Your New Servants: The Electronic "Watchmen" 123 



frequency, heating. Induction heating has been used for many 

 years. R.F. heating is induction heating "stepped up" to radio- 

 frequency speeds. Two methods of induction heating are 

 used: electromagnetic heating is used for materials, such as 

 metals, in which electric current flows easily; electrostatic 

 heating is used for materials, such as wood and plastics, in 

 which electric current does not flow easily. 



In its simplest terms the process of electromagnetic heat- 

 ing may be explained thus. A current flowing through a wire 

 sets up a magnetic field around the wire. If the wire is wound 

 into a coil, the magnetic field will flow through the coil. 

 Every time the direction of the current in the coil of wire 

 changes, the direction of the magnetic field changes too. This 

 changing magnetic field will set up or induce a voltage or elec- 

 tric pressure in a piece of metal which is inserted in the coil, 

 and cause current to flow in the metal. The resistance of the 

 metal to this flow of current causes the metal to heat. Thus the 

 term "induction heating" or heating caused by an induced 

 current. 



With nonconductive materials, such as wood, "capacity" 

 heating is used. The material to be heated is placed between 

 two metal plates. As a constantly changing current causes a 

 multitude of electrons to strike one of the plates, the electrons 

 are driven from the other plate, or "repelled" from it, just as 

 the two south poles of a magnet are repelled from each other. 

 At the next instant, when a reversal of current drives elec- 

 trons to the other plate, the electrons on the first plate are 

 repelled from it. The material between the plates, as an inno- 

 cent bystander, is caught in the thick of the argument. It feels 

 the electric stress across it as the electrons on the two metal 

 plates repel each other. The electrons within the material are 

 not easily freed from their atoms; but they do feel the tug of 

 this stress and they are displaced slightly, first to one side and 

 then to the other. This displacement causes friction and 



