7]

When rubbing briskly an ordinary incandescent lamp on a piece of
cloth and at the same time slightly revolving it, a luminous
effect is produced similar to an X-ray tube. The room must be dark
and the lamp perfectly dry to obtain good results. It appears that
the inner surface of the globe becomes charged, probably by
induction, and will sometimes hold the filament as shown in the
sketch.
--Contributed by E. W. Davis, Chicago.

[Illustration: Lamp]

** How to Make a Small Motor [428]

The accompanying sketch shows how to make a small motor to run on
a battery of three or four dry cells and

[Illustration: Details of Small Electric Motor]

with sufficient power to run mechanical toys. The armature is
constructed, as shown in Figs. 1 and 2, by using a common spool
with 8 flat-headed screws placed at equal distances apart and in
the middle of the spool. Each screw is wound with No. 24 gauge
iron wire, as shown at A, Fig. 1. The commutator is made from a
thin piece of copper, 1 in. in diameter and cut as shown in Fig.
3, leaving 8 points, 1/8 in. wide and 1/8 in.- deep. The field is
built up by using 8 strips of tin, 12 in. long and 2 in. wide,
riveted together and shaped as shown at B, Fig. 4. Field magnets
are constructed by using two 3/8-in. bolts, 1-1/2 in. long. A
circular piece of cardboard is placed on each end of the bolt,
leaving space enough for the bolt to pass through the field B, and
to receive a nut. Wind the remaining space between the cardboards
with 30 ft. of No. 22 double-wound cotton-covered copper wire. A
light frame of wood is built around the magnets, as shown at C,
Fig. 4. Holes are made in this frame to receive the axle of the
armature. Two strips of copper, 1/4 in. wide and 3 in. long, are
used for the brushes. The armature is placed in position in its
bearings and the brushes adjusted as shown in Fig. 4, one brush
touching the shaft of the armature outside of the frame, and the
other just touching the points of the commutator, which is placed
on the shaft inside of the frame. Connect the outside wire of one
magnet to the inside wire of the other, and the remaining ends,
one to the batteries and back to the brush that touches the shaft,
while the other is attached to the brush touching the commutator.
In making the frame for the armature bearings, care should be
taken to get the holes for the shaft centered, and to see that the
screws in the armature pass each bolt in the magnets a