TEE WORLD'S ADVANCE 



267 



obtained with the plain string! 



We can now take the ratio of corre- 

 sponding amplitudes in any two loops 

 and obtain the logarithmic decrement 

 exactly as we did for the string curve. 

 Suppose we take the ratio of AB to DE 

 and get 1.09 as we did for the string. 

 The logarithmic decrement per half peri- 

 od is accordingly 0.09 and 0.18 for the 

 whole period. Our set is, therefore, tuned 

 within the requirements of the radio act 

 which says, "At all stations the loga- 

 rithmic decrement per complete oscilla- 

 tion in the wave trains emitted by the 

 transmitter shall not exceed two-tenths 

 except when sending distress signals or 

 signals and message relating thereto." 



Unfortunately it is not as easy to con- 

 struct the curve for a radio transmitter as 

 was the case with the pendulum, and in 

 general other means have to be resorted 

 to in order to compute the decrement. 

 Several ingenious devices have been de- 

 veloped for this purpose, the most no- 

 table of which is the direct reading decre- 

 meter and wave meter designed by Mr. 

 Frederick A. Kolster of the Bureau of 

 Standards. This is the form of decre- 

 meter used by the U. S. radio inspectors 

 and it is mounted in a leather suit case, 

 in order to make it suitable for tran- 

 sportation on inspection tours. An excel- 

 lent description and mathematical discus- 

 sion of this decremeter by Mr. Kolster 

 himself is found in Volume 3, No. I, 

 March, 1915, issue of the Proceedings of 

 the Institute of Radio Engineers. 



To all operators who have worked in 

 congested districts the effect of a high 

 decrement is known only too well. The 

 higher the decrement the less oscillations 

 per train and the broader the wave. For 

 a decrement of 0.2 per complete period 

 there will be about 15 waves per train. 

 It is beyond the scope of this article to 

 go into the causes and effects of a large 

 value of the decrement, but it may be 

 said that the decrement will be high 

 when there is a high resistance in the 

 circuit, when there is a transfer of energy 

 back and forth between the primary and 

 secondary circuits, or when the two cir- 

 cuits are not tuned to the same wave 

 length. These two last conditions are 

 nearly of the same character. The re- 

 sult is a broad wave, and as the energy 



is not concentrated over a short space the 

 damping or decrement will be high. 

 To sum up, it may be said that the 



FIG. Z 



Diagram for Explaining the Decrement of a Pen- 

 dulum. 



logarithmic decrement is a convenient 

 measure by which the damping of a cir- 

 cuit may be determined. The clause in 

 the radio act of Aug. 13, 1912, defining 

 the maximum value of the logarithmic 

 decrement, sets a standard for the maxi- 

 mum amount of damping permitted to 

 be used by any station. It is desirable to 

 have the damping or logarithmic decre- 

 ment small in order that the emitted 

 wave will be sharp and can be readily 

 tuned out by stations not desiring to re- 

 ceive that wave. Distress signals should 

 be sent with a relatively large value of 

 the logarithmic decrement in order to 

 have a sufficiently broad wave to attract 

 the necessary attention. The correct ad- 

 justment of a transmitting set for a 

 logarithmic decrement below 0.2 is just 

 as essential as having the correct wave 

 length. 



RADIO SECTION OF THE SEPTEM- 

 BER ISSUE. 



The Radio Section of the September 

 issue will contain several short, con- 

 structional articles, as well as longer ar- 

 ticles, dealing with advanced phases of 

 wireless engineering. One of the latter 

 articles will be by Mr. A. S. Blatter- 

 man, and will cover many points that are 

 not clearly understood by the majority 

 of wireless amateurs. The policy fol- 

 lowed in the past few issues, namely, 

 that of presenting long articles of an 

 authoritative nature and prepared by 

 well-known writers in the field, will be 

 continued, although there will be quite a 

 number of short contributions similar to 

 those that characterized the Radio sec- 

 tion of previous issues. 



