POPULAR SCIENCE 623 



two substances highly charged with electricity, the one carry- 

 ing positive electricity, the other negative electricity, are 

 brought sufficiently close together, a spark may be observed 

 to pass from one to the other, as a result of which the materials 

 are found to possess no charge whatsoever, provided the 

 initial amounts of positive and negative electricity were the 

 same. It is possible, therefore, to discharge the electricity 

 from a substance by neutralisation with electricity of the 

 opposite sign. It follows that an uncharged or electrically 

 neutral substance does not necessarily contain no electricity 

 at all ; it may really possess equal amounts of the positive and 

 negative kinds. Such early observations and deductions do 

 not give us, however, much information about the mechanism 

 of the process of electrification. The charges of electricity 

 here considered reside essentially on the surface of the materials 

 examined. The first evidence that electricity could be trans- 

 ported through the body of a solid was obtained when an 

 electrical current was propagated through a wire by joining 

 the two ends of the wire to the poles of a battery. That an 

 electric current, or electricity in motion exists under these 

 conditions is shown by its effect on a small magnet suspended 

 close to the wire, the magnet being rotated through a certain 

 angle when a given quantity of electricity passes through the 

 wire. The modern view of an electric current is that it con- 

 sists of a stream of very tiny material particles called electrons, 

 or corpuscles, each of which carries an electric charge. The 

 sign of this charge has been shown in various ways to be nega- 

 tive. These electrically charged electrons existed in the wire 

 all the time ; their movement from one end to the other was 

 brought about by joining the wire to the battery, one pole 

 of which acts like a reservoir head applying a certain elec- 

 trical pressure or potential which causes the electrons to 

 stream just as water streams from high pressure to low. Since 

 the electrons are thus shown to be present in the wire, they 

 must constitute part of the atoms of which the wire is itself 

 composed. In virtue of the fact that it allows electricity to 

 pass through it, a metallic wire is called a conductor, its con- 

 ducting power being due to the presence of electrified particles 

 distributed throughout its mass. But wire is by itself an 

 electrically neutral piece of matter. It follows therefore, that 

 there must be exactly as many positively charged particles as 



