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DENTINAL ANESTHESIA BY ELECTRICAL OSMOSIS. 197

throngli metallic conductors. The resistance of these is varied by the
character of the metal, the cross section, and the distance. For certain
purposes other substances are employed to eifect greater resistance than
the metals.
The current strength flowing in a circuit is equal to the pressure
divided by the resistance.
The resistance equals the pressure divided by the strength.
The pressure equals the strength multiplied by the resistance. In
elementary terms :
Amperes = volts ^ ohms.
Ohms — volts -^ amperes.
Volts = amperes X ohms.
AVatts = volts X amperes.

It follows from the formula that the amount of power and the cost
of producing it is the same whether the current is of large amperage at
low voltao:e or of small ampera cent lamp may be supplied by 100 volts at ^ ampere or by 50 volts at
1 ampere—the result in each case being 50 watts.
Comparative Illustration.—Given a current of 100 volts at fifteen
amperes, and we wish to use only |- ampere, the resistance to be put
in the circuit is found thus : 100 v ^2^a — 40 o.
In case we have 2^ amperes under 7 ohms resistance, it requires
17.V volts to move this degree of amperage through the given resistance,
thus : 2^ a X 7 r = 17|- v. If one has a current of 110 volts, and de-
sires to use a ^-horse-power motor, the least amperage required is Iywo)
which is found by dividing 186 watts by 110. These examples make
plain the means of determining the character of current required for
any given purpose.

Electrical force may be produced from its source in galvanic cells by
arranging them in series or in multiple. If in series the voltage is
the sum of the volts of the cells so arranged, and the amperage is that
of each of the cells. If joined in multiple, the strength in amperes
is the sum of the amperes of the cells, and the voltage is that of one
cell.
Fig. 152^ represents the arranging of cells in series, the positive of
one with the negative of the next. In case each cell has a voltage of
2 and an amperage of 1 the electro-motive force of 5 cells will be 10
volts at 1 ampere.
Fig. 153^ represents the joining of cells in multiple. Here all the
^See Dented Cosmos, December 1896, p. 998. =* Ibid.