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P. 165
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THE DECAY OF THE TEETH. 139
a silk thread in a salt solution, or in any liquid which conducts,
we obtain a conductor. Xow, it is not the particles of the thread
which transmit the current, but the particles of the liquid ; and
ifwe were to bring a piece of metal in contact with this moistened
thread, the potential of the particles of silk would not be changed.
This is what we have in the living human tooth,—a non-con-
ductor permeated by a conductor. If we were able to construct
a tooth of glass and fill the pulp-cavity, the canals, and tubules
of that tooth with a 0.75 per cent, solution of table-salt (which
has about the same specific resistance as the tissues of the body),
then we would have an electrical instrument similar to a living
human tooth. Such an instrument would transmit a current of
electricity; in other words, it would be a conductor just as the
wet silk thread is, but the substance of the tooth—the glass
could not receive any potential, even by contact with gold or
any other metal.
It has just been stated that dry dentine, as well as enamel, is
a non-conductor. Although the fact that the constituents of
which dentine is composed are themselves non-conductors justi-
fies this conclusion, it nevertheless appeared desirable to test the
question by direct experiment. This test was made in the follow-
ing manner
A cross-section of dentine -^-^ millimeter thick was inclosed in
a circuit of three Siemen's cells. The galvanometer used had a
multiplicator with 16,000 turns of wire and a resistance of 5000
Siemen's units. When the circuit was closed, not the slightest
deflection of the needle could be observed.
The section of dentine was placed l^etweeu
the ends of two wires which had a diameter
of 1.9 millimeter under a pressure of about
two grams to the square millimeter.
This experiment was then varied by inclos-
ins: three sections in a circuit in a manner
illustrated in Fig. 55, so that the surface of
contact was three times as great and the resist-
ance consequently only one-third of that in the above experi-
ment. The deflection remained, however, zero; in other words,
the resistance was infinitely great.
:
THE DECAY OF THE TEETH. 139
a silk thread in a salt solution, or in any liquid which conducts,
we obtain a conductor. Xow, it is not the particles of the thread
which transmit the current, but the particles of the liquid ; and
ifwe were to bring a piece of metal in contact with this moistened
thread, the potential of the particles of silk would not be changed.
This is what we have in the living human tooth,—a non-con-
ductor permeated by a conductor. If we were able to construct
a tooth of glass and fill the pulp-cavity, the canals, and tubules
of that tooth with a 0.75 per cent, solution of table-salt (which
has about the same specific resistance as the tissues of the body),
then we would have an electrical instrument similar to a living
human tooth. Such an instrument would transmit a current of
electricity; in other words, it would be a conductor just as the
wet silk thread is, but the substance of the tooth—the glass
could not receive any potential, even by contact with gold or
any other metal.
It has just been stated that dry dentine, as well as enamel, is
a non-conductor. Although the fact that the constituents of
which dentine is composed are themselves non-conductors justi-
fies this conclusion, it nevertheless appeared desirable to test the
question by direct experiment. This test was made in the follow-
ing manner
A cross-section of dentine -^-^ millimeter thick was inclosed in
a circuit of three Siemen's cells. The galvanometer used had a
multiplicator with 16,000 turns of wire and a resistance of 5000
Siemen's units. When the circuit was closed, not the slightest
deflection of the needle could be observed.
The section of dentine was placed l^etweeu
the ends of two wires which had a diameter
of 1.9 millimeter under a pressure of about
two grams to the square millimeter.
This experiment was then varied by inclos-
ins: three sections in a circuit in a manner
illustrated in Fig. 55, so that the surface of
contact was three times as great and the resist-
ance consequently only one-third of that in the above experi-
ment. The deflection remained, however, zero; in other words,
the resistance was infinitely great.
