296 THE ELECTROCARDIOGRAM 



mercury followed by a smaller downward movement demon- 

 strated a similar but greater ventricular diphasic response. It 

 is not necessary to expose the heart and lay non-polarisable 

 electrodes on it in order to see this diphasic response by the electro- 

 meter. The right arm may be considered as electrically con- 

 tinuous with the base and the left leg (or arm) with the apex of 

 the heart. Connection of the arms by non-polarisable electrodes 

 to the electrometer gives results similar to those obtained from 

 animals by direct " leading." 



Clinicians seem to prefer the more sensitive string galvanometer 

 as an instrument for electrocardiographic work, in spite of its 

 great expense and the difficulty of analysing its records. The 



AURICLE 



VENTRICLE 



FIG. 63. Record of the electrical variations in the beating heart of a tortoise, 

 taken by a capillary electrometer (after Gotch). 



instrument at present generally employed is substantially that 

 invented by Ader and modified by Einthoven. The earlier forms 

 of string galvanometer were almost useless as a means of registering 

 the rapid alterations in the electrical state of the heart. Any 

 recording apparatus for such work must be as " dead beat " as 

 possible moving in exact accordance with the exact potential 

 difference developed and having no period of vibration of its own. 

 As its name implies the moving part of the string galvanometer is 

 a string or fibre. The string (C, Fig. 64) which is an extremely 

 light fibre of silvered glass, quartz, or platinum is stretched 

 between the poles (N, S) of a powerful electromagnet. When a 

 current passes along a fibre, the fibre is deflected at right angles 

 to the magnetic field, the amplitude of the excursion depending 

 on the magnitude of the potential difference causing the current ; 

 and the direction of the deflection (observer's left or right) depend- 

 ing on the direction in which the current is passing. If the current 



