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 sealed off.  The principal advantage of this construction was that it allowed of reaching extremely high vacua, and, at the same time use a large bulb.  It was found, in the course of experiences with bulbs such as illustrated in Fig. 29, that it was well to make the stem s near the seal at e very thick, and the leading-in wire w thin, as it occurred sometimes that the stem at e was heated and the bulb was cracked.  Often the outer globe L was exhausted only just enough to allow the discharge to pass through, and the space between the bulbs appeared crimson, producing a curious effect.  In some cases, when the exhaustion in globe L was very low, and the air good conducting, it was found necessary, in order to bring the button m to high incandescence,