@article{Scovil_1959,
title={Three-Level Masers as Heat Engines},
volume={2},
ISSN={0031-9007},
url={http://dx.doi.org/10.1103/physrevlett.2.262},
DOI={10.1103/physrevlett.2.262},
number={6},
journal={Physical Review Letters},
publisher={American Physical Society (APS)},
author={Scovil,
H. E. D. and Schulz-DuBois,
E. O.},
year={1959},
month=mar,
pages={262–263} }