230 



OUR PHYSICAL WORLD 



cost of production. Furthermore, some batteries discharge 

 undesirable fumes. 



The gravity battery already described is long-lived, and gives 

 a very steady current. It is much used for telegraph and tele- 

 phone lines, though in large plants dynamos are now replacing 

 batteries. 



In a common style of bichromate battery, a zinc rod is 

 immersed in dilute sulphuric acid held in a tall, porous earthen- 

 ware cup at the center of the battery jar. Outside this cup is 

 the solution of potassium bichromate and at 

 the periphery of the jar is a cylindrical sheet 

 of copper. The porous cup prevents the 

 mingling of the bichromate solution with the 

 acid, but permits the passage of the current 

 and of the hydrogen, which, when oxidized 

 by the bichromate, forms water. This battery 

 gives a current of considerable voltage (2 volts) , 

 and permits intermittent use without much 

 deterioration. It is serviceable for running 

 electric lights that are only occasionally used 

 (Fig. 93, p. 224). 



The Bunsen cell has a carbon rod im- 

 mersed in nitric acid in a porous cup at the 

 center of a battery jar, while a zinc plate is 

 immersed in sulphuric acid outside the porous cup. This battery 

 is inexpensive to run, and gives a current of good voltage and 

 great constancy, but unfortunately gives off disagreeable fumes 

 of nitrous oxide. Still, it is a serviceable battery for furnishing 

 the current for electrical experimentation. 



Since it is not always convenient to use a battery containing 

 liquid, the so-called dry battery has been devised (Fig. 97). 

 This is familiar in the pocket flash light and in the bicycle head- 

 light, and is now used for ringing door bells and for similar 

 domestic purposes. Such a battery is really not a perfectly 

 dry cell, but the moisture which is essential to any battery is 



FIG. 97. Diagram 

 of a dry battery. 



