Tuning Forks & Helmholtz Resonators



© Brooke Clarke 2017
Background
Description
    Tuning Fork
        Testing
    Helmholtz Resonator
Examples
    Simple
    Teletype Motor Speed Test
    American Time Products 200 Hz
    Melpar 10HDN6 Frequency Standard
    Hathaway Instruments Inc. BACR20 A4
    RCA Crystal Unit VC-5-M
    US Army Signal Corps Frequency Meter TS-65D/FMQ-1
    Accutron
    General Radio Audio Oscillator
    Resonance Box
    Lab Demonstration Electromagnetic Tuning Fork
    Electromagnets
Patents
Related
References
Links

Background

Started this after seeing the eBay listing titled: Helmholtz Sound Synthesizer (Ref 1) by Max Kohl AG (Wiki) - The Earliest Electronic Keyboard.  Although I don't think the claim about being the first electronic keyboard is correct.  Two key concepts that make up the Synthesizer are electromagnetically driven tuning forks (Wiki) and Helmholtz Resonators (Wiki).  While working on this web page I've read in some free (very old) Google books that the Sound Synthesizer did not do a very good job of mimicking vowel sounds.  It takes about twice as many sound sources to do a passable job.

The devices on this page operate mostly in the audio range, but are closely related to quartz crystals that operate at higher frequencies and with higher Quality factors (Wiki).  See Related below for my web pages relating to Crystals.

Description

Tuning Fork

Tuning forks have many uses such as:

  • tuning musical instruments (Piano Frequencies: 27.5, 55, 110, 220, 440, 880, 1760, 3520, 261.626 Hz)
  • in clocks and watches (Bolova Accutron (Wiki, below, 360 Hz)
  • Medicine (128 & 256 most common Powers of 2)
  • Quack medicine see: Faradic Chakra
  • Police Radar Gun (Wiki) calibration (Wiki: Doppler)  55 MPH: 10.525GHz=1726 Hz=3961 Hz, 24.150GHz:   35.5GHz: =5823 Hz; fd = 2 v fo / c )
  • as a gyroscope (often as a MEMS (Wiki)
  • Interrupter for other circuits or source for divider or other Science (50,  60 or 400 Hz power line, Powers of 2: 64, 128, 256, 512, 1024, 2048, 4096, 32768 Hz; Teletype: 87.6, 96.19, 180 Hz)

The tuning fork is a resonator that produces a tone with less harmonics than other types such as strings.  This can be further enhanced when, after striking the fork, you touch the tines near the bottom of the "Y" to damp out the higher harmonics.

Note that the tines move away from each other then move closer to each other.  The stem moves axially (up and down). 

In the early days of electronics (after vacuum tubes were available in the mid 1920s) electromagnetic drives were used to keep tuning forks oscillating and sense the vibrations so that they could be made to operate continuously.

x


The Villard Interrupter (Ref 2) is used in the Max Kohl Synthesizer

Testing

I just happened to have some Tattoo Machine Coils on the desk as well as some magnets stuck to the legs.  By combining them with a DMM and can measure the frequency of a tuning fork that has some ferrous metal.  The procedure is to place a permanent magnet on the core of the coil this way any movement of a ferrous object near the coil will generate AC electricity.  Now for the Fluke 87 IV:

Turn on to VAC
Press Back light (to make it easier to see the LCD)
Press Hz to measure frequency
Press Min/Max to record peak values
Press Min/Max to display the maximum value.

Strike tuning fork and hole one tine near the coil core.
Fig 1 There was a question about VPS vs. Hz.
You can see that the fork marked 87.6 VPS is actually 87.54 Hz, so in this case VPS = CPS = Hz.
Measuring
                      Frequency of Tuning Fork with DMM
Fig 2 Fork marked 523.3 Measures 523.3.  This fork has been tuned, See below.
Measuring
                      Frequency of Tuning Fork with DMM

Helmholtz Resonator

Hermann von Helmholtz (Wiki) was a pioneer in many areas. Also see my Helmholtz coil.
Note that the Helmholtz Resonator (Wiki) is much smaller than a 1/4 wavelength tube type resonator.  A common example it a beer or soft drink bottle.

