1. A. Ellet and R. Zabel, Phys. Rev., No. 37 (1931).
2. H. G. Kolsky et al., Phys. Rev., No. 80 (1950).
3. A. V. Bulyga, Semiconductor Thermoelectric Vacuum Gauges [in Russian], Énergiya, Moscow (1966).
4. G. Grigorov and S. Ivanov, Pribory i Tekh. Éksperim., No. 3 (1964).
5. S. Ivanov, Sofia University Yearbook, Physics [in Bulgarian], vol. 60 (1965?66).