TY  - RPRT
TI  - Letter Circular 409: specification for proving rings for calibrating testing machines (superseding Letter Circular 294)
PY  - 1934
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 409
DO  - 10.6028/NBS.LCIRC.409
ER  -