Is that radioactive lume on those tabs?

I haven't seen one of these in operation since the early 70s! Great to see the inside. In either 1973, or 74, in the UK we were having problems with both coal supplies (miners strikes) and oil, so the power grid was having some issues. I was working as an electronics engineer in one of the production studios at the BBC Television Centre in West London. Although we had two separate feeders from the grid, emergency diesel generators for News, and a supply from the nearby Barclays Bank data centre (they had a pair of Olympus gas turbines that could generate 4 MW), it was decided to bring in a couple of big containerised generators to power the central control area, and at least one production studio. My studio was out of service for maintenance on the HVAC (2MW of AC!), so I and a colleague got assigned to look at the generators and liaise. The generators were a big diesel and an alternator in a 40 foot container. The control panel had two of those frequency meters on. One was connected to the alternator output and the other to the feeder to the load. the generator had a governor, the idea was to match the frequency of alternator output to the frequency of the feeder. At the time I didn't think about how they worked, so it was fascinating to see your demo and how accurate the indication is. They work like tuning forks! I would love to see how they are made. Edited for spelling 28/01/21

This kind of "old" tech always amaze me because is fully made using physics properties, nowdays we mostly use DSP even for trivial tasks.

I'm consistently mesmerized by shutter effects. The moment it started, I knew that wasn't what you were seeing live. I'm also blown away at the ingenuity of pre-computerization engineers to think up these devices. Basically an inverse music box!

Hi Fran, just wanted to say that I suffer from extreme anxiety and your videos / voice help calm me down when things are wound up tight. Thank you for doing what you do.

Variable frequency mains is easy - signal gen, audio power amplifier and a mains transformer wired back-to-front

Ahhh, my OCD rested in sublime happiness as you used the properly sized blade screwdriver!

We used to have aircraft ground power units that had one of these on it to indicate that it was providing good 400 Hz power for the aircraft. Everything is all digital now.

👍"Electromechanical artwork." Good description.

Where I work we used to have a ground power unit that used one of these to indicate the output frequency of the 400 hz AC being supplied to the aircraft.

@Franlab I believe that AvE did a review on a vibrational tach that used the same tech. So cool to see it on a line frequency meter. Awesome stuff!

ha, awesome! I love looking at old analogue electronics and trying to brainstorm how they may have designed them to work. Very interesting. You sound quite positive at the moment, compared to roughly a year ago. I hope that continues.

Fran, great channel! These were widely used in applications where power frequency was critical to safe equipment operation - be it from a genset or the mains. The "T" in JBT is none other than RL Triplett, of Triplett Instrument. These were also commonly produced in 400hz versions for military and aerospace applications. Nice find!

I got called on a "low voltage generator" back in the 70's. Those meters will also respond to harmonics. They were running the generator at half speed.

That's astonishing! Oh, and it looks like there's actually a tiny bit of response from the 59Hz indicator.

My late father, who was an electronics technician back in the mid-60s/ early 70's for Hewlett-Packard, would have loved this channel. I think of him when I watch.

My dad had one of those on his boat to help regulate his generator. His only had about five "tuning forks": 59, 59.5, 60, 60.5 and 61 hz. He had a generator run off the shaft of the engine connected by a kind of variable transmission worked by a crank. After leaving the pier, with the engine settled at its cruising revs, a crew member would turn the crank and tune the generator to 60 hz before flipping the transfer switch. Dad had three ways of providing a/c power: "shore power" provided by an extension cord for use when the boat was tied up; an Onan 5kw diesel generator for use when at anchor; and the shaft generator for use when underway. There was a transfer switch to switch the boat's system from one mode to another. This video is the first time I had a chance to see the inside. The resonators weren't "forks" at all!

Luv the classic Ti 30 calculator!!

I have a 800Watt generator from the early 1980’s that has a smaller version of this vibrating reed meter. The gen and meter still work fine.

I love how the oscillation interacts with the camera's frame rate, you can see it cycling through the phase space.

Such a great video. I sincerely appreciate that you did not lard it with a recitation of the difference between AC and DC current, or laboriously work out on screen the current that the meter would draw. The cool little device was given center stage and you let it shine. Thanks!

You may have found a "holy grail" guitar pickup there...call Ry Cooder.

I work in the testing department for an electric utility company. I have equipment that is absolutely perfectly suited to demonstrating the other frequency tines. I'm not a youtuber but if you want to trust a random internet stranger I would love to borrow this thing so I could get you some footage of it reacting to different frequencies.

