Page 201 - My FlipBook

DENTINAL ANESTHESIA BY ELECTRICAL OSMOSIS 199

are water, carbon, graphite, and coils of wire of known high resistance,
the most effective being of German silver. In the case of the latter the
degree of resistance is regulated by the length and fineness of the wire,
the cross section being reduced to the size which will conduct the cur-
rent without excessive heating, and to that end it is graded with refer-
ence to the initial amperage of the current. In comparison with silver
as a unit German silver has a resistance of 13.92.
The carbon and graphite controllers usually are constructed in the
form of a broken ring—one pole of the battery being connected at one
end of the ring, the other pole being attached to an index which travels
over this annular disk. This method of construction gives a fine grada-
tion of current with high resistance. It may be used in connection with a
German-silver wire rheostat, where currents of great strength are used
for reasons which will appear later. In the use of high-voltage cur-
rents, such as the 110-volt circuit, it may be switched through the
coils to a nearly definite low voltage by means of a rlieostat, when
the adaptation to the case may be effected through the graphite con-
troller.
In the arrangement of the apparatus to effect electrical osmosis the
battery, the controller, the instruments of observation, and the patient
are in series. In the analysis of the course of the current it apjjears that
the patient is another element of resistance, and that dentin is more
highly resistant than the other tissues. In other words, there are two
resistances in the circuit—the controller and the tissues of the patient.
The result of the resistance of the dentin, unless the initial voltage is
small and is reduced by the controller to an infinitesimal degree, is the
occurrence of pain, which takes place with different persons at various
degrees of amperage. The approacli to intolerance of the current is
designated the " pain limit." This condition has been assumed by some
observers to be caused by the evolution of heat in the dentin con-
sequent upon the resistance of this tissue. This view is not now con-
sidered to be conclusive, as the calculated elevation of temperature at
j^ milliampere is not sufficient to account for the degree of irritation
which occurs on increasing the rate of flow. This determination
leaves two other hypotheses to account for the irritation : ((, the ten-
dency of the current to disorganize some of the anatomical elements of
the canaliculi ; and b, the osmotic pressure of the migration of the
medicaments. Here the student is not confused with consideration of
the complicated forces which are in action connected with the electroly-
sis of the cocain solution.^
The pain limit is variable with different ])ersons, and in different
' See "The Foundation Principles of Dental C'ataplioresis," Items of Interest, vol. xx.
\). 345 et scq.