where D is an alternating dynamo, C a condenser, P S the primary and secondary coils of a sparking transformer, T T two metal sheets or plates, and SB the exhausted bulbs.  The alternating or see-saw current in this case charges the condenser and excites the primary coil P, while the induced current in the secondary coil 5 charges the terminal plates T T. So long as the bulbs or tubes are kept within the space between the plates, they are filled with a soft radiance, and it is easy to see that if these plates covered the opposite walls of a room, the vacuum lamps would yield a light in any part of it.

Electric heating bids fair to become almost as important as electric illumination.  When the arc was first discovered it was noticed that platinum, gold, quartz, ruby, and diamond—­in fine, the most refractory minerals—­were melted in it, and ran like wax.  Ores and salts of the metals were also vapourised, and it was clear that a powerful engine of research had been placed in the hands of the chemist.  As a matter of fact, the temperature of the carbons in the arc is comparable to that of the Sun.  It measures 5000 to 10,000 degrees Fahrenheit, and is the highest artificial heat known.  Sir William Siemens was among the first to make an electric furnace heated by the arc, which fused and vapourised metallic ores, so that the metal could be extracted from them.  Aluminium, chromium, and other valuable metals are now smelted by its means, and rough brilliants such as those found in diamond mines and meteoric stones have been crystallised from the fumes of carbon, like hoar frost in a cold mist.

The electric arc is also applied to the welding of wires, boiler plates, rails, and other metal work, by heating the parts to be joined and fusing them together.

Cooking and heating by electricity are coming more and more into favour, owing to their cleanliness and convenience.  Kitchen ranges, including ovens and grills, entirely heated by the electric current, are finding their way into the best houses and hotels.  Most of these are based on the principle of incandescence, the current heating a fine wire or other conductor of high resistance in passing through it.  Figure 71 represents an electric kettle of this sort, which requires no outside fire to boil it, since the current flows through fine wires of platinum or some highly resisting metal embedded in fireproof insulating cement in its bottom.  Figures 72 and 73 are a sauce-pan and a flat-iron heated in the same way.  Figure 74 is a cigar-lighter for smoking rooms, the fusee F consisting of short platinum wires, which become red-hot when it is unhooked, and at the same time the lamp Z is automatically lit.  Figure 75 is an electric radiator for heating rooms and passages, after the manner of stoves and hot water pipes.  Quilts for beds, warmed by fine wires inside, have also been brought out, a constant temperature being maintained by a simple regulator, and it is not unlikely that personal clothing of the kind will soon be at the service of invalids and chilly mortals, more especially to make them comfortable on their travels.