Page 204 - My FlipBook

202 PRELIMINARY PREPARATION OF CAVITIES, ETC.

recorclod nnipcraj^e is not over three-tentlis of a miiliainiK'rc, the use of a
current of liio-li amperage is unnecessary, and it is attended with distress.
Hiii'h voltage is ecjually painful, as the endeavor to force the current
against the resistance of the dentin results in electrical irritation, as
already described.
The chlorid of silver cell is probably the one best suited for the
purpose, as its electro-motive force remains practically constant under
various conditions. The K. M. F. of each cell is about one volt ; the
internal resistance eight ohms; the strength one-fifth of an ampere.
This battery on account of its constancy and durability is largely used
in electro-medical apparatus. It is now furnished dry, and is more
acceptable as being less troublesome on this account.
The dry Leclanche battery is also one of the best forms, as it is an
open-circuit battery. As long as the circuit is open there is no action
in the cell and consequently there is no loss.
At present these two forms of galvanic battery cell ap})ear to be the
kinds best adapted for the purpose of inducing electrical osmosis.
The storage battery may also be used with advantage, but the plates
should be small ; each cell should contain but three plates to give the
proper degree of current strength. AVhen the plates are 3x3 inches
the normal amperage at eight hours' discharge is five-eighths of an
ampere. The voltage of each cell is two. This when discharged under
the resistance required for application to sensitive dentin in cataphoric
work should have a capacity for 800 applications, providing waste of
current strength does not occur from accidental short-circuiting.
The life of a chlorid of silver dry cell battery is stated to be 700
hours of cataphoric work under a high resistance of tissue, but it must
be remembered that the continuance of energy of all forms of battery is
varied by the resistance and the conversion of electrical energy into heat
by the controller which regulates the amperage and the voltage. This
principle applies to all sources of electrical force.
The coutroUer which at present ajipears best adapted to be interposed
between the battery and the anode is the AVillms controller, which, as
before stated, should be constructed with a resistance at the highest
point of at least 400,000 ohms. The gradations of resistance decrease
from this through 112 contact points. These permit a very gradual
reduction of the resistance as the switch is conveyed from point to
point in the circle. This controller also has the advantage of being
moderate in cost and easily procurable.
An important adjunct of any apparatus is a reliable villiiampercmeter.
This should have a scale to record divisions of fortieths of a milliampere,
from the fact that the amperage of the current through the dentin is fre-
quently efficient at less than two-tenths of a milliampere. The milli-