48 Applied Biophysics 



Parr and Walter ^^ describe the technical methods, and give 

 circuit diagrams of amplifiers suitable for electroencephalographic 

 recording. Traugott '*'"' discusses electroencephalograph design 

 and publishes the circuit of his amplifier. The Technical Sub- 

 committee of the Electroencephalographic Society has drawn uj) 

 recommendations for recording apparatus. 



The Marconi four-channel electroencephalograph (see figure 

 7), particulars of which, as far as the author is aware, have 

 not yet been published, consists of two double-channel amplifiers, 

 and a four-pen ink-recorder, with a variable-speed paper drive. 

 Each pen is actuated by a moving iron armature, the signal 

 w^inding being stationary (a permanent magnet field system is 

 used). Provision is made for the attachment of auxiliary equip- 

 ment, such as a cathode-ray oscilloscope, or a frequency analyzer. 

 Power supply units for operation from alternating cur- 

 rent supply mains are incorporated. The final smoothing of 

 the high-tension supplies is accomplished electronically and, 

 where necessary, electronic regulation is also employed to take 

 care of mains voltage fluctuations. Each amplifier channel has 

 a differential input and uses a common-cathode push-pull circuit 

 throughout. The time constant is controllable in four steps 

 between 1 second and 0.01 second, and the limit of high- 

 frequency response is variable between 15 cycles per second and 

 4,000 cycles per second. The upper limit of response with ink- 

 recording is 75 cycles per second. The overall sensitivity is 

 such that, at maximum gain, a 20 microvolt peak-peak input 

 produces a 20 millimeter peak-peak deflection of the recorder. 

 Inputs up to 100 millivolts peak-peak are accommodated. The 

 amplifier noise with the input short circuited and earthed does 

 not exceed 2 microvolts root mean square. 



Beevers and Furth •''• ^ devised the encephalophone which 

 converts the electrical-potential changes into sound waves. 

 Basically, this apparatus is a form of heterodyne oscillator, 

 where the brain rhythm varies the frequency of the heterodyne 

 beat note. The "alpha" and "beta" rhythms give characteristic 

 trills, while "delta" waves produce slow sweeps of tone. 



Various ways of supplementing primary inspection of the 



