THE ELECTRIC CURRENT
The ordinary statement that an electric current is flowing along a wire is
only a conventional way of expressing the fact that the wire and the space
around the wire are in a different state from that in which they are when
no electric current is said to be flowing.
In order to make laymen understand the action of this so called current,
it is generally compared with the flow of water.
In comparing hydraulics and electricity, it must be borne in mind,
however, that there is really no such thing as an “electric fluid,” and
that water in pipes has mass and weight, while electricity has none. It
should be noted, however, that electricity is conveniently spoken of as
having weight in explaining some of the ways in which it manifests itself.
All electrical machines and batteries are merely instruments for moving
electricity from one place to another, or for causing electricity, when
accumulated in one place, to do work in returning to its former level of
distribution.
The _head_ or _pressure_ in a standpipe is what causes water to move
through the pipes which offer _resistance_ to the _flow_.
Similarly, the conductors, along which the electric current is said to
flow, offer more or less _resistance_ to the flow, depending on the
material. Copper wire is generally used as it offers little resistance.
The current must have pressure to overcome the resistance of the conductor
and flow along its surface. This pressure is called _voltage_ caused by
what is known as _difference of potential_ between the source and
terminal.
[Illustration: FIG. 35.--Analogy of the flow of water to the electric
current. The water in the reservoirs A and B stands at different heights.
As long as this difference of level is maintained, water from B will flow
through the pipe R to A. If by means of a pump P the level in B be kept
constant, flow through R will also be maintained. Here, by means of the
work expended on the pump, the level in the reservoir is kept constant;
and in the corresponding case of the electric current, by the conversion
of chemical energy a constant difference of potential is maintained.]
The pressure under which a current flows is measured in _volts_ and the
quantity that passes in _amperes_. The resistance with which the current
meets in flowing along a conductor is measured in _ohms_.
=Ques. What is a volt?=
Ans. A volt is that electromotive force (E. M. F.) which produces a
current of one ampere against a resistance of one ohm.
=Ques. What is an ampere?=
Ans. An ampere is the current produced by an E. M. F. of one volt in a
circuit having a resistance of one ohm. It is that quantity of electricity
which will deposit .005084 grain of copper per second.
=Ques. What is an ohm?=
Ans. An ohm is equal to the resistance offered to an unvarying electric
current by a column of mercury at 32° Fahr., 14.4521 grams in mass, of a
constant cross sectional area, and of the length of 106.3 centimeters.
=Ohm’s Law.=--_In a given circuit, the amount of current in amperes is
equal to the E. M. F. in volts divided by the resistance in ohms; that
is:_
current = pressure / resistance = volts / ohms
expressed as a formula:
I = E/R (1)
in which
I = current strength in amperes;
E = electromotive force in volts;
R = resistance in ohms.
From (1) is derived the following:
E = IR (2)
R = E/I (3)
From (1) it is seen that the flow of the current is proportional to the
voltage and inversely proportional to the resistance; the latter depends
upon the material, length and diameter of the conductor.
Since the current will always flow along the path of least resistance; it
must be so guarded that there will be no leakage. Hence, to prevent
leakage, wires are _insulated_, that is, covered by wrapping them with
cotton or silk thread or other insulating material. If the insulation be
not effective, the current may leak, and so return to the source without
doing its work. This is known as a _short circuit_.
The conductor which receives the current from the source is called the
_lead_, and the one by which it flows back, the _return_.
When wires are used for both lead and return, it is called a _metallic
circuit_: when the ground is used for the return, it is called a _ground
circuit_. An electric current is said to be:
1. _Direct_, when it is of unvarying direction;
2. _Alternating_, when it flows rapidly to and fro in opposite
directions;
3. _Primary_, when it comes directly from the source;
4. _Secondary_, when the voltage and amperage of a primary current have
been changed by an _induction coil_;
5. _Low tension_, when its voltage is low;
6. _High tension_, when its voltage is high.
A high tension current is capable of forcing its way against
considerable resistance, whereas, a low tension current must
have its path made easy.
=Production of the Electric Current.=--To produce a steady flow of water
in a pipe two conditions are necessary. There must first be available a
hydraulic pressure, or, as it is technically called, a “_head_” of water
produced by a pump, or a difference of level or otherwise.
In addition to the pressure there must also be a suitable path or channel
provided for the water to flow through, or there will be no flow, however
great the “head,” until something breaks down under the strain. In the
case just cited, although there is full pressure in the water in the pipe,
there is no current of water as long as the tap remains closed. The
opening of the tap completes the necessary _path_ (the greater part of
which was already in existence) and the water flows.
