MEASUREMENT OF PULSATION BY ELECTRICITY. 59 



current is closed at the instant at which the muscle is 

 irritated, and to interrupt this current at the instant at 

 which the contraction of the muscle begins. 



This experiment also was first effected by Helmholtz. 

 The apparatus used for the purpose is shown in fig. 20, 

 in the altered form used by du Bois-Reymond. From 

 a fixed stage rises a column to which a strong vice for 

 the reception of one end of the muscle is attached in 

 such a way that it can be moved upward or downward. 

 The lower end of the muscle is fixed by means of a 

 connecting piece i h with a lever which can be turned 

 on the horizontal axis a a'. The lever is prolonged 

 below into a short rod which, passing through a hole 

 in the stage, supports at its foot a scale plate for 

 weighting the muscle. On the fore-end of the lever 

 are two screws p and q, the former of which ends below 

 in a platinum point resting upon a platinum plate, 

 while the latter is extended into a point of copper- 

 amalgam, immersed in a capsule of quicksilver. The 

 platinum plate and the capsule of quicksilver are iso- 

 lated from the stage and from each other, the latter 

 being conduc^ively connected with the vice Jc^ the former 

 with Jv, 



If the current which is to act on the swingfino- mas*- 

 net is inserted between k and Jc\ it passes through the 

 quicksilver capsule, through the portion of the lever be- 

 tween p and g, through the platinum plate, &c., as long 

 as the muscle does not contract. As soon, however, as 

 the muscle contracts, it interrupts the current between 

 p and the platinum plate. If the apparatus is so ar- 

 ranged that the current is closed at the moment at 

 which any irritant affects the muscle, then this current 

 will circulate until the muscle, in contracting, again 



