than Edison had done. For
one thing, Edison believed the action of his transmitter as due to a
property of certain poor or 'semi-conductors,' whereby their electric
resistance varied under pressure. Hughes taught us to understand that
it was owing to a property of loose electrical contact between any two
conductors.

The soft and springy button of lamp-black became no longer necessary,
since it was not so much the resistance of the material which varied as
the resistance at the contacts of its parts and the platinum electrodes.
Two metals, or two pieces of hard carbon, or a piece of metal and a
piece of hard carbon, were found to regulate the current in accordance
with the vibrations of the voice. Edison therefore discarded the soft
and fragile button, replacing it by contacts of hard carbon and
metal, in short, by a form of microphone. The carbon, or microphone
transmitter, was found superior to the magneto-electric transmitter of
Bell; but the latter was preferable as a receiver to the louder but
less convenient chemical receiver of Edison, and the most successful
telephonic system of the day is a combination of the microphone, or new
carbon transmitter, with the Bell receiver.

The 'micro-tasimeter,' a delicate thermoscope, was constructed in 1878,
and is the outcome of Edison's experiments with the carbon button.
Knowing the latter to be extremely sensitive to minute changes of
pressure, for example, those of sonorous vibrations, he conceived the
idea of measuring radiant heat by causing it to elongate a thin bar
or strip of metal or vulcanite, bearing at one end on the button. To
indicate the effect, he included a galvanometer in the circuit of the
battery and the button. The apparatus consisted of a telephone button
placed between two discs of platinum and connected in circuit with the
battery and a sensitive galvanometer. The strip was supported so that
one end bore upon the button with a pressure which could be regulated by
an adjustable screw at the other. The strip expanded or contracted when
exposed to heat or cold, and thrust itself upon the button more or less,
thereby varying the electric current and deflecting the needle of
the galvanometer to one side or the other. The instrument was said to
indicate a change of temperature equivalent to one-millionth of a degree
Fahrenheit. It was tested by Edison on the sun's corona during the
eclipse observations of July 29, 1875, at Rawlings, in the territory of
W