ELECTRICITY AND MAGNETISM 



133 



electric spark, also, ignites the vapor of gasolene in the gas 

 engine of a motor boat or an automobile. 



One of the commonest forms of cells is the dry cell, which is 

 very convenient to handle because it contains no liquid 

 which might be spilled. It is filled with a paste containing 

 moist substances which act chemically upon one of the 

 metals. Liquid cells, when used up, must have one of the 

 electrodes or the liquid replaced by new materials; but 

 the dry cell, when used up, must be replaced by an entirely 

 new cell. Dry cells are much used in operating doorbells 

 and in automobiles. 



Statical electricity accumulates in charges on various 

 bodies, and as soon as proper connections are made, it dis- 

 charges instantly. Current 

 electricity, on the other hand, 

 has a continuous, steady flow. 

 It is used in the wires of 

 telephone and telegraph 

 systems, whereas a charge 



of statical electricity is useless 



. . FIG. 67. A MAGNETIC COMPASS 

 for such service, because it is 



gone in an instant. 



143. Effects of an Electric 

 Current. Some of the effects 

 of current electricity are 

 similar to those of statical 

 electricity, but since its 

 quantity and intensity can be better regulated, current 

 electricity is of much greater use. The effects of electricity 

 are heat, light, and magnetization, all of which can be 

 produced with a battery of a few cells. Electricity from a 

 battery is, however, seldom used to produce any of these 

 effects for practical purposes. Such effects are produced by 

 a method which is described later. 



A simple way to determine whether a current is passing 



1. The box surrounding the 

 magnetic needle is usually made of 

 brass; why not of iron? 2. Should 

 the box rest in a vertical or horizontal 

 position when used to determine 

 direction? Why? 3. Why are pipes 

 in a magnetic laboratory made of 

 brass or copper? 



