ABSTRACTS OF TECHNICAL ARTICLES 547 



Probability in Engineering.^ E. C. Molina. The purpose of this 

 paper is to emphasize the practical value of probability theory in 

 engineering. For this purpose a short introduction on probability 

 theory as such is followed by a discussion of three problems from the 

 domain of engineering with which the author is most familiar, namely, 

 telephony. 



The first problem deals with the switching, or trunking, of telephone 

 calls. It illustrates the part played by probability theory in deter- 

 mining the amount of equipment an engineer must install in anti- 

 cipation of deviations from normal or average service demands. 



The bearing of probability theory on problems wherein one is 

 confronted with the cumulative effect of a multitude of small indepen- 

 dent discrepancies is indicated by the second problem presented in the 

 paper. A long distance telephone circuit equipped with repeaters at 

 several points is analyzed with reference to the cumulative effect of 

 slight voltage variations in the battery supply at each repeater station. 



The third and last problem is one on sampling. It introduces the 

 engineer to the practical significance of inverse or a posteriori prob- 

 ability. 



Direct- Current Amplifier Circuits for Use with the Electrometer 

 Tube}^ D. B. Penick. A number of balanced, single-tube, direct- 

 current amplifier circuits are compared, which are applicable to the 

 four-element, low grid-current vacuum tube. The balance equations 

 are stated for the most generally useful circuit, and magnitudes of the 

 tube characteristics involved are given for the Western Electric No. 

 D-96475 Tube. Experimentally determined values of circuit con- 

 stants observed under balance conditions are also given. The sta- 

 bility of the circuit is discussed, and a convenient procedure for 

 obtaining a balance by experimental methods is suggested. 



Infernal Dissipation in Solids for Small Cyclic Strains. ^^ R. L. 

 Wegel and H. Walther. This paper presents the results of in- 

 vestigations of dissipation of energy in vibrating solids, mostly metals, 

 by means of longitudinal and torsional vibrations of cylindrical rods. 

 The amplitudes of strain used have been kept between 10~^ cm. /cm. and 

 10~* cm. /cm., in which range the dissipation of energy is proportional to 

 the square of the strain. The specific dissipative property of a material 

 is expressed in three dififerent ways : (I) Equivalent viscosity or the ratio 



9 Elec. Engg., April, 1935. 



^^ Rev. Set. Instruments, April, 1935. 



" Physics, April, 1935. 



