732 



Pohl's Commutator or Reverser (Fig. 239) consists of a block of paraf- 

 fin or wood with six mercury cups, each in connection with a binding- 

 screw (not shown in the figure). Cups i and 6 and 2 and 5 are connected 

 by copper wires, which cross each other without touching. The bridge 

 consists of a glass or vulcanite cross-piece a, to which are attached two 

 wires bent into semicircles, each connected with a straight wire dip- 

 ping into the cups 3 and 4 respectively. With the bridge in the posi- 

 tion shown in the figure, a current coming in at 4 would pass out by 

 the wire connected with i, and back again by that connected with 2, in 

 the direction shown by the arrows. When the bridge is rocked to the 

 other side so that the bent wires dip into 5 and 6, the direction of the 

 current is reversed. The cross-wires may be taken out altogether, and 

 the commutator used to send a current at will through either of two 

 circuits, one connected with i and 2, and the other with 5 and 6. 



Du Bois -Reymond's Short- 

 circuiting Key. A cheap and 

 convenient form is shown in 

 Fig. 240. 



Time - Markers Electric Sig- 

 nal. It is of importance to know 

 the time relations of many 

 physiological phenomena which 

 are graphically recorded ; for 



Fig. 239. Pohl's Commutator. 



Fig. 240. Short-Circuiting Key. 



example, the contraction of a skeletal muscle or the beat of a heart. 

 For this purpose a tracing showing the speed, of the travelling sur- 

 face in a given time is often taken simultaneously with the record 

 of the movement under investigation. For a slowly-moving surface 

 it is sufficient to mark intervals of one or two seconds, and this is 

 very readily done by connecting an electro-magnetic marker (such 

 as the electric signal of Deprez) with a circuit which is closed and 

 broken by the seconds pendulum of an ordinary clock or a metronome 

 (Fig. 88, p. 195). Special clocks have also been constructed which 

 permit of the time intervals being varied. For shorter intervals a 

 tuning-fork is used, which makes and breaks a circuit including an 

 electromagnetic marker, or writes on the drum directly by means of 

 a writing-point attached to one of the prongs. 



Amoeboid movement (p. 16) is the most primitive, the least 

 elaborated form of contraction. The maximum velocity of the 

 movement has been reckoned at o-ooS millimetre a second. Stimu- 



