46 



LABORATORY EXERCISES 



FIG. 20. 



a. Prepare a strip of sheet zinc and one of sheet copper, each 

 about 3 x 10 cm., and make a small hole near one end of each 

 strip. Through each hole put one end of a piece of copper wire 

 about 20 cm. (8 in.) long, bend the part that is through the 

 hole, and with a hammer flatten the wire tightly against the 

 strip. Then tack the strips of metal on opposite 

 sides of a block of wood (Fig. 20) long enough 

 to reach across the glass. 



b. Half fill the glass with dilute sulphuric acid, 

 and put the free ends of the two metal strips into 

 it. Note what happens. From which metal do 

 bubbles appear to rise? Touch the tips of the 

 copper wires, held a little distance apart, to the 

 tip of the tongue. What evidence do you get 

 that a current is traveling through the wires? 



c. Join the free ends of the two wires, and place them over a 

 compass or a floating magnetized needle (c/. Exercise 42). 

 What is. the result? 



d. If your simple cell is too weak to give good results, put 

 into the dilute sulphuric acid about 10 cu. cm. of a saturated 

 solution of potassium dichromate, and try the experiments 

 again. What are the results now? 



EXERCISE 45 

 THE SAL AMMONIAC CELL 



Apparatus and Materials. A sal ammoniac cell, ready to set up^. 

 wooden paddle, battery that operates house doorbells. 



a. Examine the construction of a "sal ammoniac" cell. 

 The common form is also called a "carbon cylinder" cell. If 

 the cell is not ready to use, set it up according to the directions 

 on the jar. Usually about 150 g. (5 ounces) of solid am- 

 monium chloride are used. This is mixed with water, and 



