ELECTROCARDIOGRAMS 



271 



ord obtained may be quantitative in character, it is further necessary 

 that the movement of the string be standardized. This is done by as- 

 certaining to what extent the string moves when a current of known 

 voltage is sent through it and by altering the tension of the string so that 

 one millivolt of current causes an excursion of one centimeter of the 

 string shadow on the photographic plate. It would take us beyond the 

 confines of this volume to go in any greater detail into the technic in- 

 volved in taking electrocardiograms, but it may be said that this is by 



Fig. 81. Electrocardiographic apparatus as made by the Cambridge Scientific Materials Co. Con- 

 tact electrodes are shown, but the immersion electrodes described in the context are preferable. 



no means difficult, provided the instructions which are supplied with 

 the instrument are carefully followed. In practice the taking of elec- 

 trocardiograms is indeed quite a simple matter, and the extremely im- 

 portant information which they give us concerning the mechanism of 

 the heartbeat and the evidence of myocardial disease should make their 

 employment a universal practice in all cardiac clinics. Some of these 

 clinical applications are described elsewhere (page 278). 



What particularly interests us here is the contour of the electrocardio- 

 gram in a normal person (Fig. 82). It will be observed that there are 



