368 



ELECTRO-PHYSIOLOGY 



CHAP. 



metal wheel, but in circuit with one another, are the two points 

 (brushes) forming closure of the galvanometer, which at a certain 

 point of the revolution pass through the mercury pools of two 

 isolated steel cups (g l , (f\ or over amalgamated copper contacts 

 included in the galvanometer circuit (B^, B\ The pools (contacts) 

 are movable, so that the duration of the simultaneous dip, 

 i.e. duration of galvanometer closure (T), can be altered within 

 a wide margin. Now, instead of extending this interval over the 

 surface of the curve of variation as above, the distance of 

 the time (T) from the moment of excitation (t, t l ), etc., is 

 regulated in Bernstein's instrument by alteration of the slider 



FIG. 115. Bernstein's differential rheotome (seen from above). 



which carries the exciting contact. The whole arrange- 

 ment of the experiment resembles the diagram (Fig. 116). 

 Owing to the complicated structure of the gastrocnemius, the 

 sartorius is better adapted for the study of the negative variation, 

 its demarcation current being compensated to start with. In 

 consequence the galvanometer magnet remains at rest during 

 rotation, and is only deflected when there is an alteration of the 

 muscle current during the time (T). If the slider is then 

 arranged, as in Fig. 116, so that the closure of the circuit 

 of the primary coil occurs at the same moment at which 

 the galvanometer circuit is broken by the two contacts, a 

 complete revolution occurs before the closure of the muscle 