Examples

Simple

These are tuning forks that need to be manually tapped to get them going.

Fig 1 Set of Tuning Forks
Fig 2 64 Hz Weighted "C" Tuning Fork
Note the Allen screws that hold the weights in place are steel.
They can be sensed by the combination of an electromagnet, permanent magnet and DMM.
64 Hz
                      Weighted "C" Tuning Fork
Fig 3 320 Hz generic Aluminum
Came with the Resonator Boxes (see below)
320 Hz
                      generic Aluminum Tuning Fork
Fig 4 Sheffield Ragg England C.523.3 Universal Low
Notice tuning mark at the base of the tine at at the joint with the yoke.
There's another tuning mark on the other tine.  The base of the stem is a round cone.
Very nice.
Sheffield
                      Ragg England C.523.3 Universal Low Tuning Fork


Teletype Motor Speed Test

VPS = Vibrations Per Second, which in clock circles is half a cycle. But that does not apply here.
87.6 V/sec * 60 sec/min = 5256 Vibrations/minute.
Note there is a slit in each vane and the pair of vanes act as a shutter to provide a stroboscopic effect when looking at gear wheels.
Used to adjust motor speed in Teletype (Wiki) equipment.

Teletype
                      Motor Speed Test 87.6 VPS
87.6 VPS
When tested on an Android using audio spectrum analyzer apps the peak is:
177.65 Hz or 86.1 Hz.
TM 11-2222 says motor: 2102 RPM and 368.1 Operations Per Minute
96.19 VPS fork for 2308 RPM & 404 OPM

180 Hz NSN: 5815-00-224-9717

American Time Products 200 Hz

The 7-pin base matches a 7-pin tube socket.
Marking:
NSN: 5825-00-736-8521
Resonator, Tuning FO
9LA900-85-M-8204
date Code: 8517
03040
Pin Out: Pins 1 & 4 = 468 Ohms,  Pins 6 & 7 = 950 Ohms
It's not clear which is the drive and which is the sense winding, but it's likely that the higher resistance coil is for sense.


1
2
3
4
5
6
7
1
-
OL
OL
468
OL
OL
OL
2
-
-
OL OL OL OL OL
3
-
-
-
OL OL OL OL
4
-
-
-
-
OL OL OL
5
-
-
-
-
-
OL OL
6
-
-
-
-
-
-
950

7
-
-
-
-
-
-
-


American Time Products 200 Hz Tuning Fork

Melpar 10HDN6 Frequency Standard 60 cps

I remember the defense contractor Melpar (Wiki) when I was working (1960 - 1990).
Top marked:

Model
10HDN6
Freq. CPS
60
Tolerance +/- .1%
Temp deg C
+65
0
Frequency
Standard

Patent No.
3106124
MELPAR
[logo]

Fig 1
Melpar
                      Tuning Fork 60 cps 10HDN6 Frequency Standard 60
                      cps
Fig 2
Melpar
                      Tuning Fork 60 cps 10HDN6 Frequency Standard 60
                      cps

This is a hermetically sealed 60 Hz frequency standard based on a tuning fork design.

Operation

With +5 VDC input with the can pressed to your ear you can hear the tuning fork at a pitch much higher than 60 Hz.  With +15VDC input the output is about 5.0 VAC at 60.00 Hz as measured on a DMM.

3106124
                              Tuning forks, William P Asten, Melpar Inc,
                              Oct 8, 1963
3106124
                              Tuning forks, William P Asten, Melpar Inc,
                              Oct 8, 1963

3106124 Tuning forks, William P Asten, Melpar Inc, Oct 8, 1963, 84/457, 984/260, 310/25, 84/409, 116/DIG.300 
The patent covers the sheet metal tuning fork and how it's packaged, but not the oscillation circuit.
The main idea is to use sheet metal stamping instead of machining to make the fork itself.
After abrading each tine so it's frequency is slightly below the desired frequency the can is sealed in a chamber with adjustable pressure.
The fork frequency is measured and as the pressure goes down the frequency goes up and can is sealed at that pressure where the fork is at the desired frequency.

