HEAT AND LIGHT FROM ELECTRIC CURRENTS 201 



the energy used in forcing water through pipes. An old dry 

 cell may often be improved for a short time by punching a 

 hole in the top and adding water. 



There are only two practical sources of electricity, primary 

 cells and dynamos. The former are useful where small, 

 intermittent currents are desired for doorbells, toys, clocks, 

 telephones, and spark coils; the latter are required for the 

 cheaper energy for domestic and commercial purposes. Dry 



BRASS SCREW POSTS 



FIG. 98. Dry cells 



A, longitudinal section of a dry cell ; B, simple flashlight which uses a dry cell: 

 C, pocket flashlight similar to B except that it is inclosed 



cells of the usual size are now manufactured in the United 

 States at the rate of about fifty millions each year. Great 

 quantities of smaller cells are used in flashlights. 



215. How to make a common kind of electric circuit. The 

 question often arises, What is the best way to connect dry 

 cells ? In figure 96 the current is considered as leaving the 

 carbon (known as the +, or positive, terminal) of the right- 

 hand cell, passing along the wire to the fuse, then through 

 the wires and back to the switch, then to the zinc (known 

 as the , or negative, terminal) of the left-hand cell. The 

 electricity passes through the cell from zinc to carbon, but 



