TY  - RPRT
TI  - Letter Circular 686: photoelectric cells; selenium cells; thermopiles (supersedes Letter Circular 515)
PY  - 1942
PB  - National Institute of Standards and Technology
CY  - Gaithersburg, MD
SN  - LCIRC 686r1942
DO  - 10.6028/NBS.LCIRC.686r1942
ER  -