The fork and it's mount is made from NI-SPAN-C (tm International Nickel Corp). Curie point 160 deg C and a negative temp coefficient  of elasticity.  By proper heat treating the zero temp coefficient  of elasticity can be made to occur at 15 deg C and is positive on either side of that temperature.


2874602 Apparatus for maintaining constant the vibration frequency of a tuning fork, Asten William P, (not assigned), Feb 24, 1959, 84/409 - This drive circuit was designed add temperature compensation to a  NI-SPAN-C sheet metal tuning fork, i.e. it matches the above patent.

Hathaway Instruments Inc. BACR20 A4 Tuning Fork

This is similar to the Melpar Tuning Fork frequency standard above except the output frequency is 10.51 kc.  To me this seems like a strange frequency.
Label:
Tuning Fork
Freq.  10.51 KC    Tol.   +.01%
Cat. 58802-3A      Ser.   040
Hathaway Instruments Inc.,    Hathaway Denver
5800 East Jewell Avenue, Denver 22, Colorado
A Division of Hataway Instruments Inc.
Dec 6, 1961
{inspection stamp B2/337

Fig 1
Hathaway Instruments Inc. BACR20 A4 Tuning
                      Fork
Fig 2 Hand written:
1, 2 - Pwr
3 Gnd    4 Out
Pins stamped 1 to 5 left to right
Hathaway Instruments Inc. BACR20 A4 Tuning
                      Fork

2687338 Synchronous time system for oscillographs, Davis William L, Hathaway Instr Company, Aug 24, 1954, 46/107.4, 250/208.4, 324/88, 318/85 - Tuning fork ((a & 9B) based. makes time marks on paper or film oscillographs.

RCA Crystal Unit VC-5-M

This is a quartz crystal, but from the same 1960s time frame as these tuning fork oscillators.
This may be a CR-15/U, CR-16/U, CR-29/U and/or CR-30/U Crystal Unit (Ref 5) also it uses the HC-5 Crystal Holder.

Label:

Crystal Unit

Reg U.S.
Pat Off.
RCA
Marca
Registrada

See Patent License Notice on Carton
MODEL VC-5-M
Serial  1480
Frequency  100 KC
Calibrated at 30 (deg) C
Radio Corporation of America
Camden  N.J. U.S.A.

Stamped on side:

M-254141-1
[A5]

Inspection stamp SC/831P
On Bottom:

red box [TEST 17]
67
79 
Maybe tested in 1967 and retested in 1979?



Fig 1 There is a rubber gasket between the bottom
and the black case.
RCA Crystal Unit
                      VC-5-M
Fig 2 Two bottom pins are ground.
RCA Crystal Unit
                      VC-5-M


2326923 Art of mounting piezoelectric crystals, Bokovoy Samuel A, Rca Corp, Filed: Sep 30, 1941, Pub: Aug 17, 1943, 310/354, 310/344 
3073975
Crystal Unit, Robert R Bigler, Edward M Washburn, Rca Corp, Jan 15, 1963, 310/344, 310/348 

US Army Signal Corps Frequency Meter TS-65D/FMQ-1

This is a test set to calibrate the TMQ-5 Radiosonde receiver.  Inside there is a white powder like coating on some of the parts.  Maybe this unit was in a fire where a dry chemical extinguisher was used?

Manual TM 11-6625-407-14 Operator's, Organizational, Direct Support, and General Support Maintenance Manual,
Frequency Standard TS-65C/FMQ-1 and TS-65D/FMQ-1
(NSN 6625-00-649-4279)
Headquarters Department of the Army
October 1973

Label:
Signal Corps                     U.S. Army
Frequency Standard TS-65D/FMQ-1
Serial No.  X100  Order No. 35756-PH-53
          Precision Associates Inc.
                   115 VAC 60~

Photos

Fig 1 Handle up.

