260 



THE CIRCULATION OP THE BLOOD 



quired amount of current, called the compensating current, to bring the 

 strong shadow back to the zero or midposition. In order that the rec- 

 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 



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

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



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 

 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 266). 



