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196 PRELIMINARY PREPARATION OF CAVITIES, ETC.

GENERAL PRINCIPLES INVOLVED IN THE METHOD.
The application of electricity requires the consideration of the
general principles or laws governing its transmission.
The source of this force is to be found in chemical transformation.
Under the laws of the correlation of force it is capable of being con-
verted into heat, light, magnetism, and me(;hanical power, and may be
used to disorganize substances, wlien its action is called electrolysis. Its
movements are constant in their direction, viz, from bodies of high to
those of low potentiality.
In perfectly conducting substances electricity moves with entire free-
dom under any electro-motive force however small.
In perfectly non-conducting substances electricity will not move
under any electro-motive force however great.
In imperfectly conducting substances electricity moves only on the
exhibition of intense electro-motive force, the force varying according
as the substance is a more or less indifferent conductor.
Electricity has two elemental properties. These are defined as cur-
rent strength, designated by the term amperage ; and electro-motive
force, which is termed its voltage.
The active energy of electricity depends upon the first property,
its distribution upon the latter. Since it must be assumed that few
bodies are perfect conductors, this force or pressure is of that degree
which may be required in any given case to move the active energy,
the amperage, against the resistance it meets with.
The unit of strength is the ampere.
The unit of pressure is the volt.
The unit of resistance is the ohm.
The unit of poicer is the watt.
A volt represents the electro-motive force (E. M. F.) required to
impel one ampere of current through one ohm of resistance.
An AMPERE of current is so much as will deposit 0.00118 gram of
silver per second when passing through a standard solution of nitrate
of silver—or which will decompose 0.09326 milligram of water in one
second. Hence the ampere is the measure of rate of flow of an electri-
cal current, and in connection with the voltage measures the energy of
the current.
The unit of resistance (ohm) is that degree of resistance which
will permit the passage of one ampere of current at one volt of
pressure.
The watt is the power exerted by one ampere of current at one volt
of pressure.
In the economic application of electricity its transmission is effected