Page 202 - My FlipBook

200 PRELIMINARY PREPARATION OF CAVITIES, ETC.

teeth of the same jxn-soii. With sonio it is reached Avitli the first
influx of the current at low voltage with a recoi-d of" ^'|, inillianipere,
this low record indicating high resistance of dentin and ])erinitting but
slow increase of the force until after cocain has diminished the sensitiv-
ity of the irritated surface. AVith others the ])ain limit may not be
reached with a voltage of 20 and a recorded amperage of ^V to y^,y milli-
ampere. In respect of electrical irritation there must be taken into
account also the high nervous sensitivity of some persons, as with these
there usually appears greater susceptibility to electrical irritation.
The following table of calculated resistances shows the resistance
in ohms, and the liability to the generation of heat in the dental tissues
in view of their density, or the tendency to disorganization as ])reviously
stated, and it suggests that care be used in the application of electrical
force for the purpose under consideration.

niilliampere in circuit the olims are .S7,500.
Witli 15 volts initial pressure at x*o
" 15 " " J^ " " " 150,000.
" 10 " " t1j " " " 25,000.
" 10 " " tV " " " 100,000.
" 5 " " T% " " " 12,500.
" 5 " " tV " " " 50,000.

As the resistance of the body including the dental tissues varies from
10,000 to almost 70,000 ohms, it would appear necessary that the con-
troller should have at the highest ])oint a resistance of not less than
400,000 ohms. This degree of resistance is required to obviate the
effect of impulse which may occur in closing the circuit. Occasionally
slight shock is felt at 600,000 ohms.
The varying resistance of the current through the tissues depends
upon the density of the dentin, the distance traversed, the condition
of the surface of the skin, and the thickness of the adipose tissues.
The average resistance of the patient as recorded by Dr. W. A. Price
is about 25,000 ohms from cavity to hand, and the difference of resistance
from tooth to hand and cheek to hand is from 3000 to 5000 ohms. He
reports one case where the resistance from cavity to hand with a 40 per
cent, solution of cocain was 28,500 ohms, which on placing the pad on
the cheek was reduced to 23,000 ohms.
Dr. Price further places the average resistance from hand to tongue
at 9000 ohms, and from cheek to tongue at from 3000 to 7000. This
w^ould make the resistance of the dentin nearly 20,000 ohms. An
exact determination of the resistance of the skin in any given case
would enable a very close approximation for the dentin to be calculated.
The condition of the cavity as to relative moisture and the degree
of saturation of the pledget of cotton containing the anesthetizing agent