U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

NIST Museum & Archives

A Barred Experiment: The NBS designed prison bar tester

Share

Title image for the A Barred Experiment: The NBS designed prison bar tester exhibit.

In 1934, the Federal Bureau of Prisons requested help from the National Bureau of Standards (NBS) in inspecting a shipment of tool-resistant prison bars, some of which had been found to be defective.

Tool-resistant prison bar consisted of a tough, but relatively soft, outer matrix surrounding a core of hardened steel of a different composition. The differing hardness prevented using ordinary tests for hardness.

Raymond L. Sanford at NBS developed a non-destructive technique which compared the magnetic properties of a test bar with a reference specimen of known quality. Differences in structural properties would cause differences in magnetic properties.

Magnetic comparator device for testing prison bars, circa 1930s. The device is framed in a wooden box that doubles as a carrying case. The interior of the box has a control panel with various settings knobs.
Magnetic Comparator for testing prison bars, circa 1930s.
Credit: NIST Museum Collection
Close up of Magnetic Comparator for testing prison bars panel, circa 1930s.
Close up of the control panel of the Magnetic Comparator.
Credit: NIST Museum Collection
Reference specimen holder used to magnetically compare prison bars, circa 1930s. The holder is metal.
Reference specimen holder used to magnetically compare prison bars, circa 1930s.
Credit: NIST Museum Collection
Reference specimen holder with extension pole out used to magnetically compare prison bars, circa 1930s.
Reference specimen bar extended from the sample holder.
Credit: NIST Museum Collection
Journal of Research of the National Bureau of Standards Research Paper 894 FIGURE 8.- Comparator with accessory apparatus connected for testing a bar in the experimental grille.
Magnetic comparator and accessory apparatus testing a bar in an experimental grille. Circa 1936.
Credit: NIST Digital Archives (NDA)

The Bureau of Prisons used the testing apparatus to locate a large number of defective bars at the United States Industrial Reformatory in Chillicothe, Ohio, which was then under construction.

The NBS program helped identify problems in the manufacture of prison bar, and the magnetic technique would also be applied to test drill steel. The prison in Chillicothe, still in use today, was made more secure through the NBS project.

References

  1. NIST Digital Archives (NDA)
  2. Raymond L. Sanford, “An alternating current magnetic comparator, and the testing of tool-resistant prison bars.” Journal of Research of the National Bureau of Standards, Vol. 16, No. 6, Research Paper 894, p. 563-574, June 1936. http://dx.doi.org/10.6028/jres.016.036.

Example of how to reference this exhibit:
NIST Museum. 2024. A Barred Experiment: The NBS designed prison bar tester. Gaithersburg, MD: National Institute of Standards and Technology. Online. https://www.nist.gov/nist-museum/barred-experiment

Author. Year. Exhibit Name. Place published: Publisher. Online. URL.

Contacts

NIST Research Library & Museum

Created July 15, 2024, Updated July 25, 2024