which resulted usually in light-sources of
scientific interest but not of practical value. One of the latest is the
tungsten arc in an inert gas. By means of a heating coil, a small arc is
started between two electrodes consisting of tungsten, but this as yet
has not been shown to be practicable.

Another type of filament lamp was developed by Nernst in 1897. It was an
ingenious application of the peculiar properties of rare-earth oxides.
His first lamp consisted essentially of a slender rod of magnesia. This
substance does not conduct electricity at ordinary temperatures, but
when heated to incandescence it becomes conducting. Upon sufficient
heating of this filament by external means while a proper voltage is
impressed upon it, the electric current passes through it and thereafter
this current will maintain its temperature. Thus such a filament becomes
a conductor and will continue to glow brilliantly by virtue of the
electrical energy which it converts into heat. Later lamps consisted of
"glowers" about one inch long made from a mixture of zirconia and
yttria, and finally a mixture of ceria, thoria, and zirconia was used.
The glower is heated initially by a coil of platinum wire located near
it but not in contact with it. Owing to the fact that this glower
decreases rapidly in resistance as its temperature is increased, it is
necessary to place in series with it a substance which increases in
resistance with increasing current. This is called a "ballasting
resistance" and is usually an iron wire in a glass bulb containing
hydrogen. The heater is cut out by an electromagnet when the glower goes
into operation. This lamp is a marvel of ingenuity and when at its
zenith it was installed to a considerable extent. Its light is
considerably whiter than that of the carbon filament lamps. However, its
doom was sounded when metallic filament lamps appeared.

An interesting filament was developed by Parker and Clark by using as a
core a small filament of carbon. This flashed in an atmosphere
containing a vapor of a compound of silicon, became coated with silicon.
This filament was of high specific resistance and appeared to have
promise. It has not been introduced commercially and doubtless it cannot
compete with the latest tungsten lamps.

Electric incandescent lamps are the present mainstay of electric
illumination and, it might be stated, of progress in lighting. Wonderful
achievements have been accomplished in other modes