272 The Outline of Science 



ting it flow. Their atoms do not easily part with electrons. 

 Glass, vulcanite, and porcelain are very good insulators for this 

 reason. 



What the Dynamo does 



But even several cells together do not produce the currents 

 needed in modern industry, and the flow is produced in a different 

 manner. As the invisible electrons pass along a wire they pro- 

 duce what we call a magnetic field around the wire, they produce 

 a disturbance in the surrounding ether. To be exact, it is through 

 the ether surrounding the wire that the energy originated by the 

 electrons is transmitted. To set electrons moving on a large scale 

 we use a "dynamo." By means of the dynamo it is possible to 

 transform mechanical energy into electrical energy. The modern 

 dynamo, as Professor Soddy puts it, may be looked upon as an 

 electron pump. We cannot go into the subject deeply here, we 

 would only say that a large coil of copper wire is caused to turn 

 round rapidly between the poles of a powerful magnet. That 

 is the essential construction of the "dynamo," which is used for 

 generating strong currents. We shall see in a moment how mag- 

 netism differs from electricity, and will say here only that round 

 the poles of a large magnet there is a field of intense disturbance 

 which will start a flow of electrons in any copper that is intro- 

 duced into it. On account of the speed given to the coil of wire 

 its atoms enter suddenly this magnetic field, and they give off 

 crowds of electrons in a flash. 



It is found that a similar disturbance is caused, though the 

 flow is in the opposite direction, when the coil of wire leaves the 

 magnetic field. And as the coil is revolving very rapidly we get 

 a powerful current of electricity that runs in alternate directions 

 an "alternating" current. Electricians have apparatus for 

 converting it into a continuous current where this is necessary. 



A current, therefore, means a steady flow of the electrons 

 from atom to atom. Sometimes, however, a number of electrons 



