202 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



is no arcing at the surfaces; but with continued operation and accompanj'ing 

 arcing, the activating material is burned away, and the surface returns to the 

 inactive condition if no activating vapor is supplied. 



Arc voltages, which are independent of current and of ambient gas, as far as 

 tested, have been measured for a number of metals and for carbon; the arc 

 voltage for carbon is quite erratic in the range between 20 and 30 volts, but for 

 each of a number of metals the arc voltage is stead}'. 



Arcing at noble metal surfaces, similar to that induced by carbonaceous 

 material from organic vapors, can be produced also by insulating particles or 

 msulating fihns. The active condition gradually disappears with continued arc- 

 ing, unless there is a steady supply of insulating material to the surface. 



The mmunum arc current has been measured to be 0.6 amp for active silver 

 and for carbon, and 0.03 amp for inactive sih-er. These are the currents at 

 which an established arc is extinguished. 



Iron-Silicon Alloys Heat Treated in a Magnetic Field.* M. GoertzK 

 Jl. Appl. Phys., 22, pp. 964-965, July, 1951. 



Heat treatment in a magnetic field has been found effective for iron-silicon 

 alloys between two per cent and ten per cent silicon, the highest maximum per- 

 meabilit}' being obtained at about 6.5 per cent silicon. In a smgle crystal of this 

 composition, magnetized parallel to a (100) dhection, the hysteresis loop is 

 squared by the magnetic anneal and the maximum permeability is kicreased 

 from 50,000 to 3,800,000, the highest value yet reported. 



Domain Boundary Motion in Ferroelectric Crystals and the Di-electric 

 Constant at High Frequency. C. Kittel^ Letter to the Editor. Phys. 

 Rev., 83, p. 458, July 15, 1951. 



A Method for Determining the Propagation Constants of Plastics at 

 Ultrasonic Frequencies.* H. J. McSkimin^ J. Acoust. Soc. Am., 23, pp. 

 429-434, July, 1951. 



A pulse technique particular!}- suited to dissipati^'e materials is described for 

 measuring attenuation and phase-shift constants of plastics, usmg either trans- 

 verse or longitudinal waves in the frequency range of 5-50 mc. 



A tliin wafer of the material under test is placed between two identical fused 

 sihca buffers; and waves generated by quartz crystals at the ends of the assem- 

 bly are transmitted simultaneously through the specimen in both dh-ections. 

 Comparison of transmitted and reflected components by means of a special 

 balancing circuit provides information from which the complex propagation 

 constant can be calculated, and hence dynamic rigidities and viscosities. 



Illustrative data for polyethylene and Nylon are given. 



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

 1 Bell Tel. Labs. 



