from the inductive action of the leading-in wire by providing the stem
with a tube or coating of conducting material. It seems beyond doubt
that the best among metals to employ for this purpose is aluminium, on
account of its many remarkable properties. Its only fault is that it
is easily fusible, and, therefore, its distance from the incandescing
body should be properly estimated. Usually, a thin tube, of a diameter
somewhat smaller than that of the glass stem, is made of the finest
aluminium sheet, and slipped on the stem. The tube is conveniently
prepared by wrapping around a rod fastened in a lathe a piece of
aluminium sheet of the proper size, grasping the sheet firmly with
clean chamois leather or blotting paper, and spinning the rod very
fast. The sheet is wound tightly around the rod, and a highly polished
tube of one or three layers of the sheet is obtained. When slipped on
the stem, the pressure is generally sufficient to prevent it from
slipping off, but, for safety, the lower edge of the sheet may be
turned inside. The upper inside corner of the sheet--that is, the one
which is nearest to the refractory incandescent body--should be cut
out diagonally, as it often happens that, in consequence of the
intense heat, this corner turns toward the inside and comes very near
to, or in contact with, the wire, or filament, supporting the
refractory body. The greater part of the energy supplied to the bulb
is then used up in heating the metal tube, and the bulb is rendered
useless for the purpose. The aluminium sheet should project above the
glass stem more or less--one inch or so--or else, if the glass be too
close to the incandescing body, it may be strongly heated and become
more or less conducting, whereupon it may be ruptured, or may, by its
conductivity, establish a good electrical connection between the metal
tube and the leading-in wire, in which case, again, most of the energy
will be lost in heating the former. Perhaps the best way is to make
the top of the glass tube, for about an inch, of a much smaller
diameter. To still further reduce the danger arising from the heating
of the glass stem, and also with the view of preventing an electrical
connection between the metal tube and the electrode, I preferably wrap
the stem with several layers of thin mica, which extends at least as
far as the metal tube. In some bulbs I have also used an outside
insulating cover.
The preceding remarks are only made to aid the experimenter in the
first trials, for the difficulties which he encounters he may soon
find means to overcome in his own way.
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