CURRENT ELECTRICITY 485 



then said to be polarized. If cells are to be of practical 

 value, they must not quickly polarize ; that is, a way must 

 be found to get rid of the hydrogen bubbles. This is gen- 

 erally done by putting some substance into the cell that will 

 unite with the hydrogen and thus keep the copper strip free 

 of hydrogen bubbles. Many kinds of cells have been in- 

 vented which do not readily polarize. 



The so-called dry cell (Figure 155) is most used at the 

 present time. It consists of a zinc can lined on the inside 

 with porous paper. In the center is a carbon rod. Packed 

 around the carbon and filling the can is usually 

 a moist mixture of sal ammoniac, manganese 

 dioxide, granulated carbon, plaster of Paris, and 

 generally small quantities of other materials. In 

 this cell the sal ammoniac acts upon the zinc 

 somewhat as the sulphuric acid did in the simple 



.. . , FIGURE 155 



cell first mentioned, and the manganese dioxide 

 unites chemically with the hydrogen bubbles and thus re- 

 moves them from the carbon rod. The plaster of Paris 

 keeps the cell in rigid shape and the granulated carbon 

 helps to keep the contents porous so that action may go on 

 freely within the cell. 



In voltaic cells the copper or carbon strip is called the 

 positive electrode or pole, and the zinc is called the negative 

 electrode or pole. 



Experiment 157. Connect a positive and a negative pole of two 

 dry cells by a fairly heavy copper wire. Attach a similar piece of 

 wire to each of the other poles and connect these pieces by means 

 of a short, very fine, iron wire. (Figure 156.) The iron wire will 

 become red hot. Now remove the fine iron wire and connect the 

 loose ends of the copper wires to the socket of a small one or two 

 candle power electric light, such as is often used to illuminate the 



