July 1948 Radio News
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio &
Television News, published 1919 - 1959. All copyrights hereby acknowledged.
Benjamin Franklin is famous for his kite-flying experiment whereby
he 'discovered' not electricity (as many
people believe), but that lightning is a form of electricity
(most people thought it was a jet of
gas). A lesser known fact about Mr. Franklin is
that he invented the lightning rod after realizing the electrical
nature of lightning. His understanding of electric fields facilitated
a implementation whereby hefty iron cabling interconnected a
tall, pointed rod installed at the tallest point on a building
and a spike driven into the ground. Lightning typically strikes
the object that is the shortest distance
(in terms of electrical field strength)
from it because the discharge can begin at the lowest voltage.
The presence of the grounded lightning rod above the highest
point on a structure effectively brings that point all the way
down to ground level, rendering it no longer as electrically
tall, thereby reducing its likelihood of being struck by lightning.
Contrary to common belief, the purpose of a lightning rod is
NOT to attract lightning and provide a discharge path
for the bolt. Benjamin (he insists I call him 'Ben') was a hero
in Philadelphia where his invention almost totally stopped the
rash of fires that occurred all around the city whenever a major
lightning storm occurred. Word quickly spread and soon thereafter
cities all over the world were being spared similar catastrophes
after installing their own lightning rods. Chalk another one
up for American ingenuity (which might
be politically incorrect to point out these days).
Mac's Radio Service Shop: A Little Lightning
By John T. Frye
The static crashes in the receivers Mac and his red-headed
assistant, Barney, were working on had been growing steadily
louder and closer together; and when there came a sudden sharp
peal of thunder, Mac laid aside his test prods.
"Let's take a break, Red," he suggested as he started closing
the switches that grounded the AM antenna, the FM dipole, and
the TV three-element beam; "that is, if you think you can tear
yourself away from your work."
"W-e-l-l-l, boss, if you insist-" Red said with a very phoney-sounding
Mac tripped the circuit-breaker that opened both power leads
going to the receiver outlets and the test equipment and settled
himself comfortably on the end of the bench. Just then there
was a brilliant flash of light accompanied by the vicious "sn-a-a-a-p!"
of a near stroke of lightning, and Miss Perkins, the office
force of Mac's Radio Service Shop, sailed through the door of
the service department with rather unlady-like haste.
"I-I just happened to think you boys might like some of these
chocolates I picked up during lunch hour," she said holding
out the open box in hands that were noticeably shaking.
Barney hopped off the stool on which he had been sitting
and offered it to her with a sweeping bow.
"You feed us candy, and we will protect you from the thunder,"
he promised as he stretched his lanky frame on the floor and
began to open and close his mouth suggestively like a three-day-old
"I don't think we need expect any customers for a few minutes,"
Miss Perkins said as she looked through the door at the rain
slashing down across the plate glass windows of the shop; "and
as for you, Mr. Smarty," she explained as she seated herself
primly on the stool and dropped a chocolate into Barney's gaping
jaws, "I am not afraid of lightning; I just don't like it."
"Anyway, you had better save your strength for answering
the 'phone when this is over," Mac told her as he helped himself
liberally to the candy.
"How's that?" Barney asked,
"The lightning," Miss Perkins explained. "It always knocks
out several sets.
"How, Mac?" Barney inquired. Before answering, Mac lazily
sketched with his left hand - it was the one with no candy in
it - the diagram of Fig. 3 on the little wall blackboard at
the end of the bench.
"There you have the two circuits by which lightning can get
into a radio - excepting, of course, the remote possibility
of a direct stroke, which is a fate that I wouldn't even wish
on a three-tube midget. You will recognize 'A' as the antenna
circuit; 'B,' the 110 volt input circuit."
Fig. 1 - General Electric lightning arrester
for use with FM antennas.
"Let's talk about 'A' first. If lightning strikes near the
antenna, it induces a voltage in the antenna wire which flows
down through the lead-in, the antenna coil, the chassis, and
the ground lead to the earth. If the current is high enough,
it will burn out the antenna coil; if the voltage is sufficient,
it will jump to other parts of the receiver and may damage them."
"How much voltage and current does a stroke of lightning
have?" Barney wanted to know.
"That varies, of course, with different strokes. It is estimated,
though, that the voltage is around 100,000 volts per foot. That
means that if the discharge leaps from a mile-high cloud to
the earth, the voltage is equal to 5280 times 100,000, or more
than five hundred million volts. The current varies from a few
thousand amperes to a few hundred thousand amperes."
Barney whistled. "If lightning strikes the antenna, then,
about all you can do is pick up the pieces - if there are any
Mac nodded agreement. "Yes, the things to protect against
are static charges and induced voltages. Even these are pretty
hefty. It has been figured that the voltage induced in a clothesline
six and a half feet off the ground by a stroke of lightning
three miles away is often around 30,000 volts, and this voltage
would be increased by 4600 volts for every foot you raised the
line. In an antenna with considerable capacity, this induced
current can become high enough to do great damage."
