776 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



across the varistor. As a result such items as ventilation, duty cycle, and 

 the like, are important. In bridge- and ring- (lattice) type modulators the 

 problem of protecting the varistor against physical breakdown is seldom 

 present, but the limiting factor is the extraneous modulation products 

 introduced into the circuit. It is therefore necessary to make detailed analy- 

 ses of the spectrum of the sum and difference products involved. In the com- 

 pandor (compressor plus expandor), operating economies are obtained by a 

 device which is dependent on the uniformity of the dynamic characteristic 

 of the varistor in its forward direction. A selected bibliography is included. 



Japan's Recovery and Telephone Service. H. F. Van Zandt^. Telephony, 

 v. 140, pp. 15-17, 46, Mar. 24, 1951. 



Growing Quartz Crystals for Military Needs. A. C. Walker^ Electronics, 

 V. 24, pp. 96-99, Apr., 1951. 



Abstract — Perfected technique gives large, perfect crystals in quantities 

 that mean eventual independence of Brazilian sources. Quartz scrap, alkaline 

 solution and seed plates are sealed into steel bomb by welding, then heated 

 to 400 C to develop 15,000 psi for optimum growth. 



Relation between Lattice Vibration and London Theories of Superconduc- 

 tivity."^ J. Bardeen^. References. Phys. Rev., v. 81, pp. 829-834, Mar. 1, 

 1951. 



Abstract — A gas of noninteracting electrons of small effective mass, 

 meff, has a large diamagnetic susceptibility. It is shown that the London 

 phenomenological equations of superconductivity follow as a limiting case 

 when nieff is so small that the Landau-Peierls theory yields a susceptibility 

 < — Jtt. Justification is given for the use of an effective mass, ms '^ 10~"* m, 

 for superconducting electrons in the lattice-vibration theory of supercon- 

 ductivity. This value is sufficiently small to show that the theory gives the 

 London equations and, as a consequence, the typical superconducting proper- 

 ties. The concentration of superconducting electrons, Us , is smaller than the 

 total electron concentration, n, by about the same ratio as the effective 

 masses, so that mjw^ ^^ ^/^, and thus the penetration depth is of the same 

 order as that given by the usual London expression. 



* A reprint of this article may be obtained on request. 



' B. T. L. 



' Southwestern Bell Telephone Company 



