MEASUREMENT OF PULSATION BY ELECTRICITY. 61 



interrupts the current. This period, which may be cal- 

 culated by the method described in the last paragraph, 

 represents exactly that which elapses from the moment 

 at which the irritant affects the muscle to that at which 

 contraction begins. 



Yet another circumstance must be taken into con- 

 sideration, in order to render actual measurements pos- 

 sible. The muscle contracts on being irritated. This 

 contraction, however, lasts only a very few parts of a 

 second, and the muscle then resumes its former length. 

 In the experiment just 

 described, the current 

 interrupted by the con- 

 traction of the muscle 

 would soon be again 

 completed, and the mag- 

 net would undergo a new 

 deflection even before 

 the first vibration was 

 finished. In order to 

 obviate this, Helmholtz 

 employed means the na- 

 ture of which is made 

 intelligible in fig. 21. This figure represents the end 

 of the lever of the apparatus already described, together 

 with the two screws p and g, the platinum plate and the 

 quicksilver capsule ; at k are the wires connecting the 

 latter with the vices. The quicksilver in the capsule 

 Hfj can be raised or lowered by means of the screw s. 

 Jf the level of the quicksilver is raised so as to immerse 

 the point q, and if it is then again lowered, the quick- 

 silver, by adhesion, remnins hanging from the amalga- 

 mated point, and is by this means drawn out in the 



21. TlIE END OF THE LEVER OF 

 THE APPARATUS FOR TIME MEASURE- 

 MENT, TOGETHER WITH THE QU1I K- 

 SILVEU CAPSULE. 



