38 Applied Biophysics 



muscle and nerve and of applying electrical treatment of known 

 character and dosage. 



Ritchie ^^ has described a simple variable "square-wave" 

 stimulator for biological work. The instrument uses two stand- 

 ard triode valves to produce impulses independently variable 

 in intensity, duration, and frequency over the wide ranges used 

 in the excitation of nerve and muscle. 



Electrocardiography 



The electrical variations produced by the heart during con- 

 traction are distributed through the body, and can be led ofif 

 from the moist skin surface of such areas as the arms and legs, 

 and recorded. 



Kolliker and Miiller -^ showed, by physiological experiments, 

 that an electrical change accompanies the beat of the isolated 

 frog's heart. \\^aller ^^ demonstrated similar changes occurring 

 in the human heart, when electrodes are applied to the limbs. 

 He used Lippman's capillary electrometer, and his experiments 

 remained of academic interest only. Einthoven ^^ introduced 

 the string galvanometer which made electrocardiography, in its 

 modern form, a clinical science. Some of the disadvantages of 

 the string-galvanometer type of electrocardiograph are : the 

 fragility of the string, the necessity of skin-current compensation, 

 and the use of nonpolarizable electrodes. 



Because of the extremely low voltage generated by the action 

 of the heart, instruments for its measurement in the past have 

 necessarily been extremely sensitive, and the recorders of these 

 have, therefore, been very delicate. The introduction of thermi- 

 onic-valve amplifiers, and the substitution of robust oscillagraphs 

 changed all this. The usual form of recorder employed with 

 thermionic-valve amplifiers was the mirror galvanometer of 

 comparatively low sensitivity. Examples of such instruments 

 are the \^ictor electrocardiograph and the Matthews electro- 

 cardiograph. 



The Both electrocardiograph works on the thermionic-valve 

 amplifier principle, but feeds a small cutting stylus which indents 



