766 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



diffraction pattern gives no due, a difference between the thermal expan- 

 sivity and the temperature dependence of the lattice parameter suggests a 

 crystallographic change. Young's modulus data disclosed another transition 

 near — 152° C. 



How to Measure Apparatus Noise. C. H. G. Gray^ Standardization^ 

 V. 22, pp. 55-56, Feb., 1951. 



Fluorocarbon Hermetic Seal Design. A. B. Haines^ Elec. Mfg., v. 47, pp. 

 113-115, Jan., 1951. 



Abstract — ^Terminal seals using molded trifluorochloroethylene resin 

 solves sealing problem in design of miniaturized 400-cycle high-operating- 

 temperature power transformers. 



Dynamic Shear Properties of Rubberlike Polymers.'*' I. L. Hopkins^ Refer- 

 ences. A.S.M.E., Trans., v. 73, pp. 195-203; disc. pp. 203-204, Feb., 1951. 



Abstract — A simple apparatus for determining the dynamic properties 

 of elastomers in shear at audio frequencies is appraised. Typical values of 

 shear modulus and viscosity for several elastomers are given, both at room 

 conditions and at 150 F. The frequencies of test range from 100 to 5250 cy- 

 cles per second, the shear moduli from 0.5 X 10^ to 480 X 10^ dynes per 

 sq cm and the viscosities from 20 to 75,000 poises. 



Production-Line Frequency Measurements. G. J. Kent^. Electronics, v. 

 24, pp. 97-99, Feb., 1951. 



Abstract — Simplified equipment allows relatively inexperienced per- 

 sonnel to make extremely accurate measurements of frequencies up to 10 mc. 

 Entire system is standardized against WWV by simple adjustments while 

 frequency measurement is being made. 



Progress in Development of Test Oscillators for Crystal Units.* L. 

 F. KoERNERi. I.R.E., Proc, v. 39, pp. 16-26, Jan., 1951. 



Abstract — Early crystal unit test oscillators as conceived some 20 years 

 ago were principally duplicates of the actual equipment in which the crystal 

 units were to be utilized, a practice which resulted in a large variety of test 

 circuits and procedures for testing. It is now recognized that a knowledge 

 of the equivalent electrical elements making up the crystal unit is essential 

 to the circuit engineer, and that the older conception of frequency and ac- 

 tivity, the latter being an attempt to express the quaUty of a crystal unit 

 in terms of a particular oscillator circuit, do not define adequately its char- 

 acteristics. The equivalent electrical circuit of the crystal unit contains 

 essentially a resistance, an inductance, and 2 capacitances, which together 



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



1 B. T. L. 



»W.E.Co. 



