ELECTRICITY SIMPLY EXPLAINED
It is easy to understand how electricity behaves and what it does if
you get the right idea of it at the start. In the first place, if you
will think of electricity as being a fluid like water its fundamental
actions will be greatly simplified. Both water and electricity may be
at rest or in motion. When at rest, under certain conditions, either
one will develop pressure, and this pressure when released will cause
them to flow through their respective conductors and thus produce a
current.
Electricity at Rest and in Motion.--Any wire or a conductor of any
kind can be charged with electricity, but a Leyden jar, or other
condenser, is generally used to hold an electric charge because it has
a much larger _capacitance_, as its capacity is called, than a wire.
As a simple analogue of a condenser, suppose you have a tank of water
raised above a second tank and that these are connected together by
means of a pipe with a valve in it, as shown at A in Fig. 28.
[Illustration: Fig. 28.--Water Analogue for Electric Pressure.]
[Illustration: original © Underwood and Underwood. First Wireless
College in the World, at Tufts College, Mass.]
Now if you fill the upper tank with water and the valve is turned off,
no water can flow into the lower tank but there is a difference of
pressure between them, and the moment you turn the valve on a current
of water will flow through the pipe. In very much the same way when
you have a condenser charged with electricity the latter will be under
_pressure,_ that is, a _difference of potential_ will be set up, for
one of the sheets of metal will be charged positively and the other
one, which is insulated from it, will be charged negatively, as shown
at B. On closing the switch the opposite charges rush together and
form a current which flows to and fro between the metal plates.
[Footnote: Strictly speaking it is the difference of potential that
sets up the electromotive force.]
The Electric Current and Its Circuit.--Just as water flowing through a
pipe has _quantity_ and _pressure_ back of it and the pipe offers
friction to it which tends to hold back the water, so, likewise, does
electricity flowing in a circuit have: (1) _quantity_, or _current
strength_, or just _current_, as it is called for short, or
_amperage_, and (2) _pressure_, or _potential difference_, or
_electromotive force_, or _voltage_, as it is variously called, and
the wire, or circuit, in which the current is flowing has (3)
_resistance_ which tends to hold back the current.
A definite relation exists between the current and its electromotive
force and also between the current, electromotive force and the
resistance of the circuit; and if you will get this relationship
clearly in your mind you will have a very good insight into how direct
and alternating currents act. To keep a quantity of water flowing in a
loop of pipe, which we will call the circuit, pressure must be applied
to it and this may be done by a rotary pump as shown at A in Fig. 29;
in the same way, to keep a quantity of electricity flowing in a loop
of wire, or circuit, a battery, or other means for generating electric
pressure must be used, as shown at B.
[Illustration: Fig. 29.--Water Analogues for Direct and Alternating
Currents.]
If you have a closed pipe connected with a piston pump, as at C, as
the piston moves to and fro the water in the pipe will move first one
way and then the other. So also when an alternating current generator
is connected to a wire circuit, as at D, the current will flow first
in one direction and then in the other, and this is what is called an
_alternating current_.
Current and the Ampere.--The amount of water flowing in a closed pipe
is the same at all parts of it and this is also true of an electric
current, in that there is exactly the same quantity of electricity at
one point of the circuit as there is at any other.
The amount of electricity, or current, flowing in a circuit in a
second is measured by a unit called the _ampere_, [Footnote: For
definition of _ampere_ see _Appendix._] and it is expressed by the
symbol I. [Footnote: This is because the letter C is used for the
symbol of _capacitance_] Just to give you an idea of the quantity of
current an _ampere_ is we will say that a dry cell when fresh gives a
current of about 20 amperes. To measure the current in amperes an
instrument called an _ammeter_ is used, as shown at A in Fig. 30, and
this is always connected in _series_ with the line, as shown at B.
[Illustration: Fig. 30.--How the Ammeter and Voltmeter are Used.]
Electromotive Force and the Volt.--When you have a pipe filled with
water or a circuit charged with electricity and you want to make them
flow you must use a pump in the first case and a battery or a dynamo
in the second case. It is the battery or dynamo that sets up the
electric pressure as the circuit itself is always charged with
electricity.
The more cells you connect together in _series_ the greater will be
the electric pressure developed and the more current it will move
along just as the amount of water flowing in a pipe can be increased
by increasing the pressure of the pump. The unit of electromotive
force is the _volt_, and this is the electric pressure which will
force a current of _1 ampere_ through a resistance of _1 ohm_; it is
expressed by the symbol _E_. A fresh dry cell will deliver a current
of about 1.5 volts. To measure the pressure of a current in volts an
instrument called a _voltmeter_ is used, as shown at C in Fig. 30, and
this is always connected across the circuit, as shown at D.
