Bell telephone system: Monographs: B-1238, Co-ordination of Power and Communication Circuits for Low-Frequency Induction, by J. O'R. Coleman and H. M. Trueblood. 19 pages, illustrations. B-1241, Analysis of the Ionosphere, by Karl K. Darrow. 34 pages, illustrations. B-1245, High-Gain Amplifier for 150 Megacycles, by G. Rodwin and L. M. Klenk. 13 pages, illustrations. B-1246, Sound Measurement Objectives and Sound Level Meter Performance, by J. M. Barstow. 42 pages, illustrations. B-1247, Helium the Superfluid, by Karl K. Darrow.23 pages, illustrations. B-1248, Rectilinear Electron Flow in Beams, by J. R. Pierce. 17 pages, illustrations. 6 pamphlets, 15 × 23 cms. New York, Bell Laboratories, 1940