Applications of Electronics in Medicine 49 



electoencephalogram have been devised, such as that by Grass 

 and Gibbs.-^ Walter ^" introduced a device to overcome the diffi- 

 culties of the foregoing method. Briefly, Walter's method is as 

 follows : A series of tuned reeds are energized by the output of 

 the electroencephalograph. These reeds act as frequency split- 

 ters, since each is tuned to a frequency in the band to be studied. 

 Each reed is provided with a fine steel contact wire, which dips 

 in and out of a mercury cup when the reed vibrates, but is just 

 out of the mercury when the reed is at rest. A high resistance, 

 a source of electromotive force, and a condenser are in series 

 with this mercury reed-contact. The condenser is charged 

 up to a potential which is a function of the total dura- 

 tion of the contact time, and, therefore, of the amount of energy 

 at the reed frequency during the specified time. An amplifier 

 is connected to each condenser in turn by a motor-driven rotary 

 switch, and this amplifier works a wide-arc recording-pen 

 across the recording paper on which the original electro- 

 encephalogram is at the same time being traced. The summation 

 epoch is chosen to be 10 seconds, so that each 10-second stretch 

 of record has traced over it a histogram of its spectrum. The 

 analysis is performed automatically every 10 seconds. The de- 

 tails of design are fairly intricate and the adjustment is critical. 

 Electroencephalographic amplifiers can be modified for use 

 in electromyography, cardiography, and as general purpose 

 biological amplifiers. 



Sound 



An audiometer is an apparatus for the measurement of hearing 

 loss. Many of these devices have been introduced. A popular 

 model of such an instrument comprises a tone source (a ther- 

 mionic-valve oscillator working on the heterodyne principle) 

 which has a frequency range of 100-10,000 cycles per second 

 continuously variable. The output of the tone source is fed to a 

 high-fidelity moving-coil ear piece via an attenuator calibrated 

 to read in hearing loss or gain. An auxiliary control auto- 

 matically corrects the reading for the variation of the threshold 



