December 1934 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
In the 1930s, electricity and
electronics were mysteries to most of the population. The concepts were relatively
new and few had a firm grasp on the technology. That reality was exploited by Hugo
Gernsback during the 1934 Electrical Exposition to challenge attendees to discover
how "the Mystery Set" radio receiver sitting on the top of an empty, clear glass case was being
powered. It was a clever ruse that reportedly stumped most people. The secret is
revealed here in this 1934 issue of his Radio−Craft magazine. BTW, my guess
is that an even smaller proportion of our current citizens would be able to figure
it out, or for that matter even realize that maybe there should be a power source
of some sort.
The Mystery Set
Would you like to build a radio novelty that will help you mystify your friends
and neighbors? This intriguing device caught the eyes of many visitors at the recent
radio exposition in New York City. It will make an interesting and arresting window
display for dealers or Service Men.
No apparent light socket connection, yet the set, mounted on a plate glass box,
lights and plays!
Read how - below.
At the recent national Electrical Exposition which was held in New York, September
19th to the 29th, Radio-Craft and associated Gernsback Publications had a large
booth where not only magazines but radio sets constructed by Radio-Craft, etc.,
were exhibited. In order to arouse interest for the 270,000 odd people who attended
the show, a number of novel devices were exhibited, and among these was a Mystery
Set designed by the writer, which created no end of discussion, and which had many
radio people and radio engineers guessing for quite a while.
The set, as shown in the illustration, comprises a grooved wooden base (about
an inch thick), in which grooves are mounted, edgewise, four pieces of plate glass;
these in turn fit into grooves on the underside of a second (three-quarter-inch
thick), piece of wood, which constitutes a top or cover. Inside the glass box thus
created there is a sign, "The Mystery Set - How Does It Work?" Placed on top of
the cover is the open chassis of a 4 tube, stock model Emerson radio receiver. An
aerial about a foot high is also provided, and to make the display a bit more humorous
a flower pot containing actual earth is cemented in position on top of the audio
transformer. The ground wire is soldered to a nail, which is driven into the "ground."
A sign proclaims the fact that the flower pot is the "Ground."
The set was placed so that it could be conveniently handled by individuals in
the crowd. You could switch the set "off" and "on," whereupon the pilot light would
light and you could tune to your heart's content and also get the various stations.
The power cord and plug were in plain sight, hanging loose at the left side. Everything
was open and above board. You could walk around the set, look through the glass,
in fact nothing was hidden.
How then could the set work? Plainly, it could not work by induction because
you cannot work a radio set in this manner. The closest scrutiny of the set did
not reveal any hidden wires. As no cement was used in joining together the panes
of plate glass all corners were visible, and revealed - nothing! How is the mystery
The answer is: Ohm's law. Not so long ago, I did a little calculating, and found
out that two No. 36 or even No. 38 copper wires connected in parallel, (or four
wires, total,) were sufficient to carry the full load of a small radio set, such
as the one shown, which only consumes approximately 40 W. The problem, therefore,
resolved itself into hiding the wires in such a way that the closest examination
would not reveal them. Since there are four corners to the cabinet, this was easily
accomplished as follows:
The 110 V.A.C. power supply was split up into its two component lines. Then,
green silk covered wire of No. 36 B. & S. gauge was fastened, by means of transparent
cellulose cement, against the inside edge of each of the quarter-inch thick plate
glass panes. Now it should be remembered that the edge of plate glass is green,
and green silk wire against green glass is quite invisible, particularly when the
wire is as fine as the one that was used. Unless you had a pretty good magnifying
glass you could not possibly see the wire. To make the job perfect, the wire was
run very close to the edge, and for this reason the wire was mistaken, by even the
keenest observer, as the real edge of the glass. As no cement was used to cement
the four pieces of plate glass together, there was actually an open space at each
of the four corners, through which you could slide a card or piece of paper. Yet,
with all this, the wire cemented on the inside edge of the plate glass remained
The four hair-fine wires were then run through needle-like holes and sealed info
shallow (1/32- inch deep) grooves in the underside of the lower wooden board. The
four grooves converged to the center, where a standard plug receptacle was recessed
into the baseboard. Two of the No. 36 wires were soldered to one terminal, and the
remaining two leads to the other. Thus, the set could be plugged in from underneath
the table (through which a hole was drilled) and the connecting wires were thus
concealed. Needless to say, the A.C. connecting cord (and plug) which hangs at the
left was disconnected from the chassis so that the plug would not be "alive" - it
then was nothing but a dummy.
Now, as to the radio signal pick-up, the set in question is rather sensitive,
and actually does pick up on the small one-foot aerial. In order to get more wire
into the limited space available for an aerial, the wooden uprights (which are about
a foot high) were wound with black wire which was then connected to the respective
ends of the aerial, one black wire serving as part of the lead-in. This gave a small
but efficient, concentrated antenna which was sufficient to pick up the programs
of a few of the more powerful stations. (In locations where the pick-up is not so
good, a superheterodyne five or six tube set would, of course, be preferable.)
This Mystery Set can be easily built, by Service Men and others. It can be exhibited
in windows or counters, and will create no end of discussion by the average onlooker
or customer who will not understand as to how the set functions. The design is so
simple that the cost of the entire device, outside of the inexpensive radio set,
comes to less than $4.00.
A few important constructional details should be noted. The grooves in the
top of the wooden base and the underside of the wooden top must be deep enough -
at least 1/2-inch in the upper, and 5/8-inch in the bottom board - to securely
hold the plate glass. After the set has been completely assembled cement can be
placed in the bottom and top grooves. The wooden top and baseboard will thus
hold the plate glass firmly - so firm in fact, that you can pick up the entire
set by grasping the upper board, although I do not advise that this be done as a
routine thing. The method of running and connecting the top ends of the fine,
green wires is as follows: exactly at the top edges of the plate glass panels
where the green wires run off, drill a hair-like hole, for each lead, at an
angle so that the fine wire will come through the top of the top board inside
the boundary of the set chassis (out of sight). The same two fine wires
connected to one socket terminal under the baseboard are also connected together
underneath the chassis and to one of the two power connections from which the
power cord was previously soldered. The remaining two fine wires solder to the
second power connection underneath the chassis.
You will have a lot of fun in building this set and mystifying the local radio
Posted September 29, 2022
(updated from original post on 9/16/2014)