CHAPTER III
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.
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.
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_.
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.
=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_.