US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 2 The lid can be separated from the main chassis. The line cord is permanently attached.
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 3 Top knob is mechanical radius selection for the lamp-photocell, i.e. frequency.
Lower knob is output level.
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 4 Schematic.
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 5 Inside Top
To open remove 2 screws on rear panel,
Loosen 2 Dzus (Wiki) fasteners on front panel.
Use handle on front panel to lift chassis out of case.
Tall round can at right center marked:
64153
543
60
and is the 60 CPS tuning fork.
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 6 Inside Bottom
Note long "tail" from tuning fork that is bent over.  What's up with this?
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 7 The BLock at top contains a pilot lamp and photocell and moves up and down.  Shown almost all the way up.
US Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 8 gold color 60 CPS motor at right just behind front panel.  White stuff on wheel.
S Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1
Fig 9 2D21W Thyratron (Wiki)
Used extensively in W.W.II.
Torpedoes, VT Fuses
S Army
                        Signal Corps Frequency Meter TS-65D/FMQ-1

Bolova Accutron Tuning Fork Watch

Bulova (Wiki) invented a watch where the time keeping mechanism was a tuning fork rather than a classic balance wheel (Wiki).  It was more accurate that balance wheel watches.

This watch was left on a workbench loose and has picked up many many small ferrous metal parts.  It also has some kind of goop that needs to be cleaned.  Maybe Naphtha in the Ultrasonic Cleaner?
The tines do move, but they don't cause the gears to turn.

Fig 1 at the top: "Pat. 2971323 Et Al"
U.S.A.
Bulova
218D
There is a small screw and piece of wire to the left of the left coil held by the magnetic field.

Bolova
                          Accutron Tuning Fork Watch

Fig 2 You can see a small circlip at "9" held in place by the magnetic drum of the left tine.

olova
                          Accutron Tuning Fork Watch


Patents

2782627 Device for measuring the amplitude of the vibrations of a watch escapement, Max Hetzel, Bulova Watch Co IncFiled: May 19, 1952, Pub: Feb 26, 1957, 73/1.48, 968/771 -for testing balance wheel watches.
Max Hetzel also patented a number of devices for adjusting balance wheels and these included electromagn
ets (2707875, 2714306, 2748595).

2949727
                        Electric timepiece, Max Hetzel, Bulova Watch Co
                        Inc, Filed: Jun 19, 1953, Pub: Aug 23, 1960
2949727
                        Electric timepiece, Max Hetzel, Bulova Watch Co
                        Inc, Filed: Jun 19, 1953, Pub: Aug 23, 1960 This version only shows one electromagnet.

2949727
Electric timepiece, Max Hetzel, Bulova Watch Co Inc, Filed: Jun 19, 1953, Pub: Aug 23, 1960, 368/157, 984/260, 310/15, 368/167, 968/486, 968/482, 968/483, 84/457, 968/481 -

Cites WE patent 1560056 (an early electromagnet driven tuning fork, see below)


2888582
                        Tuning fork oscillator, Max Hetzel, Bulova Watch
                        Co Inc, Filed: Jun 19, 1953, Pub: May 26, 1959
2888582 Tuning fork oscillator, Max Hetzel, Bulova Watch Co Inc, Filed: Jun 19, 1953, Pub: May 26, 1959, 310/25, 968/481, 984/260, 968/482, 968/483, 84/409, 968/486 -  

2908174
                        Motion transformer, Max Hetzel, Bulova Watch Co
                        Inc, Filed: Oct 23, 1953, Pub: Oct 13, 1959
2908174 Motion transformer, Max Hetzel, Bulova Watch Co Inc, Filed: Oct 23, 1953, Pub: Oct 13, 1959, 74/128, 74/160, 968/79, 368/125, 74/578, 368/135, 74/144, 968/482 

This is about how to transform the vibratory motion of the tuning fork into rotary motion for the gear train.

