ARTICLES BY BELL SYSTEM AUTHORS 479 



ture are supplemented by tests at a higher frequency on reduced-scale, sim- 

 plified models. 



Wave Functions for Superconducting Electrons* ]. Bardeen.^ References. 

 Phys. Rev., v. 80, pp. 567-574, Nov. 15, 1950. 



Abstract — The observed variation of the transition temperature of mercury 

 with isotopic mass is evidence that the superconducting state arises from in- 

 teraction of electrons with lattice vibrations. The interaction term which 

 gives scattering of electrons at high temperatures contributes at low tem- 

 peratures a term to the energy of the system of electrons plus normal modes. 

 Frohlich has calculated the interaction energy at T = 0°K by second-order 

 perturbation theory. The energy is calculated here by taking wave functions 

 of superconducting electrons, which have energies near the Fermi surface, 

 as linear combinations of Bloch functions whose coefficients are functions 

 of coordinates of the normal modes. In an equivalent approximation, 

 Frohlich's expression for the interaction energy is obtained. When the 

 energy is calculated directly rather than by perturbation theory, modified 

 expressions are obtained for the energy and distribution of electrons in the 

 superconducting state. The criterion for superconductivity is h/r > ^^ IttkT, 

 where r is the relaxation time for electrons at some high temperature T 

 where tT is constant. It is shown that superconducting electrons have small 

 effective mass. 



Clampers in Video Transmission* S. Doba, Jr.^ and J. W. Rieke.^ A. I. 

 E.E., Trans., v. 69, pt. 1, pp. 477-487, 1950. 



Abstract — One of the major problems connected with the transmission of 

 television signals is the exceptionally wide video band of frequencies in- 

 volved. For the present black-and-white standards this amounts to about 

 4 megacycles. In the transmission of the television signal at video frequencies, 

 that is, noncarrier transmission, the problem is further complicated because 

 the lower limit of the frequency range extends literally to zero frequency. 



Calculation of Vowel Resonances, and an Electrical Vocal Tract.* H. K. 

 DuNN.^ Bibliography. Acoustical Soc. Am., Jl., v. 22, pp. 740-753, Nov., 

 1950. 



Abstract — By treating the vocal tract as a series of cylindrical sections, 

 or acoustic lines, it is possible to use transmission line theory in finding the 

 resonances. With constants uniformly distributed along each section, res- 

 onances appear as modes of vibration of the tract taken as a whole. Thus, 

 the fundamental mode of the smaller cavity may be affected considerably 

 by a higher mode of the larger; and in addition, higher resonances are found 



* A reprint of this article may be obtained on request to the editor of the B. S. T. J. 

 1 B. T. L. 