We still had these in all our mobile communications vans in the military through the 90's. Kind of nice to see them again!

I was so fortunate to work in Telco in the 70's, where our 53.5V 400A PSU's had those - could stare at them and wonder how they worked. Thanks Fran!

Amazing piece of art. I would love to see it working on different frequencies.

I knew how these worked but have never seen one apart. Interesting to see the subleties of making it work well like the shaping and distancing of the pole pieces

Thank you for all your wonderful content Fran . I'm always looking forward to your videos The time flies so fast when I watch them Im always hoping they would last longer.

Another great episode in the panel gear collection! Thanks, Fran. I have always loved really nifty switches, meters, displays, etc. That meter has a pretty odd-ball frequency range. We would expect to see something like 40 to 70 (or 45-65) range To cover the two major global mains frequencies (50Hz and 60Hz) At least your meter is new enough to be labeled in "Hertz" and not the archaic "Cycles per Second" (CPS) The general transition from CPS to Hertz was around 1960 as I was entering high-school and learning electronics in the garage.

You can just use a standard audio amplifier and a step-up transformer to drive the instrument. It will need a watt or two. 😃

Fun fact: that's how our ears work too!

Fran....I've always had a love of vibrating reed freq meters and have 4 or 5 in my stash and one running on the output of the inverter in my solar system....I'm glad there are others who enjoy such old tech stuff! How much fun!

As a pilot, I knew exactly what this was! The very first radio navigation indicators in the early 1930’s had a “tuned reed” indicator that would vibrate one or both of two reeds to indicate whether or not you are left or right of course on the old four course radio range. Commercial pilots preferred the indicator, because the alternative (used by the army) was to constantly listen to an A or N in Morse code over headphones (kind of irritating after a while). I’ve always wanted to see inside one of these type devices. Thanks!

Most of the generators I seen in the army had one. Had a knob that you could adjust the frequency with.

Since it doesn't need much current, could you drive it with an audio amplifier into a step-up transformer (such as a regular wall-wart sort of transformer used in reverse)?

I maintained ground power generators in the Army and we used these to set the RPM of the engines to 60 Hertz. They were mounted on the Control Panel. You need 60 Hertz to keep the clocks and electric motors synchronized. Love watching you Lab.

No dispute that Fran has some of the coolest vintage tech on her lab. I love this thing. Plus anything in bakelite is gonna be cool

55 to 75 Hz. It's just the digits are for even frequencies and soans 56 to 74 Hz.

AvE did a video on something very similar, it was an RPM meter, looked the exact same but square.

I've seen those on some older (60:s,70:s I think) generator sets, they were used to set the engine RPM for correct frequency (50 Hz here in Europe). When I served in the Finnish military in the early 90:s we still used portable (ok, relatively portable, weighed about 100 kilograms and needed two men to carry. Fortunately they had nice folding carrying handles so not too bad unless you had to carry them for a long distance) gasoline generators that had those and the starting procedure included checking and, if necessary, adjusting the frequency before plugging in the load. They also had 2-stroke engines so loud, inefficient and smoky and you had to remember to mix oil in the gasoline. Also saw one on a larger diesel generator (200 KVA or thereabouts) made in the 60:s that was used to run a rock crusher at a company I worked for in the early 2000:s. By the way my father had a TI calculator just like that back in the 70:s, I loved playing with it when I was a child.

Ms. Fran, this is so analogue. I really did not expect this to be so non-intricate. I LOVE you involved musical tones describing this. Love you like a sister Big hugz.

yeah I love these things, I found one on ebay a while back

When I was an intern at the erstwhile Uitenhage municipality electricity department in South Africa during my summer university break in 1997, I saw one of those vibrating reed frequency meters in the electrical distribution control room. The municipality didn't have its own generation, so the meter didn't serve a purpose. An engineer explained to me how it worked, which I found interesting. They also still used an analogue chart recorder for logging the total load of the town onto paper rolls. It looked a bit like an EEG or a seismograph. The tech was already considered obsolete at the time, but it worked. Years later, I'd seen similar frequency meters and chart recorders in the control rooms of decommissioned coal-fired power stations.