2971323
                        Electronically-controlled timepiece, Max Hetzel,
                        Bulova Watch Co Inc, Filed: Jun 19, 1953, Pub:
                        Feb 14, 1961
2971323 Electronically-controlled timepiece, Max Hetzel, Bulova Watch Co Inc Filed: Jun 19, 1953, Pub: Feb 14, 1961, 368/157, 74/142, 310/25, 968/481, 318/130, 331/156, 968/486, 331/116.00M, 968/482, 984/260, 968/483

2929196
                        Electric timepiece, Max Hetzel, Bulova Watch Co
                        Inc, Filed: Mar 12, 1956, Pub: Mar 22, 1960
2929196 Electric timepiece, Max Hetzel, Bulova Watch Co Inc, Filed: Mar 12, 1956, Pub: Mar 22, 1960, 368/167, 968/482, 310/27, 318/132, 968/486  

This is a dual electromagnet design where the left coil is in the collector circuit and the right coil is in the base circuit.

206 is described as a permanent magnet.

2960817
                        Electrical timepiece, Max Hetzel, Bulova Watch
                        Co Inc, Filed: May 12, 1955, Pub: Nov 22, 1960
2960817 Electrical timepiece, Max Hetzel, Bulova Watch Co Inc, Filed: May 12, 1955, Pub: Nov 22, 1960, 368/125, 310/25, 968/486, 74/142, 331/116.00M, 331/156, 968/482, 968/481, 318/132, 368/167, 318/128, 968/487 -  

1106: Cup shaped permanent magnets (South pole) and a central rod (North pole) part of tuning fork.
Sc: Sensing Coil
D: Driving Coil
B: 1.3 Volt Mercury coin cell Battery
TR: Germanium Transistor The characteristics of the transistor stabilize the drive current
C: 2 uF Capacitor
R: 2 Meg Ohm Resistor





3057147
                        Motion converter, Max Hetzel, Bulova Watch Co
                        Inc, Filed: Jul 30, 1956, Pub: Oct 9, 1962 3057147 Motion converter, Max Hetzel, Bulova Watch Co Inc, Filed: Jul 30, 1956, Pub: Oct 9, 1962, 368/125, 310/21, 368/167, 74/126, 310/25, 968/482, 968/481 - how to get vibrations from tuning fork into gear train.  


3070951 Frequency-adjustable tuning fork type vibrator for an electrically energized timepiece, Max Hetzel, Bulova Watch Co Inc, Jan 1, 1963, 368/167, 84/409, 331/156, 84/457, 968/486, 368/200 - adjust tuning fork frequency to extremely close limits. done by adding a wire that can be bent to tune the frequency.

General Radio 1000 CPS Audio Oscillator 213B

The 831-A and 213B are both 1000 Hz tuning fork oscillators.  The 213 runs on 6 VDC.

The manual for the 831-A says 4.5 VDC for normal operation, but for increased output voltage and higher harmonics you can use up to 8 VDC.  A circuit modification to add a resistor in series with the output microphone to lower it's current might be required to minimize microphone carbon granule packing.

The 213-B runs at 1000 Hz (common audio test frequency and used for oscillograph timing marks) and the 213-C runs at 400 Hz (aircraft AC mains frequency)

Description

The coil on the lower tine generates a field to cause the tine to be magnetized.  The "U" shaped transformer core at the tips of the tines applies the drive signal.  The chrome plated dome above the fork is a carbon microphone element that senses the tine movement.  The fork is marked 1000 ~.

As received it's not operational.  After removing the chrome metal microphone cover it was seen that the spring was not making contact with the corbon element.  By slightly stretching the spring it was wiggled onto the mike element.
The black tape that's wrapped around the drive electromagnet is loose and touching the bottom of both fork tines adding friction.  Maybe enough to keep it from running?

Fig 1
General Radio
                        1000 CPS Audio Oscillator 213B
Fig 2
General Radio
                        1000 CPS Audio Oscillator 213B
Fig 3 Carbon microphone Spring inside chrome metal cover.
General Radio
                        1000 CPS Audio Oscillator 213B

Resonance Box

This is a classical physics experiment.  But . . . I don't think the correct tuning fork was included.  The two boxes do not couple.
These boxes came with a couple of modern 320 Hz aluminum tuning forks with rectangular stems that are too big to fit into the hole in the spools.

