The phenomenon of resonance has many examples in physics, engineering, and everyday life, and the reed frequency meter is one of them. Surprisingly, such instruments are still in use here and there, although they are certainly considered rarities today. I picked up this specimen, manufactured in 1961, for a few coins at a swap meet.
Invented by Hermann Frahm, the vibration-based reed frequency meter is also called a Frahm frequency meter. On the instrument’s dial, in a window you can see a row of small rectangle; when powered on, one or more of these windows will start to vibrate.
This device was used to check the frequency of 50 Hz AC mains in the voltage range of 220–380 V. Its measurement range is 46–54 Hz with a resolution of 0.5 Hz. It can be connected directly to the 230 V mains and draws 4.85 mA during operation. Since today the grid frequency is very precisely 50 Hz, the reed with the red tip at the “50” mark will always vibrate when plugged into the mains, with its neighboring reeds vibrating slightly as well (see image on the right). The instrument could still be useful for an island-mode power plant, a home hydro turbine, or a diesel generator, where hitting 50 Hz approximately can be considered a good result.
How does it work?
Disassembling the instrument reveals a sturdy frame holding 17 “reeds” (spring plates) of varying lengths behind the small front windows.
Each reed also carries a small weight (a round disc). Each reed has a different natural frequency. Behind the reeds is an electromagnet. One pole piece runs parallel to the reeds with a few millimeters’ air gap, while the other pole connects to the frame holding the reeds. The mains voltage is applied directly to the coil. The alternating current through the coil generates an alternating magnetic field that causes the reeds to vibrate. This magnetic field is weak and can only bend the springs imperceptibly. However, you can hear that something is happening: the instrument hums softly.
The reed whose natural frequency is close to the magnetic field’s frequency (which is the mains frequency) goes into resonance and vibrates vigorously. If the frequency changes, this reed’s oscillation dies down, and another reed—whose natural frequency matches the new frequency—starts vibrating. Each reed has some bandwidth; the frequency does not need to match perfectly. If the frequency lies between two adjacent reeds’ resonance points, both will vibrate, but the one closer to the actual frequency will oscillate with greater amplitude.
Advantages of such frequency meters:
- Inexpensive
- Relatively fast (stabilizes within about 1 second)
- Easy to read (a quick glance shows if the frequency is correct)
- Measurement is independent of waveform shape
- Measurement is independent of voltage level (though sufficient voltage is needed for visible vibration)
Disadvantages:
- Cannot read values between two adjacent reeds; limited resolution
- Accuracy depends on how precisely each reed’s resonance frequency is tuned
Why 50 Hz?
Today, most countries use a mains frequency of 50 Hz, but there are exceptions. For example, in the United States, Canada, Mexico, Cuba, Central America, Brazil, the Bahamas, the Philippines, South Korea, Saudi Arabia, and some other places, the mains frequency is 60 Hz. Interestingly, Japan uses both: west of Nagoya, it’s 60 Hz; east of Tokyo, it’s 50 Hz (a historical reason).
In Europe’s interconnected grid, power plants operate in synchronization, meaning every generator produces electricity at the same frequency and phase. This synchronization is essential for inter-country power transmission and grid stability (ensuring generation equals consumption). Therefore, frequency must be kept very accurate. In practice, grid frequency fluctuates within ±0.05 Hz—too small for our reed meter to detect. A deviation of half a hertz, which this instrument can show, indicates a catastrophic problem! If the frequency drifts from 50 Hz, system operators must intervene immediately to maintain stability.
At the end of the 19th and early 20th century, there was no standardization—quite the opposite, chaos reigned. Many companies supplied electricity, each setting frequencies to suit their generators and prime movers. Frequencies ranged from 16⅔ Hz to 133⅓ Hz. For arc lamps and incandescent lamps, this was not a big issue, but it created problems for electric motors and made interconnecting different systems impossible. Although efforts toward unification began, as late as 1918 London still had electricity supplied at ten different frequencies.
In the U.S., generators typically had 8 poles and ran at 2,000 RPM, producing f = n·p / 120 = 133⅓ Hz. However, Tesla’s induction motors required lower frequencies (20–30 Hz), unlike arc lamps that worked better at 133⅓ Hz. A General Electric study concluded that 40 Hz would be a good compromise between lighting and motor needs. As a result, several 40 Hz systems emerged. For example, in Germany, AEG (the German subsidiary of GE) powered the 1891 Frankfurt International Electrotechnical Exhibition with a 40 Hz system. Observing flicker in lamps, AEG raised its standard to 50 Hz in 1891. With its dominant position in Europe, AEG’s standard prevailed there. At the same time in the U.S., Westinghouse found arc lamps worked slightly better at 60 Hz and adopted it. GE initially tried 50 Hz in 1893 but switched to 60 Hz a year later to stay competitive with Westinghouse.
International standardization began in the mid-1920s and was completed only after World War II. However, some railways (e.g., ÖBB, SBB, Deutsche Bahn) still use 16.7 Hz for traction power, a legacy of the old 16⅔ Hz system. Some North American industrial plants still have 25 Hz networks, a remnant of early Westinghouse equipment. Conversely, to allow higher motor speeds and smaller transformers, some modern facilities and data centers use 200 or 400 Hz systems, previously reserved for aircraft, spacecraft, and submarines. Such high frequencies cannot be transmitted economically over long distances, so these systems are confined to a building or vehicle.
Interestingly, frequency meters are still made today that look like old reed meters but have no moving parts—fully electronic, with LED displays. Well, the old reed frequency meter lives on in a modern form thanks to its easy readability. Still, let’s appreciate this old mechanical piece, because it’s built to last—and who knows, we might need it again someday…