STRING GALVANOMErER 



385 



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Records from these three pairs of leads dilTcr from one aiiotlier, 

 and information may be gained from these differences as to the 

 state of the myocardium at various 

 parts. 



(3) String Galvanometer. 

 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 instrument at present 

 generally eiuployed 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 appara- 

 tus for such work must be as " dead 

 beat " as possible — ^moving in exact 

 accordance witli 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. 91), 

 which is an extremely light fibre of 

 silvered glass, quartz, or platinum, is 

 stretched between the poles (A'^, 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 direc- 

 tion of the deflection (observer's left 

 or right) depending on the direction 

 in which the current is passing. If 



the current passes in the direction of the arrow, from top to 

 bottom of the diagram, the fibre will bend outwards, i.e. in the 

 direction of the arrow a. Reversal of the direction of the current, 

 of course, causes reversal of the movement of the fibre. The 

 excursions of the string can be observed by means of the reading 



B. 25 



