244 



millivolt, and tliis potential difference would actually exist between 

 the places of derivation, if tliese were not connected to the <ialvano- 

 meter. 



If the body is connected to a second galvanometer, the detlections 

 of tiie first will lie diminished, and this will be increasingly the case 

 in projiorlion us the second galvanometer possesses less resistance. 

 By a third connoction the results aie again reduced, and I he (piestion 

 therefore arises : How is the seusili\iiy of the three galvanometers 

 to be rcgidalcd, so thai liiey will siniullaneousiy inscril)e curves 

 whicii will fullil llie c(Uidilions re(|uired : The centimetres of the 

 ordinales luusi always represent the milli\olts of the potential oscil- 

 lations which occur between two points t)f derivation of a body, 

 when the body itself is still free from ail connections. 



As long as only one galvanometer is connected to the body, at 

 the siulden application of c millivolts in the circint the image of the 

 string must be deflected by ,■ ems. If thei-e are three galvanometers 

 coiuiected to the body at the same time, l)y the application of e 

 millivolts the deflection must be more than e cms. In a particular 

 ease with the simultaneous derivations I. II, and III, we will call 

 the deflections required E^, E^, and E,. These detlections can be 

 calculated by means of the laws of distribution of currents from 

 the potential difference e applied each time, the resistances of the 

 body /j, /, and /,, and the galvanometer resistances v,, r/,, and (/,. 



The I'esult may be obtained in the simplest and at the same time 

 most practical wa}', by using the method of the equilateral triangle. ') 



In this model of the human body the resistances of the body in 

 the three derivations are equal. If l■^, 4, and /, really differ from 

 one another, they can be made e(pial by means of rheostats, or in 



the adjustments a mean resistance / ^= ^ may be used. 



In almost all cases this last method which is simpler in practice, is 



amply accurate enough. 



The gahanometer-resisfances must be actually made equal to each 



other, by the addition of rheostat resistances to the two smallest 



ones. We then write ij^ =i (j^ ^ (/, = (j. 



I 

 If := a. the deflection required is for each of the three string 



images E =^ e {1 -\- a) centimetres. 



We may remark in passing that at the application of e millivolts 



1) Gomp. 'Ueber die Richtung und die manifeste Grosse der Poteiitialschwan- 

 liungen im menschlichen Herzen", u. s. w. Pflüger's Archiv fiir die ges. Physio- 

 logie. Bd. 150. p. •iTo, 1913. 



