LABORATORY GUIDK IN PHYSIOLOGY. 



plate. Touch the tongue with one wire, then with 

 the other. Touch the tongue with both at once. 

 Bring the two free ends of the wires into contact 

 with the binding posts of a detector; note results. 

 Touch the ends of the wires together, if the condi- 

 tions are favorable a minute spark may be seen 

 on touching and on separating the two poles. What 

 conclusions are to be drawn? 



(4) Define element or cell as used in this connection . 

 Define plate, pole, electrode. The zinc is arbitra- 

 rily taken as the positive plate and the copper as 

 the negative plate. The pole which is attached 

 to the negative plate is the positive pole, and that 

 which is attached to the positive plate is the nega 

 tive pole. The positive pole or electrode of a gal- 

 vanic cell or of a battery is called the anode, while 

 the negative pole or electrode of a cell or of a bat- 

 tery is called the kathode. 



b. Keys. (1) Show and describe the simple contact 

 key (Fig. 7-k), the mercury key (Fig. 3), and the Du 

 Bois-Reymond key (Fig. 4). 



(2) Two ways of using the D.u Bois Reymond key. 

 1st. As a simple contact key (PI. I Fig 1.) 

 2d. As a short circuiting key (PI. I Fig. 2.) 



c. The commutator. Most convenient for the physio- 

 logical laboratory is Pohl's commutator (Fig. 5). 

 This instrument may be used for the following pur- 

 poses: 



(1) To change the direction of the current. Set 

 up apparatus with cross bars in place as 

 shown in PL I Fig. 3. Which is the anode 

 when the bridge is turned toward a b? Which 

 is the anode when the bridge is turned toward 

 c d? 



