Tracor model 304D Rubidium Frequency Standard, Object ID no. 1981.0076.01
This object is a rubidium vapor absorption frequency standard, serial no. 105, with clock movement. The Tracor instruction manual and circuit diagrams are in the curator's file.
Tracor Instruments first put this atomic-absorption clock on the market in 1970. In its physics, electronics, performance and price, it is similar to the first commercial model introduced by Varian Associates a decade earlier (see object ID no. 1980.0511.02, the Varian V-4700A rubidium-vapor frequency standard). It is, however, less than half the size and weight. In the “Atomic Clocks” exhibition, the object was “exploded” and sectioned in order to display the clock components in their functional relationships. The principles upon which it operated are essentially those underlying the ammonia absorption and atomic beam clocks.
If the arrival time of a radio signal can be determined with accurate instruments, such as atomic clocks, aboard a ship or satellite, then the distance from the radio transmitter is easily calculated. The LORAN C navigational system, introduced in 1961, used ground-based radio transmitters. The Global Positioning System, introduced in 1980 and designed to be fully operational by 1987, achieves wider geographic coverage by using satellite-based transmitters.
Brief description of an atomic clock
Electromagnetic waves of very specific and consistent frequencies can induce atoms to fluctuate between two energy states, and by measuring that frequency we can determine the “tick” of an atomic clock. A second in a cesium clock, for example, is defined as 9,192,631,770.0 cycles of the frequency that causes the cesium atom to jump between those states. Different atoms “tick” at different rates – strontium atoms tick about 10,000 times faster than cesium atoms – but all atoms of a given element tick at the same rate, making atomic clocks much more consistent than clocks based on macroscopic objects such as pendulums or quartz crystals.
(Ref. Steven Jefferts, physicist, National Institute of Standards and Technology
There are different types of atomic clocks, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clocks include:
•Cesium atomic clocks employ a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.
•Hydrogen atomic clocks maintain hydrogen atoms at the required energy level in a container with walls of a special material so that the atoms don't lose their higher energy state too quickly.
•Rubidium atomic clocks, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.