The times. [volume] (Richmond, Va.) 1890-1903, 1892-10-16
How Electricity Is Measured.

How Electricity is Measured.
Amperes, ohms and volts have almost
Become household words. In the news
papers they are to be seen every day.
and since applied electricity has almost
reached the prominence of popular
cult, the literary and semi scientific jour
mails handle the terms peculiar to the
science with seeming confidence in the
ABILITY OF their readers to understand
thei- that too often is misplaced. Very
many intelligent persons have not yet had
occasion TO learn what amperes and ohms
ana volts mean to the electrical engineer
and how he makes use Of them. They
are the Fundamental electrical units,
ana clear understanding of what they
represent and their relations to each
other is an indispensable preliminary to
any Inquiry into the fascinating subject
OF electrical mee surements. The rela-
tons between the volt. the ohm and the
ampere are fixed and immutabe. and on
these relations depend the remarkably
accurate calculations and measurements
that the workers in electrical science
have evolved.
The volt is the unit of electrical pres
sure. or, as it is often called. electromo-
tive force. The ordinary Daniel battery,
containing zinc and copper elements in
solutions of zinc and copper sulphate,
gives an electrical pressure of one volt:
in other words, the pressure available
between the two poles of this type of
gaivanic cell was adopted as the unit of
electrical pre- ssure and was named
volt. after Volta, the renowned Italian
physicist.
The ohm is tie unit of electrical re-
sistance. All bodies offer certain
of IN ha -
an electric current. Those which offer
very high resi- stance are termed non
conductors or insulatOrs, and those which
offer relatively low resistance are
termed conductors. The insuators COM
prise such substances as glass, porcelain,
yitreous and bituminOus materials, India
rubber. gutta percha, oils and gums.
The best conductors are the metals;
water and solutions of salts and the
human body are fair conductors. In or
der that the resistance of different con-
ductors might be measured or compared
Il standard or unit of resistance had to
be determined. This was named the ohm
in honor of the German scientist who
gave to the world the laws of the cleo
trio current. \ column of mercury Il
fraction over 10G centimeters long with
a cross sectional area of one square mill
imeter has resistance of one ohm. A
copper wire of the same sectional area
and having Il resistance of one ohm
would be about sixty times rs long rs
the mercury column. as copper is much
better conductor than mercury. Inc
copper wire generally used for telephone
and telegraph lines has a resistance Of
iron wire of the same size would have
about six times the resistance. rs iron
is Il much poorer conductor of electricity
than copper. Iron wire is used rs an
electric conductor only for telegraph
lines: its cheapness and strength recon-
mend il for this work. but even for teie-
graph lines copper is replacing iron TO
great extent. In the distribution or elec,
tricity on a large scale copper Is used
exclusively, as it is for the coils OF al
most all electrical machines and instru-
ments.
The ampere is the unit of current
strength. and is so named from French
electrician. If source OF eiectrlci'y
having pressure of one volt be conhec-
ted to circuit having resistance OF
one ohm the current that will now III
that circuit has Il strength OF one am
pere The ampere is derived from the
volt and the ohm. and in order to produce
amperes we must have volts acting upon
ohms. A certain number OF volts ac'ihS
on circuit containing certain hulhpcr
of ohms must give current OF GER
tain number of amperes. II we nave
ten volts acting on one Ohm we gel w
amperes OF current flowing IN the circuit
and if we have one volt acting on one
tenth of an ohm we get ten amperes,
while if we have one hundred volts ana
ten ohns we get ten anperes. neh-
ever nm ampere is needed a volt must pc
available for every ohm OF resistance 10
force il through the circuit. Wherever
path is open to the current 1t VIII now
through that path in exact proportion 70
it's resistance. and the strength OF the
current will be proportional TO the press
IRE that drives il. Take for instance
an electric light circuit. The type OF IN
candescent lamp in most general use
requires n current of about hali an am
pere to heat the fllament to incandes-
cence. Consequently we see that the
resistance of the lamp must be twice
as many ohms as there are volts OF pres
sure in the circuit
With 40 volts pressure lamp having
a resistance of DO ohms will allow hall
an ampere of current TO pass through H.
If the pressure is allowed to diminish:
less than half an ampere of CURRENT VIII
pass through the lamp and the iiaiheh'
will grow dull: II the pressure IS lncreas,
ed the flow of current will increase and
the filament will get brighter. The now
of current. then, is dependent always on
the pressure developed at the source OF
electricity and the resistance OF the CIR
cult to which that source is corrected;
There is one other important electrical
unit, which has been adopted OF late
years, s1..e the distribution of electricity
on large scale for lighting and power
has become so general. and that Is the
unit of electrical power. This is called
the watt ard its value Is the power lie
veloped by a current of one ampere ~
pr, ssure of one volt. In other words,
one watt is IN ampere multiplied BY a
volt. An incandescent lamp consuming
half an ampere at a pressure OF iio
volts takes % watts of electrical power
Tn, watt is such convenient expression
that almost all expenditures OF eiectri'
cal power are now expressed IN waits
or la kilowatts. A kilowatt is one thou
sand watts, and is equivalent TO about
one and oue-third mechanical horse
power. A dynamo Is said to be Of SO
many kilowatts capacity; and the kilo
watt hour-that r. one thousand waits