HOW HEAT IS PRODUCED BY THE ELECTRIC CURRENT.
=145. Resistance and Heat.= We have seen that all wires and conductors
offer resistance to the electric current. The smaller the wire the
greater its resistance. Whenever resistance is offered to the current,
heat is produced. By proper appliances, the heat of resistance can be
used to advantage for many commercial enterprises. Dynamos are used to
generate the current for heating and lighting purposes.
[Illustration: Fig. 182.]
Fig. 182 shows how the current from two strong cells can be used to
heat a short length of very fine platinum or German-silver wire.
The copper conductors attached to the cells do not offer very much
resistance.
It will be seen from the above that in all electrical work the sizes
of the wires used have to be such that they do not overheat. The coils
of dynamos, motors, transformers, ampere-meters, etc., etc., become
somewhat heated by the currents passing through them, great care being
taken that they are properly designed and ventilated so that they will
not burn out.
[Illustration: Fig. 183.]
[Illustration: Fig. 184.]
=146. Electric Welding.= Fig. 183 shows one form of electric welding
machine. The principle involved in the art of electric welding is
that of causing currents of electricity to pass through the abutting
ends of the pieces of metal which are to be welded, thereby generating
heat at the point of contact, which also becomes the point of greatest
resistance, while at the same time mechanical pressure is applied
to force the parts together. As the current heats the metal at the
junction to the welding temperature, the pressure follows up the
softening surface until a complete union or weld is effected; and, as
the heat is first developed in the interior of the parts to be welded,
the interior of the joint is as efficiently united as the visible
exterior. With such a method and apparatus, it is found possible to
accomplish not only the common kinds of welding of iron and steel, but
also of metals which have heretofore resisted attempts at welding, and
have had to be brazed or soldered.
[Illustration: Figs. 185 to 189.]
The introduction of the electric transformer enables enormous currents
to be so applied to the weld as to spend their energy just at the point
where heating is required. They need, therefore, only to be applied
for a few seconds, and the operation is completed before the heat
generated at the weld has had time to escape by conduction to any other
part.
Although the quantity of the current so employed in the pieces to be
welded is enormous, the potential at which it is applied is extremely
low, not much exceeding that of the batteries of cells used for ringing
electric bells in houses.
[Illustration: Fig. 190.]
=147. Miscellaneous Applications.= Magneto Blasting Machines are now
in very common use for blasting rocks, etc. Fig. 184 shows one, it
being really a small hand dynamo, occupying less than one-half a cubic
foot of space. The armature is made to revolve rapidly between the
poles of the field-magnet by means of a handle that works up and down.
The current is carried by wires from the binding-posts to fuses. The
heat generated by resistance in the fuse ignites the powder or other
explosive.
_Electric soldering irons_, _flat-irons_, _teakettles_, _griddles_,
_broilers_, _glue pots_, _chafing-dishes_, _stoves_, etc., etc., are
now made. Figs. 185 to 189 show some of these applications. The coils
for producing the resistance are inclosed in the apparatus.
[Illustration: Fig. 191.]
Fig. 190 shows a complete electric kitchen. Any kettle or part of the
outfit can be made hot by simply turning a switch. Fig. 191 shows an
electric heater placed under a car seat. Many large industries that
make use of the heating effects of the current are now being carried
on.