n the tube T was placed a coil C of aluminium
wire, the ends of which were provided with small spheres t and t_1 of
aluminium, and reached into the U tube. The tube T was slipped into a
socket containing a primary coil through which usually the discharges
of Leyden jars were directed, and the rarefied gas in the small U tube
was excited to strong luminosity by the high-tension currents induced
in the coil C. When Leyden jar discharges were used to induce currents
in the coil C, it was found necessary to pack the tube T tightly with
insulating powder, as a discharge would occur frequently between the
turns of the coil, especially when the primary was thick and the air
gap, through which the jars discharged, large, and no little trouble
was experienced in this way.

[Illustration: FIG. 32.--ELECTRO-DYNAMIC INDUCTION TUBE.]

[Illustration: FIG. 33--ELECTRO-DYNAMIC INDUCTION LAMP.]

In Fig. 33 is illustrated another form of the bulb constructed. In
this case a tube T is sealed to a globe L. The tube contains a coil C,
the ends of which pass through two small glass tubes t and t_1, which
are sealed to the tube T. Two refractory buttons m and m_1 are mounted
on lamp filaments which are fastened to the ends of the wires passing
through the glass tubes t and t_1. Generally in bulbs made on this
plan the globe L communicated with the tube T. For this purpose the
ends of the small tubes t and t_1 were just a trifle heated in the
burner, merely to hold the wires, but not to interfere with the
communication. The tube T, with the small tubes, wires through the
same, and the refractory buttons m and m_1, was first prepared, and
then sealed to globe L, whereupon the coil C was slipped in and the
connections made to its ends. The tube was then packed with insulating
powder, jamming the latter as tight as possible up to very nearly the
end, then it was closed and only a small hole left through which the
remainder of the powder was introduced, and finally the end of the
tube was closed. Usually in bulbs constructed as shown in Fig. 33 an
aluminium tube a was fastened to the upper end s of each of the tubes
t and t_1, in order to protect that end against the heat. The buttons
m and m_1 could be brought to any degree of incandescence by passing
the discharges of Leyden jars around the coil C. In such bulbs with
two buttons a very curious effect is produced by the formation of the
shadows of each of the two buttons.

Another line of exp