298 



THE ELECTROCARDIOGRAM 



of the fibre. The movements of the fibre are, as we have seen, 

 parallel to the plane in which the lens is set, and therefore when 

 the fibre moves towards the reader (in the diagram) the result 



will be a corresponding alteration in 

 the position of the shadow spot. A 

 continuous record of these positions 

 is formed on the moving sensitised 

 surface. 



The records (Figs. 66 and 67) show 

 vertical and horizontal markings as 

 well as the electrocardiogram itself. 

 The horizontal markings enable one 

 to find by inspection the potential dif- 

 ference generated at different phases 

 of the cardiac cycle. The space be- 

 tween each line is generally 1 mm.= 

 1/10,000 volt (Einthoven's standard). 

 The lines are engraved across the 

 width of the cylindrical lens. When 

 illuminated they produce shadows 

 forming lines along the length of the 

 record. The vertical lines, shortened 

 to ticks at the foot of the records 

 illustrated, are a measure of time- 

 in the cases given=l/30 of a second. 

 They are produced by the inter- 

 ruption of the focused beam of light 

 by a serrated wheel (Fig. 65) so 

 that for a short interval no light falls 

 on the whole (or on part of the sensi- 

 tised surface) as it is travelling past 

 the slit. In consequence, a sharp line 

 falls on the record. 



Before a record can be taken, it is 

 necessary to know the resistance of 

 the subject's body and the magnitude 

 of the " skin-current." The latter 

 factor is a relatively large and fairly 

 constant potential difference caused 

 by the glandular activities of the skin. It has to be counter- 

 balanced by sending an equal current through the fibre in the 

 opposite direction. The resistance of the body to the passage of 



