James P. Wightman Lehigh University Bethlehem, Pennsylvonio
A
Demonstration of Evacuation
frequently-asked question concerns the operation of the mercury diffusion pump which is a common component of many vacuum systems. A visual demonstration of two cycles in the production of a vacuum in a system containing such a pump is described herein. The two cycles correspond to a high and a low pressure range; the mechanical pump alone is operative in the former whereas both the mechanical and diiusion pumps are operative in the latter. The requisite apparatus is shown in the figure. The particular design of mercury diffusion pump shown is singularly convenient for this demonstration. The room should be darkened to achieve optimum viewing conditions. The system is evacuated to approximately lo-= mm Hg and then exposed to atmosphere by opening and quickly closing stopcock A . Consequently the mechanical pump changes sound, and condensation of mercury occurs due to the increase in pressure of the system. A Tesla coil is immediately placed in the vicinity of C and the system sparked. Initially a green cloud of newly-evaporated mercury appears over the surface of the heated mercury a t C. The green color is the 5460 A line of mercury, electronic excitation being produced by the electrical discharge of the Tesla coil. The green cloud advances randomly upward from C. A streamlined, conical-shaped flow pattern is eventually established with base a t D and apex a t F. The duration of the pattern is directly proportional to the pumping speed of the mechanical pump. If the pattern is too short-lived, stopcock A must be kept open slightly during the demonstration. The flow pattern described above is observed only in the first cycle, i.e., in the high pressure range where evacuation of the system is due only to the mechanical pump. It should he emphasized that the diffusion pump is not. operative in this range; a minimum presmm Hg must be established in the fore sure