I've seen reed type frequency meters similar to this that are used as tachometers on small engines to set idle speed, and adjusting the engine's maximum RPM for the governor. They are completely mechanical, and are held against the engine itself to pick up the vibrations which in turn make the reeds vibrate accordingly. They are quite precise, and also quite expensive.

That is so cool. Imagine tuning this, need to have a gray beard and your tongue at the right angle.

If it were truly old, it would probably say "Cycles" instead of "Hertz" IMO.

These indicators are a delight! The repair shop I worked in had several in the old meters drawer, for 60Hz and for 400Hz power.

10:45 just hook it up to an old gasoline generator and you'll see it move around, they are not typically super frequency stable

I wonder if they made harmonicas, too! Thanks for this :-)

Thanks, Fran! That was such a great piece of old school kit!

I just became one of your Patreons. You're real and "down to earth". I hope next month when I start my channel I can learn something from you.

The early days of MEMS... ;)

I love my wiggle tachometer.

I remember seeing these sold as surplus in the electronics mags in the '70's. They weren't expensive either. Never seen one working. Beautifully simple!

Cool! I found one of these a while back, but it was set for the 400 hz frequency that an airplane's power system uses. I hooked up my signal generator to a small audio amp and a vacuum tube output transformer to power it. It was really interesting to see how each of the reeds reacted to differing frequencies.

PLEASE ADD SUBTITLES SO I CAN WATCH THIS thx

so, a VERY limited spectrum analyzer. :0

There was a similar instrument in the repair shop I worked at in the mid 1960s. This shop serviced electric shavers. It had the same "reeds" for indicators and you would press it against the running shaver and the reeds would oscillate at the "speed" of the cutting heads.

that is so fascinating Fran, never knew they existed, what a neat and functional concept.

Just use a pa amp and a standard 12v transformer backwards to create variable frequency 120v ac

4:07 did...did you say "nook-u-lar"? Maybe I need more coffee.

Thanks Fran. I was in the Army in 1971 and I used a 35 kw generator to power the test equipment van for the Pershing missile. The generator had a simiar gage and was used to trim the frequency to exactly 400 hz by adjusting the govenor. I never knew how it worked. I was guessing it was the vibration of the diesel engine. Now I know how that mysterious gage works. 50 years a mystery and solved.

A piece of art indeed. And pure genius. One of those things that the moment you see it and get it you know you could have thought of that yourself. In a million years probably, or years, months, weeks of trying dead end ideas, but still. Awesome. Brilliant in its ''simplicity'

hi Fran, i would use a transformer and an audio amp :3 then use a function generator to vary the frequency kind regards Lara :D

I’m really loving these latest batch of videos

Fran, just connect a power transformer with a 12V secondary backwards. Connect the 12V winding to the speaker output of an audio amp and the 120V winding on the meter and feed an oscillator into the amp. The meter draws less than a watt so no problem. I have seen many of those meters on standby generators in telephone exchanges I used to work in here in New Zealand. The meters had the brand name Framm, probably from the UK as the telephone switching gear came from there. One telephone exchange I worked in had 3 diesel powered standby gen-sets. When a mains power cut happened we had to get the first one going and set the voltage and frequency (with the tine meter) then connect it online. Next get number two started and set voltage and frequency but it needed to be in phase with the first one before connecting online. We had a pair of light bulbs which pulsed, one on the line and the other on the local alternator and we had to get the timing just right before throwing the (totally exposed) knife switch to connect online. The same procedure to get the third one running. Once online they keep each other perfectly in phase of course. The knife switches had badly scarred and melted bits on the contacts due to techs getting the timing wrong.

I have an ex-military ISO Comms shelter which has a pair of these on the main power input panel. One is set is for 60Hz and the other is for 400Hz. This shelter was built in the mid eighties for the military. It was usually powered by huge gensets and one way to monitor generator issues was by watching the frequency meters. A lower frequency signaled the generator was running slow and would also cause a dip in voltage and the opposite was true if one observed a higher frequency, the voltage old run high as well. The 60Hz side was used for powering the lighting and climate control system while the 400 Hz system was used for powering a special 28VDC power supply for the radios. These were both three phase systems. At 400Hz the transformers needed to provide the low side are far smaller than those needed for 60Hz systems. Also the filter caps are physically smaller since everything runs at the higher frequency. Its just like in modern switching supplies that can provide high output current without large transformers and caps found in the classic linear power supplies.

