Washington

123

American Museum of Radio and Electricity, Bellingham, WA

48° 45′ 5.25″ N, 122° 28′ 49.39″ W

CQD

Tune your car radio to KMRE-LP 102.3 FM as you drive into Bellingham, Washington, and you might think you’ve passed through a time warp—KMRE-LP plays recordings from the media collection of the American Museum of Radio and Electricity. You’ll find yourself listening to radio programs from the 1920s to the 1950s.

Far from being a pile of dusty old equipment only of interest to true radio enthusiasts, the museum is a small and delightfully presented collection of radio equipment that tells the story of radio from early experiments with electricity through the Golden Age of Radio, before television displaced the radio as the focal point of most living rooms (see Figure 123-1). To set the mood for visitors, the museum has a mockup of an 18th-century laboratory (as might have been used by Benjamin Franklin) and a full-scale copy of the Titanic radio room, including a Marconi radio set from another White Star Line ship. The last message sent from the Titanic’s radio room was CQD in Morse code—CQ meant that the message was of interest to anyone receiving, and the D indicated distress.

Figure 123-1. Part of the collection: vacuum tubes; courtesy of Christopher Bellevie

There’s also a mockup of a 1930s living room with a working 1936 Zenith radio set. Visitors can listen to classic radio and news broadcasts from the 1930s, including the Lone Ranger and Green Hornet. And there’s a working vintage telephone that visitors can use to call the museum front desk with any questions.

Naturally, the museum has a large exhibition of radio equipment, and it’s worth booking a tour to get a complete understanding of the equipment on display.

The most recent display is a hands-on exhibit explaining static electricity. The Static Electricity Learning Center is an ideal stop for children, and contains spark machines for generating static electricity, Leyden jars (an early type of battery that could be recharged), and electroscopes.

Probably the most fun thing to do at the museum is play the 1929 Theremin. The Theremin was one of the first electronic instruments, and is played without being touched at all. The eerie Theremin sound was made famous by films like Spellbound and The Day the Earth Stood Still.

There’s also a fine surviving example of Giuseppe Zamboni’s 18th-century Perpetual Motion Machine—actually a pendulum that swings between the terminals of a battery. Zamboni built one that ran until well after his death.

If you are serious about radio, there’s too much here to see in a day, but for a quick introduction, the museum’s explanatory posters can be digested in a couple of enjoyable hours.

Practical Information

Information about the American Museum of Radio and Electricity can be found at http://www.amre.us; tours should be booked in advance by calling the museum. The KMRE-LP radio station can be heard over the Internet via SHOUTcast: http://www.shoutcast.com/.

Beat Frequencies and the Theremin

In 1919, Russian inventor Lev Sergeivich Termen (who went by the name Léon Theremin in the West) created the Theremin musical instrument after having worked on proximity sensors. The Theremin has two controls (in the form of antennas), one for pitch and one for volume, neither of which is touched by the musician. But both controls use the hands of the musician as one half of a capacitor in an LC resonant circuit (for more on LC circuits, see page 232).

To generate variable pitch, a Theremin contains a pair of oscillators, generating radio frequency signals. One oscillator creates a fixed frequency; the other is variable. The variable oscillator works by attaching an antenna to an LC circuit. When the musician’s hand approaches the antenna, a small capacitance is created from the antenna to the ground through the musician. This human capacitance changes the capacitance in the LC oscillator, thus changing its frequency. When the musician moves his hand, he varies the capacitance and hence the pitch.

Both oscillators are working with radio frequency, so to get an audible frequency the two signals are heterodyned (which is just a fancy way of saying combined). In a Theremin the two frequencies are subtracted to create a beat frequency. The beat frequency is equal to the difference between the two frequencies. By choosing the fixed frequency and the range of the variable frequencies, the Theremin is able to produce a wide range of sounds from small changes in capacitance.

Figure 123-2 shows two sine waves, as might be produced by the two Theremin oscillators, interfering with each other to produce a new wave with a different frequency.

Figure 123-2. Two waves combine to create a beat frequency

The Theremin uses a similar process to control volume.

Heterodyning radio signals is very common—household AM and FM radios use the same principle. When you tune into KMRE-LP 102.3 FM, the radio superimposes a signal at 113 MHz to translate the received signal to 10.7 MHz. The rest of the radio works with the 10.7 MHz signal to produce the sound heard from the loudspeakers.

This has the advantage that almost the entire radio is designed just to work with 10.7 MHz signals; only the first piece of tuning apparatus, called the front end, needs to tune across the FM dial.