( 264 ) 



motion at the limit of aperiodioity, and obtain a time of deflection 

 of about 15 seconds. 



We remark liere that thread n". 18 may easily be so feebly 

 stretched that its time of deflection becomes about one minute, by 

 which the sensitiveness is increased to c,, = 40 mm. per nncrovolt. 

 Since, as is proved by the photograms, 0.1 mm. can still be read 

 off, with thread 18 a P.D. of 2.5 X 10--'volt can actually be 

 demonstrated. Also with this feeble tension of the thread the zero 

 point remains constant, while the image of the quartz thread remains 

 sharp over a pretty long part of the scale. It may be considered 

 remarkable that one should be able to displace so slowly with the 

 greatest regularity a suspended little thread of only a few thousanths 

 of a milligramme weight. 



II B. We now come to the methods in which the deflection or 

 the galvanometer must be aperiodic and at the same time quick. 



These methods in the first place find an application in electro- 

 technics, e. g, for investigating the shape of the oscillations of potential 

 and current obtained by means of dynamos, interrupters, induction 

 apparatus, etc. For these purposes the oscillograph is already used 

 with good results, which instrument possesses a considerably smaller 

 sensitiveness than the string gahanometer, but yet can be of excellent 

 service in the measurement of stronger currents. 



In the second and for our purpose most important place the methods 

 mentioned under II B find their application in electrophysiology. 

 Here in many cases the string galvanometer cannot be replaced by 

 any other instrument. 



A number óf electrophysiological investigatioiis of the most various 

 kind can be made with the same string. So in the laboratory 

 the same string n". 18 is now used for investigating the electro- 

 cardiogram, cardiac sounds and sounds generally, retinal currents 

 and nerve currents. Yet we will briefly discuss here the conditions 

 which must be fulfilled by a string, chosen from a number of available 

 strings, in order to yield the best results in a certaiji electrophysio- 

 logical investigation. 



Let us begin Avith the tracing of the human electrocardiogram. 

 The current may here be derived from both hands. The hands and 

 lower arms are immersed in large porous pots, filled with a solution 

 of Na 01, placed in glass vessels, containing a solution of Zn SO^. 

 In the zinc sulphate solution are amalgamated zinc cylinders, connected 

 by connecting wires with the galvanometei'. Under these circumstances 

 the ohmic resistance of the human body varies with different persons 

 from 1000 to 2000 ohms, an amount considerably smaller than the 



