ABSTRACTS OF TECHNICAL ARTICLES 323 



Some of the properties of the waves and the apparatus used in studying 

 them are described in this article. 



Recent Development in Hill and Dale Recorders. ^^ L. Vieth and C. F. 

 WiEBUSCH. A new sound-on-disk recorder has been developed in 

 which is used the principle of feeding part of the output of the system 

 back to the input of the associated driving amplifier in properly 

 controlled relationship. The use of this principle, which is widely used 

 in feedback amplifiers, replaces the usual practice of providing dis- 

 sipative elements for the control of an electrically driven vibrating 

 system. Heretofore no practical application of feedback to electro- 

 mechanical systems has been made, possibly because the requirements 

 for stable operation of such systems are difficult of achievement. 

 Through recent developments these requirements have been satisfac- 

 torily met. The new recorder is capable of recording on wax or direct- 

 recording material without appreciable effect upon its characteristics, 

 which include uniform response from 30 to 12,000 cps. and exceptional 

 freedom from distortion. The recorder is extremely simple and affords 

 easy means for field calibration from the feedback element, whose 

 output is in direct proportion to the stylus velocity. These means also 

 make available a monitoring voltage which, properly amplified, gives a 

 precise aural picture of the stylus behavior during recording. 



Internal Friction in Solids — ///. Experimental Demonstration of 

 Thermoelastic Internal Friction.^* C. Zener, W. Otis and R. Nuc- 

 kolls. In order to demonstrate the presence of thermoelastic internal 

 friction, the authors measured the internal friction of a copper reed 

 over a wide frequency range (50 to 4000 cycles/sec). They obtained a 

 maximum precisely at the predicted frequency. The observed varia- 

 tion of internal friction with frequency proves that, over a wide 

 frequency range, the internal friction due to the flow of heat back and 

 forth across a reed is of a larger order of magnitude than that due to all 

 other causes. Independent experiments of Bennewitz and Rotger on 

 wires of silver, aluminum, brass, steel, and glass are shown to furnish 

 an equally striking demonstration of thermoelastic internal friction. 



13 Jour. S. M. P. E., January 1938. 

 ^^Phys. Rev., January 1, 1938. 



