1252 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



is given, and the measured and calculated slopes are shown to be in fairly 

 good agreement. 



Microwave Spectrum in NO2. K. B. Mc Afee, Jr.^ Letter to the Editor. 

 Phys. Rev., v. 82, p. 971, June 15, 1951. 



A Simple Electronic Differential Analyzer as a Demonstration and Labora- 

 tory Aid to Instruction in Engineering. M. H. Nichols^ and D. W. Hagel- 

 BARGER^ //. Engg. Education, v. 41, pp. 621-630, June, 1951. 



Telecommunications. H. S. Osborne^. Ordnance, v. 36, pp. 87-90, July- 

 August, 1951. 



Triangular Permutation Numbers. J. Riordan^. References. Am. Math. 

 Soc, Proc, V. 2, pp. 429-432, June, 1951. 



Measurements of Dynamic Internal Dissipation and Elasticity of Soft 

 Plastics.* H. C. RordenI and A. Grieco^. //. Applied Phys., v. 22, pp. 842- 

 845, June, 1951. 



Abstract — In order to measure the mechanical properties of soft plastics 

 over wide frequency and temperature ranges two new techniques have been 

 devised. The first one, which operates in the frequency range of a few cycles, 

 uses a horizontal oscillating pendulum. The shear impedance of the sample 

 is measured by mounting a small pad of the material between the vibrating 

 pendulum and a fixed platform and determining the change in frequency 

 and the change in the decrement caused by the sample. From these measure- 

 ments the shear mechanical resistance and reactance of the specimen can 

 be determined. The other technique, which is applicable in the frequency 

 range from 100 cycles to 10,000 cycles, makes use of a vibrating tuning fork. 

 Two identical samples are mounted between a stationary weight and the 

 moving tines, and the shear mechanical impedance is determined by deter- 

 mining the change in frequency and change in decrement caused by the 

 specimen. These two techniques have been applied to measuring the shear 

 properties of a number of soft plastics including Pyralin, Koroseal, Keldur, 

 polyvinyl butyral, Thiokol, and gum rubber. All of these show relaxation 

 effects. The polyvinyl butyral appears to be approaching a crystalline elastic 

 stage at the low frequency of 1000 cycles, while gum rubber remains in a 

 quasi-co'nfigurational stage from 2 cycles to 1000 cycles. 



The Mobility of Electrons in Silver Chloride.* J. R. Haynes^ and W. 

 Shockley^ References. Phys. Rev., v. 82, pp. 935-943, June 15, 1951. 



Abstract — Techniques are described which utilize the ''print out effect" 

 to obtain both the direction and velocity of photoelectrons in silver chloride 

 crystals in an electric field. Hall mobility of the electrons is calculated from 

 their change in direction produced by crossed electric and magnetic fields. 



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

 » Bell Tel. Labs. 



