sufficiently increased, the electrode
would be brought to a high temperature as well as if the gas were at
very low pressure; in fact, exhausting the bulb is only necessary
because we cannot produce (and possibly not convey) currents of the
required frequency.

Returning to the subject of electrode lamps, it is obviously of
advantage in such a lamp to confine as much as possible the heat to
the electrode by preventing the circulation of the gas in the bulb. If
a very small bulb be taken, it would confine the heat better than a
large one, but it might not be of sufficient capacity to be operated
from the coil, or, if so, the glass might get too hot. A simple way to
improve in this direction is to employ a globe of the required size,
but to place a small bulb, the diameter of which is properly
estimated, over the refractory button contained in the globe. This
arrangement is illustrated in Fig. 28.

[Illustration: FIG. 28.--LAMP WITH AUXILIARY BULB FOR CONFINING THE
ACTION TO THE CENTRE.]

The globe L has in this case a large neck n, allowing the small bulb b
to slip through. Otherwise the construction is the same as shown in
Fig. 18, for example. The small bulb is conveniently supported upon
the stem s, carrying the refractory button m. It is separated from the
aluminium tube a by several layers of mica M, in order to prevent the
cracking of the neck by the rapid heating of the aluminium tube upon a
sudden turning on of the current. The inside bulb should be as small
as possible when it is desired to obtain light only by incandescence
of the electrode. If it is desired to produce phosphorescence, the
bulb should be larger, else it would be apt to get too hot, and the
phosphorescence would cease. In this arrangement usually only the
small bulb shows phosphorescence, as there is practically no
bombardment against the outer globe. In some of these bulbs
constructed as illustrated in Fig. 28 the small tube was coated with
phosphorescent paint, and beautiful effects were obtained. Instead of
making the inside bulb large, in order to avoid undue heating, it
answers the purpose to make the electrode m larger. In this case the
bombardment is weakened by reason of the smaller electric density.

Many bulbs were constructed on the plan illustrated in Fig. 29. Here a
small bulb b, containing the refractory button m, upon being exhausted
to a very high degree was sealed in a large globe L, which was then
moderately exhausted and