[Illustration: FIG. 36.--Hydraulic analogy of the electric current. If,
say 10 gallons of water flow in every second into a system of vessels and
pipes of any shape, whether simple or more complicated as shown in the
figure, and 10 gallons flow out again per second, it is evident that
through every cross section of any vessel or pipe of the system 10 gallons
of water pass every second. This follows from the fact that water is an
uncompressible liquid and must be practically of the same density
throughout the system. The water moves slowly where the section is large
and quickly where it is small, and thus the quantity of water that flows
through any part of the system is independent of the cross section of that
part. The same condition holds good for the electric current; if in a
closed circuit a constant current circulates, the same amount of
electricity will pass every cross section per second. Hence the following
law: _The magnitude of a constant current in any circuit is equal in all
parts of the circuit._]
For the production of a steady electric current two very similar
conditions are necessary. There must be a steadily maintained electric
pressure, known under different aspects as “electromotive force,”
“potential difference,” or “voltage.” This alone, however, is not
sufficient. In addition, a suitable conducting path is necessary. Any
break in this path occupied by unsuitable material acts like the closed
tap in the analogous case above mentioned, and it is only when all such
breaks have been properly bridged by suitable material, that is, by
conductors, that the effects which denote the flow of the current will
begin to be manifested.
The necessary electromotive force or voltage required to cause the current
to flow may be obtained:
1. Chemically;
2. Mechanically;
3. Thermally.
In the first method, two dissimilar metals such as copper and zinc called
_elements_, are immersed in an exciting fluid or _electrolyte_.
[Illustration: FIG. 37.--Volta’s “Crown of Cups.” The metallic elements C
and Z each consisted of two metals, the plate C being of copper and the
plate Z of zinc. They were placed, as shown, in the glass vessels, which
contained salt water and ordinary water or lye. Into each vessel, except
the two end ones, the copper end of one arc and the zinc end of the next
were introduced, the series, however long, ending with copper dipping into
the terminal vessel at one end and zinc into that at the other. The
arrangement is almost exactly that of a modern one-fluid primary battery.]
When the elements are connected at their terminals by a wire or conductor
a chemical action takes place, producing a current which flows from the
copper to the zinc. This device is called a _cell_, and the combination of
two or more of them connected so as to form a unit is known as a
_battery_. _The word battery is frequently used incorrectly for a single
cell._ That terminal of the element from which the current flows is called
the _plus_ or _positive pole_, and the terminal of the other element the
_negative pole_.
Cells are said to be _primary_ or _secondary_ according as they
generate a current of themselves, or first require to be charged
from an external source, storing up a current supply which is
afterwards yielded in the reverse direction to that of the
charging current.
An electric current is generated mechanically by a _dynamo_. In either
case _no electricity is produced, but part of the supply already existing
is simply set in motion by creating an electric pressure_.
An electric current, according to the third method, is generated directly
from heat energy, as will be later explained; the current thus obtained is
very feeble.
[Illustration: FIG. 38.--Hydrostatic analogy of fall of potential in an
electrical circuit.]
[Illustration: FIG. 39.--Showing method of connecting voltmeter to find
potential difference between any two points as _m_ and _n_ on an
electrical circuit.]
=Strength of Current.=--It is important that the reader have a clear
conception of this term, which is so often used. The exact definition of
the strength of a current is as follows:
_The strength of a current is the quantity of electricity which flows past
any point of the circuit in one second._
_Example._--If, during 10 seconds, 25 coulombs of electricity
flow through a circuit, then the average strength of the current
during that time is 2-1/2 coulombs per second, or 2-1/2
amperes.
=Voltage Drop in an Electric Circuit.=--A difference of potential exists
between any two points on a conductor through which a current is flowing
on account of the resistance offered to the current by the conductor.
For instance, in the electrical circuit shown in fig. 39, the potential at
the point _a_ is higher than that at _m_, that at _m_ higher than that at
_n_, etc., just as in the water circuit, shown in fig. 38, the hydrostatic
pressure at _a_ is greater than that at _m′_, that at _m′_ greater than
that at _n′_, etc. The fall in the water pressure between _m′_ and _n′_
(fig. 38) is measured by the water head _n’s_.
In order to measure the fall in electrical potential between _m_ and _n_,
(fig. 39), the terminals of a volt meter are placed in contact with these
points as shown. Its reading will give the difference of potential between
_m_ and _n_, in volts, provided that its own current carrying capacity is
so small that it does not appreciably lower the potential difference
between the points _m_ and _n_ by being touched across them; that is,
provided the current which flows through it is negligible in comparison
with that which flows through the conductor which already joins the points
_m_ and _n_.