The volt makes a stop at the Fluke metrology lab


Your car battery, 12 volts. A single AA battery, 1.5. One solar panel, 17. All these objects, of course, generate direct current (dc) voltage. Ensuring a dc volt is the same wherever you go and on whatever device you own is the job of national and international standards organizations with an assist from private industry.

In the United States, the National Institute of Standards and Technology (NIST) maintains the dc volt standard among others. NIST in turn works with standards organizations worldwide.

A team from NIST visited Jeff Gust, Chief Corporate Metrologist for Fluke, in December as part of the standard body's rounds to compare the volt standard reference. NIST physicist Dr. Yi-hua Tang and associates brought their specialized testing equipment to Everett, Wash., to conduct an experiment that confirms the correct dissemination of the volt in North America. The NIST volt is compared with Fluke metrology's reference standard, which is transferred to the world's most accurate voltage transfer standards, the Fluke Calibration 732A and 732B. These are then deployed worldwide to reproduce the volt for innumerable devices.

The volt throughout history

Named after Italian physicist Allessandro Volta in the 18th century, the volt is a measurement unit of electric potential or electromotive force. Before 1972, the standard volt was kept in special chemical batteries in each country. Since then the volt has been standardized with reproducible experiments tied to natural forces.

This new method of establishing the dc volt uses a chip made with what are called Josephson junctions, named after Welsh physicist Brian David Josephson. These chips are made by sandwiching layers of superconducting material between layers of insulating material.

How physicists measure accuracy

In the experiment, the chip with the junctions is put on a probe, then immersed in a tank filled with liquid helium that is cooled to 4.2 degrees Kelvin (-269 C, -452.2 F). It is then shot with a special device that emits a microwave frequency. The chip “sandwich” then delivers a consistent step of voltages, 0-1, 1-2 and so forth. By measuring each step, you will get the most accurate volt possible.

It's probably a test you wouldn't try at home, since the chip “sandwich” alone can run about $50,000 USD. A large tank of liquid helium isn't easy to come by, either.

By conducting the experiment and then comparing it to a NIST reference the volt is accurately reproduced. The Fluke 732 calibrators then carry the load to ensure labs, manufacturers and others have an accurate dc volt. Keeping the dc volt consistent around the world ensures interoperability of machines, software and infrastructure.