176 BELL SYSTEM TECHNICAL JOURNAL 



nique is called Pulse Code Modulation or simply PCM. An eight-channel 

 system embodying these principles was developed and produced in portable 

 form for field operation. Other work carried on simultaneously by W. M. 

 Goodall (see B. S. T. J., July 1947) resulted in the development of an experi- 

 mental system using a different method of coding. 



In carrying out this new type of modulation, the speech wave applied to 

 each channel is, in effect, transmitted sample by sample, and each sample is- 

 represented by a multi-unit code employing on-or-off pulses, hence the term 

 PCM. 



This method appears to have exceptional possibiUties from the standpoint 

 of freedom from interference. Its full significance in connection with future 

 radio and wire transmission may take some time to reveal. 



Stereoscopic Drawings of Crystal Structures.^ W. L. Bond. A method is 

 presented for getting stereoscopic pairs of atomic structure views given the 

 coordinates of the atoms and cell constants. 



Properties of Liquids at High Sound Pressure.^ H. B. Briggs, J. B. John- 

 son and W. P. Mason. When sound of high amplitude is transmitted into 

 a liquid by means of a mechanical driving device, the ultimate limitation to 

 the power that can be transferred is cavitation or breakdown of the liquid 

 under high internal stresses. A study of cavitation has resulted in establish- 

 ing the following results. Under steady-state conditions, light liquids filled 

 with air cavitate when the negative acoustic pressure reaches the atmos- 

 pheric pressure. When liquids are degassed, their natural cohesive pressure 

 becomes effective and they will withstand a negative acoustic pressure. It 

 is found that the total negative pressure required to cause cavitation is equal 

 to the sum of the cohesive pressure — tensile strength — and the ambient 

 pressure. Viscous liquids have a higher cohesive pressure and a proportion- 

 ality has been established between the logarithm of the viscosity and the 

 cohesive pressure. The amount of power that a liquid can withstand 

 increases markedly as the pulse length is shortened. 



An explanation of these phenomena is attempted on the basis of Eyring's 

 theory of viscosity, plasticity and diffusion. On this theory natural holes 

 exist in the liquid into which molecules can jump, leaving holes behind them. 

 A jump occurs when the molecule has accumulated enough heat energy to 

 surmount an activation potential barrier of energy value Eo. Cavitation 

 appears to be the result of coalescing of the natural holes in the negative 

 pressure phase of the cycle. Since a molecule has to jump from a hole in 

 order that this can coalesce with another hole, the cavitation pressure is pro- 

 portional to the activation energy which in turn is proportional to the log- 



♦ The American Mineralogist, July- August 1947. 

 *Jour. Acous. Soc. Amer., July 1947. « 



