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198 PRELIMINARY PREPARATION OF CAVITIES, ETC.

positive elements arc joined toi^etlicr and similarly all the negative to
each other. The voltage now is 2 and the amperage 5.

Fig. 152.
+

S_ s

The former method of assembling the cells is designated as "high
tension," the latter method as "low tension." When the source is the

Fig. 153.

dynamo, high and low tension are produced by the strength or weakness
of the magnetic field.
For electrical osmosis the source should be from batteries in series,
for the reason that in multiple the amperage would be too great when
the voltage is of sufficient force to overcome the resistance.
The degree of electrical energy tolerated by living dentin is exceed-
ingly small, on account of the peculiar and intense pain excited by the
transmission of electrical currents through the teeth. This is shown by
the low initial voltage of the batteries used for the purpose, varying
from less than 5 to rarely more than 20. But the initial passage of a
current of as high electro-motive force as these would not be tolerable,
and must therefore be reduced by suitable methods of effecting re-
sistance.
The apparatus used for this purpose is the controller, the purpose of
which is through its resistance to diminish the energy of the current to
sufficient weakness to meet the requirements of any given case. All
forms are constructed on the principle of the use of materials which are
highly resistant of the passage of electric currents. These substances