Pittsburg dispatch. [volume] (Pittsburg [Pa.]) 1880-1923, 1890-11-09
CURRENT MEASURING
Principles Upon Which the Amme-
ter and Voltmeter Depends.
ELECTRICITY AND MAGNETISM
So Intimately Associated That They De-
termine Each Other.
HOW TO GET AT THE PRESSURE
WRITTEN FOR THE DISPATCH

is not thing that we can see, handle, touch
or weigh, but it is a condition of things
capable of doing work-a condition OF
things that can be changed, and when the
change is made, work is done. Electricity
is therefore condition of things that will
produce effects, and, not being able to see OF
handle electricity, we recognize its presence
by its known effects, and not only this, but we
also measure it by its comparative effects
under varied circumstances.
For example, if we wish to measure the
pressure of the atmosphere, we balance its
pressure against the pressure of a column
OF mercury, as in the barometer, and by
noting the effect we are able to say that the
pressure of the atmosphere is greater or less
one day than another. If we have a fire
that will melt iron, we call the heat great
that is, it is great compared with the heat of
fire that will but just melt lead. We
thus not only recognize the pressure of heat,
but we also make comparative measure
ments of its intensity by its effects under
varied circumstances. 1t is in this way that
We measure the electric current, that is, by
Its comparative effects under varied condi-
tions.
THE MAGNETIC FIELD.
In previous articles we have studied the
lines of force of a magnet and the effect that
they have on a compass or maguetized
needle free to move in all directions We
have also shown that these lines of force
have closed circuits. Now an electric cur
rent will also generate lines of force similar
in character to those of magnet. To illus
trate this fact take wire with a current of
electricity flowing through it and pass it
vertically through a smooth piece of
cardboard, or better, a fat piece of
glass, held in a horizontal position.
Then if fine iron filings are sifted
on to the surface of the cardboard or glass
it will be noticed that the filings will ar
range themselves in coneentric circles
around the wire.
These circles are closed lines of force and
do not represent any flowing motion, as of a
current, but they have direction and they
influence magnetic needle, free to move in
all directions, just as magnetic lines of force
do. The space occupied by lines of force is
called a ''feld of force or magnetic
field" or often simply 'feld," for short.
We have in a previous article shown how,
by exploring the field of magnet we
can determine and locate the poles OF mag
net. We can now also, in similar way,
explore the field of an electric current and
rent, but also its strength, as compared with
some other current.
A COMPASS IN THE FIELD.
IF on the glass plate and close to the wire
we place compass, the compass needle will
at first vibrate rapidly and finally come to
rest in q position tangent to the rings or
lines of force. If the direction of the cur
rent is reversed, the compass needle will run
hot ic hnll
tangent position, but reverse the direction
of its poles, thus showing that the lines of
force generated by the current have changed
their direction. If the current is flowing up
through the glass plate, the lines of force
will have direction opposite to that of the
hands of a clock; that is, looking down on
the plate, the direction will be from right
to left If the current flows down through
the plate, the direction of the lines of force
will be reversed.
Now the direction of the lines of force OF a
compass needle is from or out of the North
Pole and into the South Pole, and as the
lines of force of the compass and electric
current must have the same direction, it is
simple matter to determine the direction of
an electric current For example, if the
current is flowing up through the glass
plate, its lines of force will be from right to
left, and the north pole of the compass will
point in that direction. We thus lave the
following rule ior determining the direction
OF an electric current. Place portion of
the wire in a horizontal position and
compass under it. face up; the needle will
then place itself in position at right angles
to the wire, and if we look at the needle
from its south pole toward its north the cur
rent will be flowing from left to right. A
few practical applications of this rule will
enable the observer to quickly detect the
direction of an electric current without stop
ping to think of the rule or place himself in
the above mentioned positions.
MEASURING THE STRENGTH.
We have thus seen how by the effect of an
electric current on a magnetic needle we can
determine the direction of the current But
this is not all, for we can also determine its
comparative strength. For example, let us
again imagine the current flowing through
wire piercing glass plate as before and
again place the compass on the plate near
the wire. If now we fasten permanent
magnet near the compass, the compass
needle will struggle to point in two different
directions at the same time, that is in one
direction toward the permanent magnet, and
in another direction, as we have above seen,
due to the lines of force generated by the
electric current. In other words, we now
have two forces pulling the compass needle,
and if one of these is constant like the per-
manent magnet and the other variable as
the electric current can be made, the direc-
tion of the COMPASS needle will vary with
the variable force acting ON it, which, in
this case, is the current.
That is to say, if there is no current in
wire the compass needle will point toward
the permanent magnet; if there is small
current flowing through the wire, the
needle will be turned away from the magnet
little, and if the current is increased the
needle will turn still more, and thus the
position of the compass will determine the
relative amount of current flowing through
the wire. But to make this compass needle
a practical instrument Tor measuring the
strength of electric currents, it must be en-
closed in a suitable box or case with per-
mauent magnet attached and then properly
calibrated in the following manner:
GRADUATING THE INSTRUMENT.
Fix the instrument firmly with regard to
the electric wire; pass a known current
through the wire, say one ampere, and mark
the position of the needle on a suitable dial
with the figure 1. Then increase the cur
rent to- say two amperes, and mark the new
position OF the needle with the figure 2, and
so on. If then, at any future time, we have
an unknown current and wish to determine
its strength, the determination is readily
made by noting the DETECTION of the needle
and taking the reading off the scale. Of
course the position which the ceedle occu-
pies when there is no current flowing indi-
Gates the zero pointer point indicating that
there is no current flowing.
The sensitiveness of the instrument is