overall (height x width x depth): 15 cm x 10 cm x 15 cm; 5 29/32 in x 3 15/16 in x 5 29/32 in
Object Name:
gauge, vacuum
Place made:
United States: Massachusetts, Cambridge, Harvard University
Date made:
1959-1960.
Description:
Left-hand vacuum gauge - towards dissociator tube (.01.04) from Hydrogen Maser of Norman Ramsey. Object ID 1980.0594.01.05
Object was originally a component within part 1980.0594.01.01 (main steel frame with two diffusion pumps and hexapole state-selector magnet between them). The object is one of the 11 separate parts of the Ramsey hydrogen maser (1980.0594.01), which consists of an assembly of a glass hydrogen dissociator tube (.01.04), a hexapole state selector electromagnet and two vacuum pumps (.01.01), all supported by a bolted steel frame; and connected by a beam tube to a silver-plated quartz microwave cavity with tuning plunger (.01.02), surrounded by three mutually perpendicular sets of Helmholtz coils (.01.03). The cavity and coils are supported by a second smaller steel frame. The assembly was displayed in the "Atomic Clocks" exhibition. After the exhibition closed in 1988, the object was dismantled into 11 separate parts (object IDs 1980.0594.01.01 - .11).
History and basic principles
In its original form, the maser invented by Charles Townes offered no advance in accuracy nor in stability as an atomic clock over the cesium-beam frequency standard. It was Norman Ramsey and his colleagues, Daniel Kleppner and H. Mark Goldenberg at Harvard, who developed Townes’ molecular-beam ammonia maser into an atomic-beam hydrogen maser, and thus were able to achieve greatly improved stability.
In the Ramsey maser apparatus, hydrogen gas leaks slowly from a high pressure tank into the glass discharge (dissociator) tube, where an electric arc splits the hydrogen molecules into two hydrogen atoms. The atoms pass through a small hole into the evacuated region, there forming an atomic beam. Half of the atoms have an excess of microwave energy and half have a deficit (depending on the relative orientation of the proton and the electron in each hydrogen atom). In the Townes maser, the low- energy ammonia molecules were removed from the beam by an electrostatic “state-selector.” In the Ramsey maser, the low-energy hydrogen atoms are dispersed by an analogous magnetic device.
The atoms possessing an excess of microwave energy are focused upon a small opening of the quartz storage bulb inside the microwave cavity. Once inside the bulb, a hydrogen atom collides many thousands of times with the walls, which are lined with paraffin to “soften” the impact. (This paraffin coating, being rich in hydrogen, eliminates induced electrostatic forces between the wall surfaces and the hydrogen atoms bouncing off of them; forces that would otherwise perturb the sensitive oscillation frequency of the atoms.) Eventually the atom chances again upon the opening and escapes back into the vacuum. During this time – nearly one second – the atom is being stimulated by an oscillating electromagnetic field in the cylindrical cavity surrounding the storage bulb. By means of a plunger, the length of the cavity is adjusted until its resonant frequency is exactly that of the hydrogen atom: 1,420,405,758 Hz (cycles per second). Long before the hydrogen atom escapes the bulb, it has radiated away its excess energy, which joins in exact phase with the electromagnetic field of the cavity. Thus the microwave oscillations are sustained by the atoms themselves.
The frequency of the microwave vibrations of hydrogen, and of other atoms, is altered by a magnetic field. Therefore the microwave cavity is surrounded by wooden hoops on which are wound coils of wire. Electric currents in these Helmholtz coils produce a magnetic field in the storage bulb equal and opposite to the earth’s, thus cancelling its effects.
