Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1890-12-07

temperature the glass will most likely
crack and sooner or later a leak will be
formed and air will enter the exhausted
globe. Now to use a technical expression,
platinum has the same "coeficient expan-
sion" as glass. This means that Tor a given
increase in temperature platinum and glass
will expand AN equal amount This Is the
only metal known that has the same ''coeffi-
cient of expansion" as glass, and although it
is expensive, yet without It, incandescent
lamps could not De made.
WHY AIR IS TAKEN OUT.
The objec. OF the vacuum in the globe of
an incandescent lamp is To prevent combus-
tion. When we burn wood or coal VIC do not
destroy anything that Is matter; we simply
cause a change in the condition of things.
Wood when burned is not destroyed as TAR
as matter is concerned; it has simply
changed its condition. The carbon of the
wood unites with the oxygen of the air to
form carbonic acid gas, so that when wood
is burned its condition is simply changed,
part of il being burned into the condition OF
carbonic acid gas and the rest into ash.
Now, carbonic acid gas is composed of
carbon and oxygen-buti if there is no oxygen
present when carbon is heated to a white
heat the union of these two elements cannot
take place. 1t will now be perfectly clear
that if we pump the air, which is largely
composed OF oxygen, out Of the globe OF an
incandescent lamp no combustion can take
place; that is, there will be no oxygen there
to combine with the carbon flament when
it is brought to white heat through the ac-
ton Of the electric current.
HEATING THE FILAMENr.
There now remains to be explained how
the electric current causes the carbon fila
ment to become white hot. This action is
simply a transformation OF energy. If we
rub our hands together we transform some
oi the energy of the body into heat, friction
being the cause of the Transformation. The
condition of the body is one form of energy
and heat is another form of energy. We
can so rub two sticks of wood together that
the energy of motion can be transformed
into the energy o: heat to such an extent as
to cause the wood to ignite. If we force
water rapidly through a small pipe the
water can be made to boil. The heat of the
water in this case is due to the friction be-
tween the rushing water and the sides of the
pipe.
1t is Just 50 with the electric current. If
we force a current of electricity through
wire the electrical energy will be trans
formed, through friction, into heat energy
and the wire becomes warm. IT the current
is sufficiently great the wire can be readily
melted. In the incandescent lamp, there
fore, it is the friction between the electric
current and the filament that causes the
white heat, and as no combustion can take
place the flament will stand this great heat
for a long time before it will 60 to pieces.
However, as no vacuum can be perfect, the
tilament will finally 60 to pieces. The
average life of a good incandescent lamp is
about 900 hours.
CALCULATIONS ON LIGHTING.
From business standpoint it is necessary
to know how many iucandescent lamps can
be worked with one horse power of energy.
Now, heat and electricity being forms of
energy from one of which the other can be
obtained, the above information is calcu-
lated in the following manner: The unit of
electrical energy is the watt" and it takes
TAG watts to equal one horse power. The
watt is the product of one volt of pressure
Into one ampere of current. To that ii we
know the pressure in volts and current In
amperes needed for any incandescent lamp,
we can at once calculate the electrical
energy. For example, if lamp requires
80 volts of pressure and one ampere OF cur
rent to bring it up to its proper candle
power, its energy in watts will be found by
80 multiplied by 1, or 30 watts. If the
lamp requires lUU VOLS anu o nalI an am
pere, its energy in watts will be the same as
before; that is, 100 multiplied by n, or 90
I watts, and as it takes TAG watts to equal
| one horse power, the number of lamps, al
| above, that can be worked with one horse
power is calculated by TAG, divided by 80, OF
nearly Is lamps.
THE LAMPS GIVE OUT HEAT.
Many people think that incandesceni
lamps do not give out heat. This idea is,
however, a mistake. for although the heat
given out by such a lamp is not nearly as
great as the heat from gas jet of equal can
| die power, yet some heat is necessarily given
of. 1t would be impossible to have white
| hot filament, such as there is in an incan
| descent lamp, without heat.
However, leaving expense and convent
| ence out of the question, the incandescen
| lamp has this one overwhelming advaptag,
over every other method of lighting, namely
| it does not give off any products of combus
| tion and so does not vitiate the atmosphere
1t follows from this that thsee lamps cannot
smoke or blacken the wall paper.
SCIRE FACIAs.