The interior dimensions are:
73mm wide x 38.5mm high x 173 mm deep

The box is probably just a 1/4 wave resonator so the depth is given by (velocity of sound) / (4 * frequency). So. frequency of these boxes is:
F = 340 meters/second / (4 * 0.173 meters) = 491 Hz. (This is neither a power of 2 or a Chakra frequency. Not sure what this frequency is used for?
Maybe these would work with 440 Hz forks?
For 320 Hz the quarter wave length is  0.2656 meters (10.457")
This can be confirmed by striking the 320 Hz tuning fork and holding it near the open of a paper towel roll where the other end is blocked.  Inserting a dental floss box into the closed end a little improves the resonance since the paper towel roll is 11" long and the desired length is about 10 - 1/2".

Also the location of the fork support seems questionable, maybe it should be centered on the width, like it is now, but with the center of the stem at the open edge?

Fig 1
Tuning Fork
                      Resonance Box


 

Electromagnets

See the Magnets web page for the Tattoo Machine Coil electromagnets.

Lab Demonstration Electromagnetic Tuning Fork -

This probably was used as a demonstration in Physics lectures and probably fits lab stand clamps.
There are 4 contact points on the fork and the opposing adjustable contact can be moved to any of them.

DOA
The electromagnet is not working, it's shorted.  Tests:
1. Connected to a lab DC power supply and with .25 Amps flowing the voltage drop is small and there is no magnetic attraction.
2. Using the Fly Back Tester shows a shorted turn.
3. Unwrapped wire and found no problems.  Wire dia = 0.036 i.e. 19 AWG and the length for 0.2 Ohms is about 25 feet which looks correct.  So it just needs a lot of current.  Connected to the HP 6038A power supply set for 9 Amps max and the fork ran, but sparks between the tine nearest the contact and the metal coil frame.
Turning down the voltage the fork runs on about 3 Volts and about 2 Amps (average).  When connected to a Tattoo power supply with frequency readout it shows 59 or 60 Hz (see Fig 5).  So the reading of 387.6 Hz on the smart phone app was actually a 6.5 times harmonic (seems strange for the half part of that, but that may just be a problem with the smart phone spectrum analyzer app and or poor microphone sensitivity at 60 Hz.

Fig 1
Lab
                      Demonstration Electromagnetic Tuning Fork

Fig 2  The overall length is 17".  The tines are about 13-1/4" long and 3/4" x 3/8"
Using an audio spectrum analyzer on my Android phone the frequency is 387.6 Hz.
This is not a musical note on the equally tempered scale. False reading since cell microphone is not sensitive to 59 Hz
Lab
                      Demonstration Electromagnetic Tuning Fork

Fig 3  The rods are all 1/2" diameter.
Lab
                      Demonstration Electromagnetic Tuning Fork

Fig 4  Using the Fly Back Tester shows a shorted turn. Is really OK.
Lab
                      Demonstration Electromagnetic Tuning Fork

Fig 5  Using a tattoo power supply it's easy to see the actual frequency.
YouTube Video: https://youtu.be/BdmS8Ny88b0
Lab
                      Demonstration Electromagnetic Tuning Fork -60 Hz

Patents

A number of the early patents reference telegraph multiplex patents because they used tuning forks and/or stepping motors.

