November 1935 Short Wave Craft
Wax nostalgic about and learn from the history of early electronics. See articles
from Short Wave Craft,
published 1930 - 1936. All copyrights hereby acknowledged.
As mentioned many times in the past, some things never change regarding
the basics of electricity and electronics. Resistance, inductance,
and capacitance are examples. When first starting out in this science,
an effective introduction to the fundamentals can often determine
whether a person sticks with it or finds another area of interest
to pursue as a hobby and/or vocation. Analogous examples of voltage
and water pressure, resistance and the diameter of a water hose,
inertia in a spinning mass opposing a change in rate and an inductor
opposing a change in current, etc., are presented along with some
good sketches of the principles.
See Part 1,
Part 2, and
Radio Amateur Course
No. 3 - Resistance, Inductance, and Capacity
This is the third lesson in our Amateur Radio Course and it will
deal with resistance, capacity, and inductance, as concerned with
radio circuits. In order to understand how a vacuum tube oscillates,
how tuned circuits work, and the function of a tuning condenser,
it is necessary to become familiar with these three very important
When electrical current flows through a wire or some other conducting
medium it encounters resistance or opposition, the same as the flow
of material substances. For instance, a certain amount of water
can be forced through a length of one-inch pipe with a definite
pressure. In other words, the size of the pipe offers resistance
to the flow. The larger the pipe becomes, the greater the amount
of water can be forced through it at a definite pressure, or the
larger the pipe becomes, the less its resistance would be.
This holds true in conductance of electricity inasmuch as a fine
wire or conductor offers a greater amount of resistance than a heavy
conductor. The resistance of a conductor is inversely proportional
to its cross sectional area and with some materials, in fact most
of them, the resistance also increases as the temperature rises.
In Fig. 1 we have a diagram showing how the size
of a pipe governs the amount of water that can be forced through
it. In Fig. 2 we have resistors connected in series; in Fig. 3 they
are connected in parallel; the formulae are given in the text. In
Fig. 4 is the hydraulic analogy for the action of a condenser when
alternating current is applied to it. Fig. 5 shows the magnetic
fields and direction of current flow in straight wires and coils;
also the right-hand rule is given, where, if the thumb points in
the direction of the current flow, the four fingers will curve around
the conductor in the direction of the magnetic field.
In dealing with resistance in electrical circuits, we have what
is known as Ohm's Law. In Ohm's Law, we have to consider three things:
First, the flow of electricity, which is current; second, the force
or pressure, which is voltage; and third, the resistance which the
flow of electricity encounters. Three letters are assigned to the
above, and they are:
I = Current
E = Voltage (EMF)
R = Resistance
The formulas for finding the resistance, voltage, or current,
where either two of the three are known are as follows:
I = E/R
R = E/I
E = R x I
When two or more resistors are connected in series the total
value of the resistance is the sum of all the resistors. In other
words, three 5-ohm resistors in series would have a total resistance
of 15 ohms.
However, when resistors are connected in parallel the method
of calculation is a bit more complicated. For instance, if we have
three resistors connected in parallel, one has the resistance of
5 ohms, another of 10, and another of 20. The formula for expressing
Most of us are familiar with the now well-known condenser which
is an instrument capable of storing up a certain amount of electricity
and consists of two or more plates placed adjacent to each other,
with insulation of
either air or some other insulating medium. When a constant direct
voltage is applied between the plates of a condenser, current will
flow into the condenser, until the condenser becomes charged to
its maximum capacity. The current then ceases to flow and the condenser
is charged. Then, after the source of electricity (battery for example)
is removed from the circuit, the condenser will hold its charge
until, due to its inherent (conductivity of dielectric) resistance,
the power is dissipated.
Fig. 6 above shows measurement of resistance.
Fig. 7 shows how pressure or voltage decreases with increase in
resistance to flow of water or electric current. Fig. 8 shows action
of expanding and contracting magnetic fields. Fig. 9 shows mechanical
"spring" analogy for inductance; Fig. 10 - Fly-wheel analogy of
inductance. Fig. 11 - Analogies for condenser.
If the insulation is mica or parafined (waxed) paper, the condenser
will hold its charge for a considerable length of time. In large
condensers of one or two microfarads the charge may remain in the
condenser for several hours. This can be proved by short-circuiting
the two terminals of the condenser and noting the spark, or an ammeter
could be connected across the condenser and it would indicate the
current flowing until the condenser was completely discharged and
the power dissipated. The unit of capacity is a farad; however,
in radio work, we use considerably smaller units in our condensers.
A microfarad is one millionth of a farad, and one micro-micro-farad
is one millionth of a microfarad. The most important part of a condenser
is the dielectric or insulating material because, contrary to popular
belief, it is in the dielectric that the charge resides. When a
condenser is charged, the dielectric opposes the setting up of an
electric displacement of an electric field in the dielectric and
the charge is said to be the energy of the charging source stored
up as electro-static energy in the dielectric.
A simple analogy for the action of an electrical condenser is
a sponge, which absorbs water when placed in a cupful of it, for
example, and afterwards if pressure is exerted on the sponge, then
it gives up the water stored in it. It requires 1 coulomb (ampere-second)
to charge a condenser of 1 farad to a potential of 1 volt. A condenser
having a capacity of 1 mf. (1 mf. = 1 millionth of 1 farad) requires
a charge of 1 millionth of 1 coulomb to charge it to a potential
of 1 volt.
The coils used in radio circuits are called inductances or inductors.
