ABSTRACTS OF TECHNICAL ARTICLES 189 



a high degree of attenuation for audio-frequency waves while offering 

 low resistance to continuous or slowly pulsating air flow. For that 

 purpose ducts lined with absorbing materials are sometimes used. 

 This paper deals with sound propagation, particularly with its at- 

 tenuation constant, in such ducts. Two types of lining are considered: 

 (a) non-vibratile, i.e., a lining in which there is no wave motion propa- 

 gated in the direction of the duct axis; (b) vibratile, i.e., a lining admit- 

 ting of such motion. Methods for computing the propagation con- 

 stants in terms of the acoustic constants of the lining, are given. Some 

 experimental data are presented. Comparison of observed and 

 computed values indicates that the computational procedure is sub- 

 stantially valid up to a frequency at which the sound wave-length is 

 about twice the internal duct diameter. 



New Experimental Methods Applicable to Ultra-Short Waves }^ G. C. 

 SoUTHWORTH. This paper presents a new approach to the problem 

 of electrical measurements at extremely high frequencies. It makes 

 use of a relatively new principle whereby electromagnetic waves may 

 be propagated through the interiors of hollow metal tubes. These 

 tubes have for convenience been called wave guides. Their diameters 

 must be at least 0.585 wave-length in order that power may be propa- 

 gated. Some of the difficulties of previous methods have been in- 

 cidental to radiation and spurious coupling effects. These have been 

 largely eliminated by this method since the power resides almost 

 entirely within the pipe. Short sections of wave guide may be made 

 to resonate electrically much as organ pipes and air columns resonate 

 acoustically. A high degree of sharpness may be obtained. Such a 

 pipe may, therefore, play the role of a resonant circuit and become 

 effectively a wave meter, a frequency determining unit for an oscillating 

 vacuum tube or an impedance matching device. Specimens under 

 study may be placed inside a resonant chamber and be subjected to 

 electric fields of controlled intensities. 



Preparation of Large Single Crystals of Sodium Chloride}^ H. 

 Walther. An apparatus and a method are described, whereby 

 single crystals of sodium chloride are produced in the form, of bars 

 2 cm in diameter and 30 cm long. The crystal is drawn from the 

 melt by means of a platinum rod which is dipped into it and which is 

 raised and rotated simultaneously by a clock mechanism. An air 

 stream from a circular nozzle surrounding the growing crystal im- 

 mediately above the surface of the molten salt provides the steep 



" Jour. Applied Phys. October 1937. 

 ^^ Rev. Sci. Instruments, November 1937. 



