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36 DENTAL MEDICINE. —

rent^ the galvanic current^ both of which are dynamical, and the
static current^ which is frictional.
Galvanic electricity — galvanism, and induced electricity
faradism, are the two forms generally employed, frictional elec-
tricity being but seldom applied. What is known as electro-mag-
netism is a form of induced current, generated by the rotary in-
strument, and is not reliable. Faradism, so-called from its dis-
coverer, is generated by an instrument which is capable of apply-
ing slow or rapid shocks, and giving what is denominated a fine,
strong, induced current. It consists of coils of wire ; a small
hammer of soft iron, attached to a spring ; a pole containing a
platinum-pointed screw, the hammer breaking the current in the
coil of wire, and by rapid vibration producing shocks.
" The galvanic current is generated by a series of cells, suffi-
cient in number to cause a current of tension, which is the
resistance offered to the passage of a current.
*' One cell supplies a current, the poles of other cells being
alternately joined, and there are finally but two terminal poles."
As the current from the first cell passes through the remaining
cells, its power is increased and the effect is governed by the
number of cells belonging to the circuit. There is also what
is dominated a current of " quantity," which is generated by a
large metallic surface in the battery cell, the tension current
being generated by a number of small metal plates. The " ten-
sion current " serves the best purpose for medical use, while
the " quantity current " furnishes heat, and is, therefore, adapted
to purposes of electro-surgery. The modified Bunsen galvanic
cell is the best for medical purposes, although what is known as
the Siemens and Halske cell is extensively used ; " it consists of
an outer cell of glass, with elements of zinc and copper, a dia-
phragm of porous earthenware, and a diaphragm of papier mache,
between the solutions." The Holtz electric instrument furnishes
the best static current.
The Ruhmkorff coil is also employed, in the use of which but
one wire is brought in contact with the patient, the other con-
ductor being formed by the air, and a spark, similar to the ordi-
nary spark from the friction machine, being produced.