, the carbon will be sharply compressed, the
resistance in the local circuit will be proportionately lowered, and the
signal heard on the local sounder will be a loud one. Thus it will be
seen, by another clever juggle with the willing agent, carbon, for which
he has found so many duties, Edison is able to transfer or transmit
exactly, to the local circuit, the main-line current in all its minutest
variations.

In his researches to determine the nature of the motograph phenomena,
and to open up other sources of electrical current generation, Edison
has worked out a very ingenious and somewhat perplexing piece of
apparatus known as the "chalk battery." It consists of a series of chalk
cylinders mounted on a shaft revolved by hand. Resting against each of
these cylinders is a palladium-faced spring, and similar springs make
contact with the shaft between each cylinder. By connecting all these
springs in circuit with a galvanometer and revolving the shaft rapidly,
a notable deflection is obtained of the galvanometer needle, indicating
the production of electrical energy. The reason for this does not appear
to have been determined.

Last but not least, in this beautiful and ingenious series, comes the
"tasimeter," an instrument of most delicate sensibility in the presence
of heat. The name is derived from the Greek, the use of the apparatus
being primarily to measure extremely minute differences of pressure.
A strip of hard rubber with pointed ends rests perpendicularly on a
platinum plate, beneath which is a carbon button, under which again lies
another platinum plate. The two plates and the carbon button form part
of an electric circuit containing a battery and a galvanometer. The
hard-rubber strip is exceedingly sensitive to heat. The slightest degree
of heat imparted to it causes it to expand invisibly, thus increasing
the pressure contact on the carbon button and producing a variation
in the resistance of the circuit, registered immediately by the little
swinging needle of the galvanometer. The instrument is so sensitive that
with a delicate galvanometer it will show the impingement of the heat
from a person's hand thirty feet away. The suggestion to employ such
an apparatus in astronomical observations occurs at once, and it may
be noted that in one instance the heat of rays of light from the remote
star Arcturus gave results.

CHAPTER X

THE PHONOGRAPH

AT the opening of the Electrical Show in New York City