Electrical currents generate magnetic fields, and the amount of current determines the strength of the field. This simple principle underlies the plan for an atomic vapor-cell ammeter. Such a device would be particularly useful in electronics applications where a very small current -- for example, on the order of 100 milliamperes, roughly one-thousandth the current in household electric lines -- must be measured accurately and stabilized.
NIST’s design combines a vapor-cell magnetometer with a coil that carries the current to be measured. The coil is made up of a large number of extremely fine wires evenly spaced at intervals of a few micrometers. The effect of the magnetic field on the atomic-vapor magnetometer depends on two variables: the magnitude of the current and the spacing between the trace wires. That spacing can be measured to excellent accuracy because the traces function in effect as an optical grating whose dimensions can be readily determined by spectroscopy. Researchers expect that, once fully developed, the devices might have uncertainties in the range of parts in 106.
Just as current can be measured by its associated magnetic field, voltage can be measured by its electric field. Again, the configuration involves a vapor cell joined to an electrical device: a cell of highly excited atoms placed between two electrodes with a voltage to be measured.
The trapped atoms are excited by laser light into Rydberg states in which the outermost electrons are so far separated from the rest of the atom that they are particularly sensitive to electric fields. In the absence of an applied field (that is, without a voltage across the electrodes), an electron will have a distinctive spectral signature. But when a field is present, the spectral line will shift according to a phenomenon called the Stark effect, the mechanism of which is determined by fundamental constants.
Once again, the measured shift depends on only two variables: the magnitude of the voltage (strength of the electric field) between the two electrodes, and the distance between the electrodes. That distance can be ascertained to very high accuracy using techniques of interferometry; the shorter the distance, the larger the effect. In those circumstances, even a small change in voltage results in a very large change in spectral shift.
Voltage and current are the fundamental physical phenomena that underlie the flow of electricity. Compact, inexpensive and yet accurate ammeters and voltmeters may therefore be used in electrical power transmission and in a broad range of electronics systems.