•Tnr. STF.Tfiornos'i-.," .ix elect rich, stethoscope 535 



■J. IIakiv I)ivi:i.()Pmi:n r or nil-: l-liiu tkk .\i. Si i: iiiosc opk . 



riie earliest ilc\elopmeiit work on eleclriral stethoscopes was 

 naturally centered about the carhon transmitter and other micro- 

 phonic contact ile\ ices. In I'.IO", Kinthoven '" made records of normal 

 heart sounds anil murmurs. In HMO,'' heart sounds were reproduced 

 !>>■ a tuned mechanical rela\' consisting of a sinj^le microphonic con- 

 tact and an electromaj;nelic element. With this device, heart sounds 

 were transmitted audibly but evidently with a considerable amount 

 of distortion, over a commercial telephone line in London. The 

 normal heart sounds were amplified by Scjuicr ' for a ii;roiip of physi- 

 cians l)y means of a carbon transmitter in 1921. It is readily possible 

 to amplify the fluctuations in current in a carbon microphone by 

 means of vacuum tube amplifiers. However, the carbon microphone 

 also introduces a certain amount of noise inherent in the use of loose 

 contacts. This noise is below the threshold of audibility for the 

 [lormal use of the microphone, as in the telephone plant, but when 

 it is amplified along with the faint sounds of interest, in auscultation 

 it Incomes ver\' annoying and tends to obscure these other sounds. 

 This "microphone roar" contains components throughout the range 

 of audible frequencies and hence cannot be eliminated. \'arious 

 experimenters have, however, attempted to perfect such a device.*'^ 

 As far as we have been able to determine, such devices have not 

 satisfactorily reproduced faint heart murmurs or chest sounds. 



Of the other possible types, the electromagnetic has thus far ap- 

 peared to offer the greatest promise. In design, this resembles 

 closely the ordinary telephone receiver. This type requires a more 

 powerful amplifier than the carbon microphone but this is not a 

 serious limitation. Such a combination has been used with prom- 

 ising results to obtain graphical records of heart murmurs.'" The 

 progress made with this type of equipment for teaching purposes has 

 been outlined." The successful application of the electromagnetic 



' \V. Einthoven, " Die Reglstricrung der menschlichcn Herztone mittels des 

 Saitengalvanometers," Arch. f.d. ges. Physiol., 117:461 .April 1907; " Rin dritlrr 

 Herzton." ibid. 120:31 Oct. 1907 



«S. G. Brown, ".A Telephone Relay," Journal I. E. E. May 5, 1910. 



■ S. \V. Winters, "Diagnosis by Wireless," Scient. Amer. 124:465 June, l''.'l. 



' R. B. .-\blMjtl, "Kliniinating hiterfcring Sounds in a Telephone Transmitter 

 Stethoscope." Phys. Rn: 21:200 Feb., 1923. 



' Jarohsohn, ".Amplified .Audibility of Heart Sounds," Berlin Leiler J. .1. M. .1.. 

 80:493 V\h. 17, 1923. 



'" H. B. Williams, "New .Method for Graphic Study of Heart Murnuirs." Proc. 

 Soc. Exper. Biol, and Med., 18:179 March 16, 1921. 



" R. C. Cabot, ".A Multiple Electrical Stethoscoix: for Teaching I'ur|)Oscs," 

 J. A. M. A., 81 :298 July 28, 1923. 



