These pumps were very widely employed in the electric lamp industry at one time, but have been superseded by more modern types. A special type of oil, of low vapour-pressure, has to be employed, and it is claimed that with carefully constructed two-cylinder pumps of this pattern vacua with pressures as low as .0002 mm. of mercury have been attained.

Mercury Pumps.

In the second half of the nineteenth cen tury various types of mercury pump were introduced, which were used in all the early experiments on the discharge of electricity through gases at low pressure. The principle of all these pumps is to connect the receiver with a Torricellian vacuum created by the pump; the equalisation of pressure will then bring part of the air in the receiver into the Torricellian space. The receiver which is being exhausted is then cut off from the vacuous space, and the vacuum recreated. This process can be carried out in various ways. The simplest and earliest form of apparatus is that de scribed by Geissler in 1862. This consists simply of a vessel connected by a flexible tube to an open reservoir of mercury; the vessel can by means of a two-way tap be placed in connection either with the outside air or with the space to be exhausted. The tap being turned so as to give passage to the air, the reservoir is raised until the vessel is full of mercury; the tap is then turned so that the vessel is cut off from the air and connected to the space to be exhausted, and the reservoir is lowered, so as to tend to create a Torricellian vacuum in the vessel, into which the air from the space to be exhausted rushes. This process can be repeated as often as desired, the fraction of air removed each time depend ing upon the relative volumes of space to be exhausted and the vessel.

A

type of pump possessing many advantages over the Geis sler pump was devised by Tiipler; it avoids all taps, the connec tions and disconnections being made by the mercury itself. An im

proved form of 'ropier pump is shown in fig. 2, from which the working of the pump can easily be understood. V is a cylindrical more in connection with S. The process is repeated as often as may be required. A valve at C, consisting of a glass float, the top of which is ground to fit on a seating at 0, serves to prevent the mercury ever passing over into the vessel S, while the inclined position of the cylindrical vessel V has, among other advantages, that of lessening the shock of the rising mercury against the top of the vessel. Pumps of 'ropier type were widely used for produc vessel connected by the tube T to the space to be exhausted. M is a mercury reservoir which must be of somewhat greater capacity than V. When the mercury levels are as in fig. 2, V contains gas at the same pressure as S. The reservoir M is then raised : the mercury rises in the tube Z, cutting off V from S, and fills the vessel V, driving the gas from it down the tube T. The gas being thus expelled, M is lowered again, and the vacuous space V is once ing a vacuum in the researches of the first ten years or so of the present century, but with the coming of the Gaede rotary pump (see the part of this article devoted to MODERN METHODS AND TECHNIQUE) they became less and less popular. Pumping out with them was a tedious business. To-day they are only used when it is desired to collect the gases which are being pumped out ; a purpose for which they are excellently adapted.

To avoid the tedious raising and lowering of the reservoir, a process which may have to be carried out for an hour or two to obtain the desired vacuum, automatic mercury pumps were devised. The earliest was that of Sprengel, which consists essen tially of a narrow tube down which mercury flows from a reser voir: the top of the tube is connected to the space S to be ex hausted. If the tube is of barometric height the pressure at the top will be zero, air will pass into it from S, and will be carried down as bubbles by the mercury, and discharged into a collecting vessel or into the air, as desired. This type of pump was much improved by Gimingham in 1877, and in this form used by Crookes in his extensive investigations on electrical discharges in vacuum tubes. These pumps were not strictly automatic, in that the reservoir gradually emptied, and had to be refilled by hand. Devices were introduced by which the reservoir could be kept automatically refilled : a typical pump of this kind was that of Kahlbaum in which the flow of quicksilver was maintained by the help of a water-operated filter pump of ordinary type. The Sprengel type of pump, which came into very extensive use, like the widely known 'ropier type, was the subject of many modifications.