A description of easily-constructed apparatus that will tune in different radio stations by the addition of weights on a balance.
"Randall selected a weight, placed it on the scale ... and the signals were heard coming from the speaker."
“Weighing In” Radio Stations
By A. Benneweg, Jr., Radio News, April 1928
The Marconi of our college was Philip Randall; he was a young man, not over 25 years of age, who had recently received his E.E. degree. He had been promised the first vacant assistant-professorship in the electrical department and, while waiting for this opening, he had accepted the position of laboratory assistant. He was kept very busy with his official duties during the day but, frequently after dinner, he would retire to a corner of the laboratory and experiment with new radio apparatus of his own invention. Often a light would be noticed in the laboratory during the early hours of the morning and everyone knew that Randall was occupied in perfecting some new radio device.
From our brief description of Philip Randall, we do not wish you to gain the impression that he was an eccentric; for he was not. Randall was well-liked among the students of the college and was popular also with the fair sex. On the nights when special dances were held he was always to be found in the crowd, and he was present at all the athletic events of importance. Participation in the rehearsals of the glee club was another college activity which he enjoyed greatly. However, he had always saved at least three or four evenings each week for experimenting in the laboratory, and during the course of a year he would develop many very interesting receivers.
The Plot Thickens
It so happened that, for over a month prior to the Easter vacation, Randall appeared to be very busy with his radio experiments, and most of his associated accused him of being unduly secretive. It was noticed that he had not been present at three consecutive meeting of the glee club and that he had left the Saturday dance before 11 o'clock. In addition to these suspicious circumstances, a light was seen burning in the laboratory every evening.
Usually, when busy with his experiments in the laboratory, Randall would welcome the company of students, who were interested in his work, as often they would be able to assist him and check his results. During the period we are describing, he acted peculiarly, in this respect also; when anyone would enter the laboratory he would quickly cover up his work and plainly show he did not desire company. When questioned about his work he would avoid giving any details, but promised to hive a demonstration of his invention when it was complete.
A few days before the Easter vacation a complete change came over Randall. He seemed anxious to join everyone in having a good time and was glad to be included in all social activities. He accepted several invitations to parties and surprised all by inviting a number of students to a party at his home the coming week.
Everyone was in the best of spirits on the evening of Randall's party. Six men who were particularly interested in radio had been invited and they were accompanied by an equal number of girls from the city itself. In the early part of the evening they played bridge; later the tables were removed and they danced to tunes of an electric phonograph; and around 10:30 P.M., refreshments were served. After all had finished eating it was suggested that radio music would finish up the evening in great style. This, it seemed, was exactly what Randall had been waiting for; as he stood up in preparation for making a speech.
A Remarkable Discovery
"Ladies and Gentlemen," he began, "for the past several weeks I have been spending my spare time on a new invention which may revolutionize the entire radio industry, and tonight I will demonstrate to you a remarkable receiver which exemplifies my new ideas. In my experimenting I have found that there is a distinct relationship between wavelength and weight, and, with the data which have been discovered after hundreds of tests, I have been able to develop an entirely new system of tuning. With this receiver a different weight is used for tuning each station. When this method is used the owner does not have to learn the technique of tuning, but merely has to select a weight which is marked with the call letters of the station he wishes to hear. This weight will automatically tune in the program. Now, gentlemen, if you will assist me, I will remove the set from the closet."
When the closet door was opened all persons in the room could see the apparatus which was to be used in the demonstration. There was a large table and over it a cover which extended almost to the floor. On one end of the table there was a small radio receiver and a cone-type loudspeaker. On the other end of the table were a pair of scales and a number of weights, which were labeled with the call letters of the popular nearby broadcast stations.
"Now, gentlemen," continued Randall after the table had been moved to the center of the room, "if you suggest the call letters of a station you would like to hear, I will endeavor to show how my set operates." Someone called out "WJZ" and Randall selected a weight, place it on the scales, and turned on the filament switch. Much to the surprise of everyone, the signal from WJZ was heard coming from the speaker.
Jack Dallas was the most skeptical member of the party, and he immediately said that he considered the demonstration a fake. To prove his point he placed the WEAF weight on the scale and to his surprise the WJZ signal disappeared, but WEAF was not heard. Randall immediately explained that he should remove the WJZ weight and then WEAF could be heard. This was tried and found to be true. All persons in the room then became interested and, for the next hour, we tried placing the various weights on he scales; and, in each case, the loud speaker brought in the signal of the station which was called for.
The Secret Exposed!
When Randall was asked for an explanation of his new receiver, he lifted the table cover and disclosed a large coil spring which had been concealed beneath the table.
This sketch illustrated the manner in which the balance is attached to the spring coil.
"It is very simple," he explained. "The coil spring which you see under this table acts as a loop aerial for thee set on the table and, at the same time, it serves as the grid coil for the first R.F. amplifier tube of the set. You will notice that one end of the coil is attached to one side of the scales in such a way that, when weights are placed on the other side of the scales, they will cause the coil to expand in length. Probably you know that the inductance of a coil decreases as the separation between the turns in increased; and this principle is used in tuning the set. A heavy weight placed in the scales will greatly increase the length of the coil and, as a result, tune the set to a very low wavelength; whereas a much lighter weight is used for the high-wave station."
The schematic diagram of a receiver which employes for a loop antenna a spiral spring which is tuned by its stretching under the strain of different weights.
"Well, Randall," explained Jack Dallas, "You set provides a fine after-dinner trick, even if it isn't much value to the radio industry. Give me the full details sometime so I can fix up a set to fool the folks back home."
"Sure, I'll be glad to," Randall replied: "This arrangement is the easiest thing in the world to fix up, but it takes considerable time to make the weights of the correct size. The first thing you need is a loop-operated radio receiver, which may be tuned with a single variable condenser. IN most cases the set must have three stages of untuned radio-frequency amplification, as other systems usually require a more complicated tuning arrangement.The size of the spring coil isn't very important; as the set can be adjusted to the desired wavelength with the variable condenser and then the weights may be used to select the various stations. Twenty-five turns of aerial wire wound on a hat box are used, and they make a very satisfactory loop antenna."
By adding show to a hollow weight and lpugging it with a cork, the tension can be determined at which stations are tuned in.
"There are a number of systems which may be followed in making the weights, and none is very difficult. I used some weights which I found in the laboratory and filled them with a sufficient quantity of shop to tune in the desired station. However, pill boxes, which you can buy at any drug store, would be just as satisfactory and might make the trick even more mysterious; as they are all the same size. It is possible to add to the trick by placing a weight in both pans of the scales. Of course, the weight in one of the pans would be the same for all stations, and the weight in the other pan must be changed each time."
"Now that me know what it is, and how it works, tell us how you thought of the idea, and why," someone asked.
"Well, I'll tell you," Randall answered: "Last week, while experimenting with my new invention I dropped my fountain pen and its weight on some wires changed the inductance of the circuit enough to tune in a new station. That gave me the idea and I could not resist the temptation to have some pleasure at the expense of a few curious persons who have tried to find out what I am doing in the Lab."