[This article appeared in the January 1970 issue of Popular Electronics.]

An Experiment With Gravity
Chart These Strange Forces With Your Receiver

By Cdr. Thomas Appleby

We are all familiar with the natural phenomenon known as gravity; but most of us tend to think of gravity on the surface of the earth as being constant. In fact, it is always changing in magnitude, due mainly to the forces exerted on the earth by the sun and the moon. The variations are, of course, so minute that only in the past few years have they been detected by specially designed, highly sensitive instruments. Oddly enough, my years of research into the phenomenon have shown that the average ham radio CW receiver can apparently "detect" changes in gravity.

The effects of gravity on a receiver might account for its drifting off frequency. Even after communications receivers have had time to become thoroughly temperate stabilized, frequency drifting and periodic returning are common occurences.

Taking advantage of the effects the forces of the sun and the moon have on the earth's gravity, you can experiment on your own. All you need is a receiver with an ultra-fine scale on its tuning dial. (One that has 10 divisions for each minor division on the main tuning dial scale.) Remember that gravity variations are on the order of only 10-6 part of the weight of the mass in which they are produced. Although the ffect of the variations is greatly amplified by your receiver, the end result is still minute.

To perform the experiment, disconnect the antenna and any other leads that might pick up a signal at either 3500 or 7000 kHz. In the morning, set the tuning dial of your CW receiver to either of the above frequencies and adjust the BFO for zero beat.

Allow the receiver to warm up for several hours. Then reset the BFO for zero beat. Every half hour or so after this, see if it is necessary to retune for zero beat. Record the new dial setting and make up a graph similar to that shown here. The frequency changes you record will be very small so use an expanded scale.

The recorded frequency variations will increase or decrease, depending on whether the magnitude of gravity is increasing or decreasing, respectively. You will notice that after the sun or moon passes the zenith, the curve will begin to bend downward. Also, the curve will change from day to day because of variations in the orbits of the sun and moon.

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Updated June 26, 2004