46 Applied Biophysics 



The loudness of the heart sounds as heard in the amplifying 

 stethoscope is governed by a tone-compensated gain control, 

 which helps to correct certain deficiencies in the human ear 

 in which the auditory sensation produced by complex sounds 

 may be decidedly different in character as well as intensity 

 when the stimulating level is increased or decreased. Such a 

 device permits greater latitude in varying the intensity levels at 

 which the heart sounds are heard. 



It is easy to pick up the jugular sphygmogram by shunting 

 the microphone with a 1 mfd condenser. The shunted-condenser 

 microphone method is also used for recording the pneumo- 

 cardiogram. It is an obvious advantage to have an all-electric 

 method of recording these traces. 



The phonoelectrocardioscope is of value in teaching, research 

 and clinical medicine. 



Electroencephalography 



The technique of electroencephalography is analogous to that 

 of electrocardiography, viz., amplification and registration of 

 the electrical potentials from the brain as picked up from the 

 surface of the body. The upper limit of size of the brain poten- 

 tials as led off from the scalp approaches that of the electro- 

 cardiogram, i.e., about 1 millivolt. Potentials even greater than 

 this are obtained when leads are placed directly on the exposed 

 cortex. Discharges of this magnitude are rare, and only found 

 in abnormal conditions. 



The electrical variations generated by the brain fall into 

 certain patterns. The alpha waves, normally present in most 

 people, have frequencies in the neighborhood of 10 cycles per 

 second and amplitude of 10-50 microvolts. The beta waves have 

 a frequency of 30-40 cycles per second, but are of lower voltage. 

 Low-frequency waves, below ?> cycles per second, are called 

 delta waves, and are often of larger amplitude than either the 

 alpha or beta waves. The patterns are frequently superimposed. 

 Walter and Dovey ^^ suggest that rhythms at about 6 cycles 

 per second should be termed "theta" rhythms, and that such 



