e to generate electrical energy directly from the heat of
the combustible. The armature, or moving part of the machine, consists
in reality of eight separate armatures all constructed of corrugated
sheet iron covered with asbestos and wound with wire. These armatures
are held in place by two circular iron plates, through the centre of
which runs a shaft, carrying at its lower extremity a semicircular
shield of fire-clay, which covers the ends of four of the armatures.
The heat, of whatever origin, is applied from below, and the shaft being
revolved, four of the armatures lose their magnetism constantly, while
the other four gain it, so to speak. As the moving part revolves,
therefore, currents of electricity are set up in the wires of the
armatures and are collected by a commutator, as in an ordinary dynamo,
placed on the upper end of the central shaft.

A great variety of electrical instruments are included in Edison's
inventions, many of these in fundamental or earlier forms being devised
for his systems of light and power, as noted already. There are numerous
others, and it might be said with truth that Edison is hardly ever
without some new device of this kind in hand, as he is by no means
satisfied with the present status of electrical measurements. He holds
in general that the meters of to-day, whether for heavy or for feeble
currents, are too expensive, and that cheaper instruments are a
necessity of the times. These remarks apply more particularly to what
may be termed, in general, circuit meters. In other classes Edison
has devised an excellent form of magnetic bridge, being an ingenious
application of the principles of the familiar Wheatstone bridge, used
so extensively for measuring the electrical resistance of wires; the
testing of iron for magnetic qualities being determined by it in the
same way. Another special instrument is a "dead beat" galvanometer which
differs from the ordinary form of galvanometer in having no coils or
magnetic needle. It depends for its action upon the heating effect of
the current, which causes a fine platinum-iridium wire enclosed in a
glass tube to expand; thus allowing a coiled spring to act on a pivoted
shaft carrying a tiny mirror. The mirror as it moves throws a beam of
light upon a scale and the indications are read by the spot of
light. Most novel of all the apparatus of this measuring kind is the
odoroscope, which is like the tasimeter described in an earlier chapter,
exc