38 



EXPERIMENTAL GENERAL PHYSIOLOGY 



acid. Chemically pure zinc is acted upon very slowly by 10 per cent, 

 sulphuric acid. Join a wire to the exposed end of each plate; touch 

 the tongue with the free end of each wire separately; touch the 

 tongue with both wires simultaneously. Record results. 



(3) Place the porous cup with the zinc plate in the receptacle 

 holding CuSO 4 with the copper 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 wire into contact with the binding 



FIG. 13 



FIG. 14 



The pole changer, or the Pohl commutator. (For description 

 see text.) 



posts of a galvanoscope. Note results. 

 Touch the ends of the wires together; if 

 the conditions 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 arbitrarily 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 galvanic cell or of a battery is called the anode, while the 

 negative pole or electrode of a cell or of a battery is called the 

 cathode. 



(6) Keys. (1) Study and describe the simple contact key (Fig. 

 25, K) and the Du Bois-Reymond key (Fig. 13). (2) The two ways 

 of using the Du Bois-Reymond *key are shown in the figures: first, as 

 a contact key (Fig. 15); second, as a short-circuiting key (Fig. 16). 



The Du Bois-Reymond key. 



