KINDS OF CELLS 137 



In making a cell we may use other materials instead of zinc and 

 copper. We choose one substance that acts with the acid, and one that 

 does not. Carbon plates are often used instead of copper ones. Both 

 plates must be conductors. Solutions of other acids, of bases, or of 

 certain salts (cf. 214, 218, and 220) may be used instead of the 

 dilute sulphuric acid. 



To bring the free ends of the metals into connection, so that there 

 can be a current, is called "making," or "closing, the circuit"', the re- 

 verse is called "breaking," or "opening, the circuit" (cf. 100). A 

 number of properly connected cells is called a battery. 



151. Kinds of Cells. The voltaic cell is of little prac- 

 tical use in its simplest form, because it is soon weakened, 

 or stopped entirely (we say it is "polarized"), by the 

 bubbles of hydrogen that collect upon the copper plate. 

 The hydrogen is formed by the action between the zinc 

 and the acid. A description of DanielFs cell also called 

 the gravity cell will show one way in 

 which polarization is avoided. 



In DanielPs cell (Fig. 126) the two plates, 

 zinc and copper, are given a "crowfoot" 

 shape to increase their surfaces. The cop- 

 per is placed at the bottom of the jar in a 

 saturated solution of copper sulphate (blue 

 vitriol), while the zinc, which is in the upper A Gravity? or 

 part of the jar, is surrounded by dilute sul- Daniell > ceii. 

 phuric acid. The current causes the blue vitriol solution 

 to remain at the bottom; hence the name "gravity" cell. 



The action of the cell is as follows: The zinc reacts with the dilute 

 sulphuric acid, giving zinc sulphate and hydrogen. The hydrogen 

 moves toward the copper plate; but in passing through the layer of 

 copper sulphate the hydrogen is used up, and copper is deposited in its 