This has been nice to see in action, ive acquired one myself,inadvertently, and like yours it is of great quality. Thank you!

I love that design as well. It appears that that tang is vibrating in various modes rather than just a single mode.

A bunch of years ago I was a telephone technician and was taking shelter from a violent thunderstorm inside a central office. There was also another tech sheltering there. As my typical luck would have it, lightning struck the transformer that fed the office and all of the lights went out except a couple of emergency lights that run off the office's 48V batteries, and alarms ringing everywhere. The lightning had blasted a lot of the phone lines and we couldn't get dial tone on the office phone, but there was a special direct line to the Central central office Office, so we called in to report that the office had lost power. The supervisor at the control center got kind of excited and wanted us to start up the emergency generator, so the other guy and I went through the plasticized checklist that was wired to the front of the diesel control panel. We opened the air intake, and the baffles, primed the fuel pump, left the diesel glow plugs on for the requisite amount of time, and finally mashed the go button, and the beast started right up. Using the tachometer, we set the engine speed, and then observed that the voltage (with no load on it yet) was right at 120 where it was supposed to be, and having confirmed all of this with the control center supervisor (stupidvisor?) we were instructed to throw the switch to take the office off of the failed commercial power circuit and move it to the generator. The term "switch" doesn't do justice to the thing, this big panel had what resembled a whiffle ball bat, at least in size, mounted in the middle of it, and you unlatched the safety interlock and moved it from one position to the other, rapidly, drawing a small arc and filling the air with the scent of burned contacts. The generator responded to the load by slowing down, which we had been warned about, and we tuned up the vernier dial to get back to 120V, and stood back congratulating ourselves for a job well done. The other guy had to answer a call of nature, and the toilet was in the other end of the building, just off the fan room. He toddled off down the aisle and then I hear all this shouting and here he comes back down the aisle like his butt was on fire, which it almost was. The upshot is that we had missed one adjustment, which was not actually highlighted on the checklist. We had neglected to set the frequency of the generator using the mechanical frequency meter, to 60Hz. As a matter of fact, none of the reeds between 50 and 70 Hz was even twitching, so we had no idea what frequency we were running at. The rectifiers charging the batteries didn't care what frequency they were being fed with, and the office's fluorescent lights did look a little strange, but the fan motors . . . We had burned up the three fan motors in the fan room. Those guys really wanted to be fed with 60Hz, and not getting the right freq. they had merely sat there and vibrated and overheated. Being a volunteer fireman I was able to "control" the fire, which was really just a matter of tripping the breakers feeding the fans, but the big reason that it was up to me was that the other guy had left the building. I did kind of wonder what that funny looking gauge was about.

I remember these reed type freq meters. When I graduated with my engineering degree, I worked in Marconi’s special services decision, where I specialized on military equipment. Their rule of thumb was if it costs less than 50% of a new unit to repair and to bring into calibration, then repair it. I was surprised at how old some of the technology was that I worked on. One of them was this type of meter. Interesting how things were designed before solid state (and yes, I even worked on a lot of tube electronics!).

Very neat stuff, would look good in a nice wood enclosure, cheers.

As an Electrician's Mate in the Navy, we used those on some ship's generators!! Very robust obviously. We used it to adjust the frequency of shipboard generators. I've also seen them since then in some emergency generator installations. That number on the side, xxxx-xx-xxx-xxxx looks like an NSN (National Stock Number). Each grouping breaks down into codes for the DoD stock system. When I worked in Navy Cal. lab, these were a 'go / no-go' cal. If it read 60Hz when hooked up and wasn't obviously damaged, it passes, and otherwise failed. Nice review. :)

I love it! I use VFD drives for many of my projects. Will have to try to find a frequency meter like yours to play with and make a video!

Thank You for another great video ! Saw a few of these years ago during an equipment upgrade at a military base, along with other old school power equipment. That stuff lasts forever 💝

Hi Fran! 30 years ago, I used to see these gauges on old military generators.

Nice find, Fran. They are used on portable petroleum powered generators for power supply in the field. If you can connect it to someone's generator you should see the tines form the same wave pattern that the numerals are in. Make the generator react a bit by turning a device on and off, you will see the peak dance around until the generator stabilizes. As several other commenters have said, generators for aircraft use have those for 400 Hz. Your digits on the side of the meter are actually one stock number xxxx-xx-xxx-xxxx. The two-digit segment indicates country of origin, 00 and 01 being USA made.