"What is the best protection against these induced voltages?"
"Grounding switches would probably be best if they were always
closed during electrical storms, but people either forget or
are not around when the storm comes up; so lightning arresters
are the best bet. You have probably noticed that I keep the
shop key in that slot so I cannot get it out until the antenna
grounding switches are all closed. I forget, too!"
Barney waited until a window-rattling growl of thunder had
subsided before asking, "How are lightning arresters made?"
"There are dozens of different types. Resistor, horn gap,
carbon pile, electrolytic, coated pellet, oxide disc, neon gas
- these are a few; but the important thing is what they do.
A lightning arrester is simply a device that has a high resistance
to the passage of current until a certain critical voltage is
reached, at which time it develops a low resistance that allows
the high voltage to go to ground. After this is done, it returns
to its former condition. It is placed between the lead-in and
the ground, usually just where the lead-in enters the house."
"Most sets have built-in antennas now, though, don't they?"
Miss Perkins asked.
"Outside of FM and TV sets, yes," Mac said. "But since these
came along, people are once again trying to get their antennas
as high and as in the clear as possible - right where the lightning
likes to have them."
"I'd think a lightning arrester would upset the balance of
a twin-lead lead-in," Barney offered.
"Ordinary ones would, but the engineers are bringing out
new ones that don't." Mac opened a drawer and tossed a couple
of objects to Barney. "That long arrester is made by G.E. and
the round one is RCA. Both are made to clamp on a water-pipe,
and both are especially designed for use with twin-lead lead-ins
from FM and TV antennas. Some antennas of the folded dipole
type are arranged so that grounding the supporting mast also
grounds the antenna itself at a voltage node and so affords
lightning protection without impairing the efficiency of the
Fig. 2 - RCA lightning arrester for use with
FM and television antennas.
"How about the damage done by lightning coming in over the
"That is much more common in my experience than antenna damage,"
Mac said. "What happens is that high voltages are induced into
the power lines, sometimes at a point some distance from the
receiver, and it is piped right into the set by way of the line
cord. Once inside, it may raise all kinds of hob, such as arcing
the line switch, blowing condensers, burning out tubes, breaking
down the insulation of the power transformer - and I once saw
where it had welded the tuning condenser plates into a solid
"Does it usually do all that?" Barney asked in awe.
"Fortunately, no. The most common damages are the blowing
of line-filter condensers, ruining of line switches and associated
tone or volume controls, and burning out of antenna coils."
"Wup! Wait a minute there!" Barney exclaimed. "How is lightning
that comes in over the power line going to hurt the antenna
Mac grinned approvingly at the boy's alertness and pointed
at the diagram on the blackboard. "Remembering that half the
sets in town have the antenna posts attached to the ground because
the owners are too lazy to put up antennas, suppose a voltage
surge comes in on the ungrounded light wire and breaks through
the insulation of C1 to the chassis and proceeds
through the chassis and the antenna coil to the ground. The
surge does not have to do all the work. All it has to do is
break down the insulation of C1, and then the line
current will follow this path and burn the fine wire of the
antenna coil primary to a crisp."
Fig. 3 - If antenna terminal is connected
to an external ground, lightning surge may break down C1,
thus shorting line through the primary of the antenna coil.
"Tell him about the sets that turn themselves on," suggested
Miss Perkins, who was taking more interest in the conversation
now that the thunder had subsided to a distant growling.
"Well," Mac said, "suppose the surge does not puncture C1
but does cause the insulation of C2 to break down.
Then the line current can flow right through the transformer
primary, C2, the chassis, the ground lead, and the
ground back to the grounded side of the pole transformer, effectively
bypassing the receiver switch. People get pretty excited when
their sets turn themselves on during a thunderstorm and they
cannot turn them off.
"What is the best protection against these line surges?"
"The best protection is for the customer to pull the plug
out of the wall socket during thunderstorms, and I warn all
my customers to do that. One thing I have found helpful is to
put in d.p.s.t. switches in place of the usual s.p.s.t. type
used in receivers. By breaking both sides of the line, you increase
the arc path, and you remove the vulnerable line-filter condensers
from the line except when the set is turned on."
He was cut short by the shrilling of the telephone, and Miss
Perkins ran to answer it.
"Mac's Radio Service Shop; good afternoon!"
"You say the receiver started playing without anyone's touching
it, and now you can't turn it off? Well just pull the plug out
of the wall socket and leave it out. Do not try to plug it back
in or you may blow a fuse. We will pick it up in a few minutes."
Barney looked first at Mac's teasing grin and then at Miss
Perkin's satisfied smile.
"How smug can two people look?" he asked in mock disgust.
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no other than John T. Frye,
creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine (which
itself started as simply Radio News), and then changed its name to Mac's Service
Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his eager,
if not somewhat naive, technician assistant. 'Lessons' are taught in story format with dialogs
between Mac and Barney.
Posted July 2, 2015