In the drawings we see how an electro-magnetic field may be produced
around the wire when a current is passed through it. If the flow
of current through a conductor is constant (D.C.) a steady electro-magnetic
field is produced around the conductor. However, when alternating
current (abbreviated A.C.) flows through a conductor, the current
flow is constantly changing and likewise the field is changing.
When current begins to flow through a wire the circular electro-magnetic
field originates at the center of the conductor and travels outwardly
away from this center in constantly increasing diameters and of
course, extends into the space surrounding the wire. Until this
field becomes of larger diameter than the wire, it causes a second
current to flow in opposition to the main current.
When the current flow through the wire decreases or stops, the
circular fields collapse and are then said to cut the wire in ever-diminishing
diameters. This induces a current in the opposite direction to the
field but in the same direction as the original (exciting) applied
current, tending to prolong the flow of the exciting current.
This property of a coil or conductor to act upon itself or another
inductor in close proximity to it, is called inductance. The unit
of inductance is the henry and in most formulas it is usually designated
by the symbol "L." A henry is the inductance of a circuit in which
the induced E.M.F. is one volt; when the (varying) current travels
at the rate of one ampere in one second. Usually in radio circuits,
inductance values are indicated as one thousandth of a henry or
- one milli-henry; a millionth of a henry is known as a micro-henry.
The physical dimensions and form of a circuit, determine the amount
of inductance and it is for this reason that our radio circuits
consist of coils rather than straight wire, because a greater amount
of inductance can be obtained by coiling the wire, also allowing
considerably less D.C. resistance because less wire is used. A straight
wire, of course, would have less inductance than one of the same
length which was coiled.
Induction subdivides into two branches - self and mutual induction.
If the current passing through a coil, for example, is rising from
zero to maximum value, such as when the circuit is closed from a
battery, (or the first half of an alternation of an alternating
current) the magnetic field around the wire is expanding and while
this is taking place there is induced in the conductor a counter-current
(and counter e.m.f. or voltage) which tends to buck or oppose the
current (and voltage) which is producing the field.
As one of the diagrams shows there is electrical energy stored
up in an inductive circuit, just as if you had compressed a spring.
The opening of the circuit, and spark at the switch, corresponds
to releasing the compressed spring and heaving off the weight.
Another analogy is the flywheel. The inertia of the wheel opposes
any force to set it in motion; once in motion, the energy tied up
in the wheel tends to keep it going, if any effort is made to stop
Let us consider for a moment now the next phase of the action
taking place when the circuit is opened or when the second half
of the alternation of an applied A.C. is taking place. Now the magnetic
field around the wire or turns of wire comprising the coil is contracting
and while this occurs, the lines of magnetic force are cutting the
wire in the opposite direction and a current of opposite sign is
induced in the wire, this current being in the same direction as
the applied (exciting) current which is flowing around the wire
and creating the magnetic field.
In other words, the self-induced e.m.f. is in the opposite direction,
while the field is expanding about the wire, and tends to oppose
it while the opposite is the case when the field is contracting
and the current is then in the same direction or aids the inducing
current and acts to prevent its decay.
It will be apparent, of course, that while the current is varying
in strength or let us say increasing, the field about the coil is
expanding, and the lines of magnetic force expanding out from the
coil composed of a number of turns, will induce a current by induction
in a second coil, placed near or adjacent to the first or exciting
If we term the exciting coil No. 1, and the adjacent unconnected
coil as No. 2, coil 2 is said to have a current induced in it by
electro-magnetic induction. As the magnetic field in coil No. 1
subsides, the magnetic lines of force surrounding coil No.2 also
subsides. At the same time these lines of force cut across the turns
in coil No. 1 and induce therein an e.m.f. or voltage (also a current)
and thus we have a third e.m.f. set up by induction.
To begin with, we have the original exciting e.m.f. in coil 1;
secondly we find an induced e.m.f. in coil 2; and thirdly, there
is a reinduced e.m.f. in coil 1, due to the reaction of the magnetic
field surrounding coil 2, and this effect is what is known as mutual
The usual radio tuned circuit consists of a coil and a condenser,
namely: inductance and capacity. Coils or inductances have what
is known as inductive reactance, while condensers have capacity
reactance. When the capacity reactance minus the inductive reactance
equals zero, at some certain frequency, the circuit is said to be
When the condition known as Resonance has been established in
any given circuit whether a series or parallel type circuit: then
we know that the inductive and capacitive reactance are equal, and
that they balance each other. When this condition has been achieved
their reactive effect upon the circuit is zero. Under these conditions,
or when the circuit has been made resonant, (by the proper adjustment
of the capacity and the inductance of the circuit) any current flowing
in the circuit due to an applied e.m.f. will be that due simply
to the ohmic or direct current resistance in the circuit. Expressed
another way, the current passing through such a resonant circuit
will be given by the expression: I = E/R.
The difference between the capacitive and inductive reactance
of a circuit at some frequency is called the impedance. However,
at resonance, this is always zero, and the losses in the circuit
are due only to the usual D.C. resistance of the circuit, through
which the currents are flowing.
In Fig. 4 we see a hydraulic analogy of current flowing into
a condenser. When the piston is moved forward, the elastic partition
will bend or become curved but will not allow the liquid to be transferred
from one side to the other.
In the fourth lesson of our Amateur Radio Course, which will
appear in the following issue of Short Wave Craft, the action and
principles involved in Regeneration and Oscillating Vacuum Tube
circuits will be discussed. Don't miss the next installment.
Posted June 21, 2015