200 ELEMENTS OF GENERAL SCIENCE 



with the metal jaws, making the metal circuit continuous. 

 A closed circuit is obtained also when the button of an elec- 

 tric-bell circuit is pushed and when the switch of an incan- 

 descent lamp is turned on so that the lamp gives light. 



A short circuit can be illustrated with materials, as shown 

 in figure 96, by placing a piece of heavy copper wire on the 

 zinc of the first dry cell and on the carbon of the third dry 

 cell. Under these conditions very little current flows through 

 the usual course because the short, heavy piece of copper 

 wire permits most of the current to pass through it. In this 

 case the electricity does not perform the desired work, and 

 it may be said to be leaking, but this is not strictly true. 

 Although electricity takes all paths, the greater quantity 

 takes the path through the thick, short conductor. A short 

 circuit is produced whenever any piece of metal or other 

 conductor permits the electric current at least a large 

 portion of it to be diverted from its regular path. The 

 short circuit is a condition which electricians try to avoid, 

 since the current may become so excessive that great damage 

 results. Fuses are usually melted, and a considerable flash 

 and a startling noise may accompany the momentary .short 

 circuit. 



214. The dry cell and its uses. There are many different 

 kinds of electric cells, some of which are easily made. The 

 dry cell (fig. 98, A) consists of a zinc can which contains 

 a rod of carbon surrounded by a mixture of black manga- 

 nese dioxide and powdered carbon. Within the zinc can is a 

 paste of sal ammoniac (ammonium chloride) and water. The 

 ammonium-chloride solution or paste is the most important 

 material in the cell, but the manganese dioxide .and powdered 

 carbon are necessary for the best operation. In the zinc of 

 an old dry cell many holes may be seen. Most of the metal 

 has disappeared, since it has been consumed by chemical 

 change to furnish the energy to drive the electricity through 

 the cell and the circuit, much as coal is burned to furnish 