The angled plates make it so the magnetic force is stronger at one end and weaker at the other. This is needed because the tines are stiffer at one end (high frequency) and more flexible at the other end (low frequency). This notion should resonate (pardon the pun) with a guitarist. The purpose is so that all the tines move up and down approximately the same distance.

When I did my service in the USAF, we had these installed in generators made in the 50's and 60's. I was always fascinated by them. We were able to adjust the governors on the engines accurately for output frequency.

Never seen one of these before, and it's Genius in it's simplicity. I just saw the first shot of the teardown, and can instantly see how it works. Love seeing stuff like this. ;) Basically a row of mini tuning forks, and the coil affects all of them, but they respond mainly to their own natural resonance.

AvE did a video on a completely mechanical variant - just hold it up close to a spinning thing and it reads the rip-ums from the vibration. Presumably it's sensitive to read even a well-balanced chuck on a mill or lathe. The tines were individually calibrated with beads of solder. I'm guessing Fran realized after posting this video that KZfaq videos tend to be light on any sound below 120Hz?

It is absolutely a piece of electro-mechanical art as you say. I'm always amazed at what engineers could do with electromechanics during that era. Of course nowadays it would be a Chinese digital display and would no doubt show the information potentially more accurately and be crystal time based etc but this is a marvel of engineering. Glad you appreciate it as I and many other do!

Those "tines" are usually referred to as reeds. It's known as a mechanical reed frequency meter. I'm sure they were common in all kinds of power generating applications, but I've seen them mostly in generators where the speed of the engine (governor) had to be adjusted to maintain a 60hz frequency. Super-simple tech, yet accurate and foolproof! I loved seeing you take it apart so we could see the insides. I've never had the opportunity to do that.

I’m jealous Fran!! That’s a real nice meter!! I run a couple of old generators for backup power here in Alaska. Most of the paint has fallen off of mine but they still work.

Amazing..! Many thanks for your time for us. :) (...btw, AGAIN, I LOOOOVEEE the mess in your workbench, LOL..! :)

As someone who uses one of these regularly on an aircraft ground power unit, I've always wondered how it works. Ours has a lot more indicators on it, and our target frequency is 400hz, so they never move anywhere near as much as yours did. Until today, I honestly thought that it had little spinny things inside, and had no idea that they actually oscillate

What a wonderful channel you have Fran.

I have a book printed in the 1930's that described all kinds of 1920's (and older) technology. There already was this kind of instrument described. I also have used it for the usual purpose of synchronizing "my" generator to the power line in the University of Technology lab. There was a little difference in the frequency resolution. For the first, the overall range was much narrower and the vibrating tines were graduated by 0.5 Hz. Moreover, you could see 3 tines vibrating -- one at high amplitude and two neighboring ones at half amplitude at perfect resonance. A little deviation caused tiny difference in the lower and higher neighboring tine amplitudes. So you could interpret around 0.25 Hz variations. However, the generator synchronizing had yet another "trick", rotating pointer meter. It slowed down when the difference between the power line and the generator approached zero. But besides the frequency, you had to tune the phase shift down to zero before hitting the connection switch. If you had a frequency mismatch, naturally a big bang and tripping breakers would result. Yet, the same bad behavior would also happen even at correct frequency, if the phase was not matching. We students were scared to hit the connecting switch, until the teaching assistant showed that the small generator could be synchronized with as much as two Hz difference when done at just before the phase shift meter was coming to perfect match. I also was dealing with the tine frequency indicator in my second summer work at an oil refinery. There was a 25 KW Diesel motor & generator for emergency situations. It had a massive flywheel and a handle. Between the generator and the flywheel there was an electric clutch. It took some 4 to 5 minutes for me to crank the fly wheel to sufficient speed that the clutch could be activated and the flywheel kicked the Diesel motor (and the generator) to speed. The system had to be tested at least once a month.

3:57 - *Click* "Ahh, yeah!" - I love and VERY much relate to that moment & sound of satisfaction, awaiting the moment of discovery!

Still in use where I work! A lot of old tech on our boat :) Interesting video, thanks.