Resistance and the Ohm.--Just as a water pipe offers a certain amount
of resistance to the flow of water through it, so a circuit opposes
the flow of electricity in it and this is called _resistance_.
Further, in the same way that a small pipe will not allow a large
amount of water to flow through it, so, too, a thin wire limits the
flow of the current in it.
If you connect a _resistance coil_ in a circuit it acts in the same
way as partly closing the valve in a pipe, as shown at A and B in Fig.
31. The resistance of a circuit is measured by a unit called the
_ohm_, and it is expressed by the symbol _R_. A No. 10, Brown and
Sharpe gauge soft copper wire, 1,000 feet long, has a resistance of
about 1 ohm. To measure the resistance of a circuit an apparatus
called a _resistance bridge is used_. The resistance of a circuit can,
however, be easily calculated, as the following shows.
[Illustration: Fig. 31.--Water Valve Analogue of Electric Resistance.
A- a valve limits the flow of water. B- a resistance limits the flow
of current.]
What Ohm's Law Is.--If, now, (1) you know what the current flowing in
a circuit is in _amperes_, and the electromotive force, or pressure,
is in _volts_, you can then easily find what the resistance is in
_ohms_ of the circuit in which the current is flowing by this formula:
Volts E
--------- = Ohms, or --- = R
Amperes I
That is, if you divide the current in amperes by the electromotive
force in volts the quotient will give you the resistance in ohms.
Or (2) if you know what the electromotive force of the current is in
_volts_ and the resistance of the circuit is in _ohms_ then you can
find what the current flowing in the circuit is in _amperes_, thus:
Volts E
----- = Amperes, or --- = I
Ohms R
That is, by dividing the resistance of the circuit in ohms, by the
electromotive force of the current you will get the amperes flowing in
the circuit.
Finally (3) if you know what the resistance of the circuit is in
_ohms_ and the current is in _amperes_ then you can find what the
electromotive force is in _volts_ since:
Ohms x Amperes = Volts, or R x I = E
That is, if you multiply the resistance of the circuit in ohms by the
current in amperes the result will give you the electromotive force in
volts.
From this you will see that if you know the value of any two of the
constants you can find the value of the unknown constant by a simple
arithmetical process. This relation between these three constants is
known as _Ohm's Law_ and as they are very important you should
memorize them.
What the Watt and Kilowatt Are.--Just as _horsepower_ or _H.P._, is
the unit of work that steam has done or can do, so the _watt_ is the
unit of work that an electric current has done or can do. To find the
_watts_ a current develops you need only to multiply the _amperes_ by
the _volts_. There are _746 watts_ to _1 horsepower, and 1,000 watts
are equal to 1 kilowatt_.
Electromagnetic Induction.--To show that a current of electricity sets
up a magnetic field around it you have only to hold a compass over a
wire whose ends are connected with a battery when the needle will
swing at right angles to the length of the wire. By winding an
insulated wire into a coil and connecting the ends of the latter with
a battery you will find, if you test it with a compass, that the coil
is magnetic.
This is due to the fact that the energy of an electric current flowing
in the wire is partly changed into magnetic lines of force which
rotate at right angles about it as shown at A in Fig. 32. The
magnetic field produced by the current flowing in the coil is
precisely the same as that set up by a permanent steel magnet.
Conversely, when a magnetic line of force is set up a part of its
energy goes to make up electric currents which whirl about in a like
manner, as shown at B.
[Illustration: (A) and (B) Fig. 32.--How an Electric Current is
Changed into Magnetic Lines of Force and These into an Electric
Current.]
[Illustration: (C) and (D) Fig. 32.--How an Electric Current Sets up a
Magnetic Field.]
Self-induction or Inductance.--When a current is made to flow in a
coil of wire the magnetic lines of force produced are concentrated, as
at C, just as a lens concentrates rays of light, and this forms an
intense _magnetic field_, as it is called. Now if a bar of soft iron
is brought close to one end of the coil of wire, or, better still, if
it is pushed into the coil, it will be magnetized by _electromagnetic
induction,_ see D, and it will remain a magnet until the current is
cut off.
Mutual Induction.--When two loops of wire, or better, two coils of
wire, are placed close together the electromagnetic induction between
them is reactive, that is, when a current is made to flow through one
of the coils closed magnetic lines of force are set up and when these
cut the other loop or turns of wire of the other coil, they in turn
produce electric currents in it.
