References

Owen W. Richardson, (1929) Thermionic phenomena and the laws which govern them. Nobel Lecture
             http://nobelprize.org/nobel_prizes/physics/laureates/1928/richardson-lecture.pdf
Child C. D., Phys. Rev. (Series I) 32, 492 (1911);
             I. Langmuir, Phys. Rev. 2, 450 (1913).
Ejs http://www.um.es/fem/EjsWiki/ accessed 2009 September
Hemenway C.L., Henry R.W., Caulton M. PHYSICAL ELECTRONICS.. pp 59-60 Ed.: John Wiley and Sons, Inc. 1962