Hey Fran, you got a tube amp(I hear it played all the time, in the laaaaab) you got a cell phone, a frequency generator app is free, disconnect the speaker driver, hook the freq analyzer to the output of the output transformer, hook your phone to the amp input, viola! Variable frequency on the cheap, or use your guitar, notes A1-D#. I use a freq app on my phone to my car amp to hunt down resonant panels, and to find the clipping limits. A valve amp is just a low power var. freq. drive, and loading the output xfmr. to a 16kohm vs. a 8-16ohm load for quick funsies wont harm either device at all.

I noticed the angled plates seem to be in a fixed location, so I was wondering how they tuned each tine to exact frequency when you mentioned that each tine has different mass. Changing the smallest and cheapest piece to tune it makes so much sense that I can't believe I thought they would spend time adjusting parts instead. Really neat piece of tech, something I would never need but I could watch all day, thanks for showing us!

Physically mount it to a speaker driver and it will oscillate mechanically. The thing is, it's actually able to do the equivalent of an FFT! Feed the driver a complex signal that's a mix of frequencies in the instrument range, and it will neatly isolate them. Also, since it uses less than 1 W, you can feed an amplified signal generator output to a suitable step-up transformer. I suspect a bass amp may be up to the task. While shaking it will yield the FFT-like effect, I'm not sure you can electrically drive it with a complex signal due to possible coil electro-mechanical design optimizations to eliminate possible line/load harmonics, though it would certainly be safe to give it a try! As for the finger resonances, my bet would be a depth taper on the brass mounting block, allowing the fingers to be of uniform construction.

These meters were used to set the frequency of standby power generators. This same resonant reed technology was widely used to activate early pocket pagers. The base station would transmit two or three sequential audio tones on a common RF carrier channel. The pager contained a radio receiver and several very precisely trimmed resonant reeds that would act as mechanical audio filters by vibrating when excited by the corresponding transmitted tones. Circuitry within the pager decoded the output of the reed filters and activated an aural alarm, notifying the pager user to call the office. That was it. There was no message to read or listen to, just a beep, hence the term 'beeper'.

Thanks for another great video Fran!

As a kid I got my hands on an old Heathkit RC airplane transmitter and receiver circa 1965. It worked the same way as this device except there were electrical contacts over each of the tines. The transmitter would transmit tones that translated to the same frequencies as the music-box-like combs in the receiver. If the transmitted frequency matched the comb tine frequency the tine would vibrate up and down making contact with the electrical contacts. This would make pulses that would operate the servo motors in the aircraft. One tone for rudder left another for rudder right etc. Very cool.

So, I am very familiar with these. We used them on Aircraft External Power Supply units up until we got fancy new computer controlled units about 5 years ago. They are absolute genius little gauges. I remember that it was always a source of pride to be able to tune in the power supply to the right frequency in the least amount of time. Great video as always :) I wonder if this is from a military parts stockpile from when all of the old manual power carts got retired. Probably not, but it wouldn't surprise me.

Hi Fran. You're right. They built them the way they did because that was the way they did it! And how the technology of the time allowed. I'm finding all these "elder" gadgets to be rivotoling fascinating. I wish, where possible - or the buyer would allow - that you could put source links in. Regardless of that little niggle, please keep up the great old stuff. You are on a real roll right now! Love it!

In the ‘60s, before we had the Web, the way you found out who manufactured something you needed was to look in the Thomas Register, several four-inch-thick volumes that had ads for all sorts of companies. When I was 11 years old, I used to pore over the copy that the library had, and would write to companies for more information about products that interested me. One of those companies was the Herman H. Sticht company, which made vibrating-reed frequency meters like this one. A few days later, I got a call from a sales rep-apparently they looked up my last name and address in the phone book-which scared hell out of me. After that, I would start each letter with “I am only 11 years old, but I am very interested in your products.” Perhaps surprisingly, it did not reduce the number of mail responses I got. People tend to be kind towards kids. I’ve always wished I could find an application for such a meter and buy one, to sort of repay them for helping in my early engineering education.

I have a large "tuned reed" frequency meter rated for 15 to 45 periods per second at five volts mounted on a stand which has a switched resistance with five settings for voltage ranges going from 30 to 50 volts and 160 to 250 volts. Inside it has a date of 1923. The dial is 7 inch in diameter and one reed painted red at the 25 position while the others are all white suggesting that the meter was aimed at the motor traction generating, early electric trains and trams ran at 25 HZ here in the UK. I got the meter from a whole lot of old equipment thrown out by Cambridge university who I suspect put the variable voltage device on the meter as it looks more homebrewed than the meter itself.