It is the mutual induction that takes place between two coils of wire
which makes it possible to transform _low voltage currents_ from a
battery or a 110 volt source of current into high pressure currents,
or _high potential currents_, as they are called, by means of a spark
coil or a transformer, as well as to _step up_ and _step down_ the
potential of the high frequency currents that are set up in sending
and receiving oscillation transformers. Soft iron cores are not used
in oscillation inductance coils and oscillation transformers for the
reason that the frequency of the current is so high the iron would not
have time to magnetize and demagnetize and so would not help along the
mutual induction to any appreciable extent.
High-Frequency Currents.--High frequency currents, or electric
oscillations as they are called, are currents of electricity that
surge to and fro in a circuit a million times, more or less, per
second. Currents of such high frequencies will _oscillate_, that is,
surge to and fro, in an _open circuit_, such as an aerial wire system,
as well as in a _closed circuit_.
Now there is only one method by which currents of high frequency, or
_radio-frequency_, as they are termed, can be set up by spark
transmitters, and this is by discharging a charged condenser through a
circuit having a small resistance. To charge a condenser a spark coil
or a transformer is used and the ends of the secondary coil, which
delivers the high potential alternating current, are connected with
the condenser. To discharge the condenser automatically a _spark,_ or
an _arc,_ or the _flow of electrons_ in a vacuum tube, is employed.
Constants of an Oscillation Circuit.--An oscillation circuit, as
pointed out before, is one in which high frequency currents surge or
oscillate. Now the number of times a high frequency current will surge
forth and back in a circuit depends upon three factors of the latter
and these are called the constants of the circuit, namely: (1) its
_capacitance,_ (2) its _inductance_ and (3) its _resistance._
What Capacitance Is.--The word _capacitance_ means the _electrostatic
capacity_ of a condenser or a circuit. The capacitance of a condenser
or a circuit is the quantity of electricity which will raise its
pressure, or potential, to a given amount. The capacitance of a
condenser or a circuit depends on its size and form and the voltage of
the current that is charging it.
The capacitance of a condenser or a circuit is directly proportional
to the quantity of electricity that will keep the charge at a given
potential. The _farad,_ whose symbol is _M,_ is the unit of
capacitance and a condenser or a circuit to have a capacitance of one
farad must be of such size that one _coulomb,_ which is the unit of
electrical quantity, will raise its charge to a potential of one volt.
Since the farad is far too large for practical purposes a millionth of
a farad, or _microfarad_, whose symbol is _mfd._, is used.
What Inductance Is.--Under the sub-caption of _Self-induction_ and
_Inductance_ in the beginning of this chapter it was shown that it was
the inductance of a coil that makes a current flowing through it
produce a strong magnetic field, and here, as one of the constants of
an oscillation circuit, it makes a high-frequency current act as
though it possessed _inertia_.
Inertia is that property of a material body that requires time and
energy to set in motion, or stop. Inductance is that property of an
oscillation circuit that makes an electric current take time to start
and time to stop. Because of the inductance, when a current flows
through a circuit it causes the electric energy to be absorbed and
changes a large part of it into magnetic lines of force. Where high
frequency currents surge in a circuit the inductance of it becomes a
powerful factor. The practical unit of inductance is the _henry_ and
it is represented by the symbol _L_.
What Resistance Is.--The resistance of a circuit to high-frequency
currents is different from that for low voltage direct or alternating
currents, as the former do not sink into the conductor to nearly so
great an extent; in fact, they stick practically to the surface of it,
and hence their flow is opposed to a very much greater extent. The
resistance of a circuit to high frequency currents is generally found
in the spark gap, arc gap, or the space between the electrodes of a
vacuum tube. The unit of resistance is, as stated, the _ohm_, and its
symbol is _R_.
The Effect of Capacitance, Inductance and Resistance on Electric
Oscillations.--If an oscillation circuit in which high frequency
currents surge has a large resistance, it will so oppose the flow of
the currents that they will be damped out and reach zero gradually, as
shown at A in Fig. 33. But if the resistance of the circuit is small,
and in wireless circuits it is usually so small as to be negligible,
the currents will oscillate, until their energy is damped out by
radiation and other losses, as shown at B.
[Illustration: Fig. 33.--The Effect of Resistance on the Discharge of
an Electric Current.]
As the capacitance and the inductance of the circuit, which may be
made of any value, that is amount, you wish, determines the _time
period_, that is, the length of time for a current to make one
complete oscillation, it must be clear that by varying the values of
the condenser and the inductance coil you can make the high frequency
current oscillate as fast or as slow as you wish within certain
limits. Where the electric oscillations that are set up are very fast,
the waves sent out by the aerial will be short, and, conversely, where
the oscillations are slow the waves emitted will be long.