Classes

33 Oscillators
73 Measuring & Testing
84 Music
310 Electrical Generator
318 Electricity Motive Power Systems
331 Oscillators
324 Electricity: Measuring & Testing
368 Horology: Time MEasuring Systems or Devices
370 Multiplex Communications

Patents

   281339 Multiplex telegraph, Edward A. Calahan, The Standard Electric Manufacturing company, Jul 17, 1883, 370/304; 178/75 - Uses tuning fork to pack selector rotation
   395556 Vibratory multiplex telegraphy, S.D. Field, Jan 1, 1889, 178/47; 178/65; 178/71.13; 310/25; 335/222
   465832 System of Synchronism for Telegraphy, G.A. Gassagnes, Dec 29, 1891, 318/75; 84/457 - 
1166317 Tuning-fork and resonator, Walter Berry, Dec 28, 1915, 84/409 - mounted on the end of a 1/4 wave long can 
1524868
Method of and apparatus for electrically operating tuning forks, Knoll Lloyd M, Thomas Appleby, Feb 3, 1925, 84/409, 310/35, 310/25, 84/457 - to stabalize an oscillator  
1560056 Source of waves of constant frequency, Joseph W Horton, Western Electric Co, Nov 3, 1925, 24/76.49, 327/121, 324/76.41, 331/156, 84/409, 331/107.00R, 310/25, 333/200, 116/DIG.300 - Driving coil similar to the Eckhard (Ref 4).

1653794
                      Temperature-compensating means for maintaining
                      constant frequency in tuning forks, Whitehorn
                      Homer A, Western Electric Co, Dec 27, 1927
1653794 Temperature-compensating means for maintaining constant frequency in tuning forks, Whitehorn Homer A, Western Electric Co, Dec 27, 1927, 84/409, 84/457, 310/19 - used in multiplex telegraph system driving an impulse motor of the La Cour type (Wiki) motor.  A bi-metal strip is used either at the tine ends or the support for a horseshoe magnet.

203423 Improvement in isochronous and synchronous movements for telegraphic and other lines, Poul La Cour, May 7, 1878, 388/830; 310/163; 318/161; 318/700 -

1284219
                      Starting device for La Cour motors ("Phonic
                      Wheel"), George R Benjamin, Western Union
                      Telegraph Co, Nov 12, 1918
1284219 Starting device for La Cour motors ("Phonic Wheel"), George R Benjamin, Western Union Telegraph Co, Nov 12, 1918, 318/701, 318/400.4, 318/400.8, 310/41 -
When the Hand Lever (11) is pressed and held down and the flywheel spun clockwise, the auxiliary contacts keep the motor spinning,  Once up to speed the hand lever is released and the tuning fork provides the electromagnet switching.

Note  that opposite pairs of electromagnets  (4 & 4)are wired together.  The vertical pair (3 & 3)is driven then the horizontal pair (4 & 4).  Note the armature (2) has a lobe pitch that does not match the spacing between a pair of electromagnets.  For example the top pair (3) is shown with an armature lobe aligned with the left pole piece, but the right pole piece is not aligned with the adjacent armature lobe. 

Modern stepping motors (Wiki) work in this way.  A more complex drive waveform for each electromagnet would eliminate the need for the starting hardware.
Photos courtesy of the Poul la Cour Museum
Fig 1 Motor
Poul La Cour motor ("Phonic
                              Wheel")
Fig 2 Motor
Poul La Cour motor ("Phonic
                              Wheel")
Fig 3
Poul La Cour motor ("Phonic
                              Wheel") Wiring



203423
                              Improvement in isochronous and synchronous
                              movements for telegraphic and other lines,
                              Poul La Cour, May 7, 1878
203423
                              Improvement in isochronous and synchronous
                              movements for telegraphic and other lines,
                              Poul La Cour, May 7, 1878

203423 Improvement in isochronous and synchronous movements for telegraphic and other lines, Poul La Cour, May 7, 1878, 388/830; 310/163; 318/161; 318/700 -
Fig 1 & 2 are of the tuning fork driver for one or more motors.
Fig 3 & 4 are of the motor.
Fig 5 is for an addition that compensates for slip of one or more teeth for a higher power motor.
Fir 6, 7 & 8 all look the same to me, but are supposed to represent in-phase, lagging or advancing.

Fig 3 shows a chamber filled with Mercury that helps the motor start and get up to speed.  This may not have worked very well since Western Electric has a patent (1284219 above) titled Starting device for La Cour motors.

The system is shown here:
302502 Synchronous telegraphy, Poul La Cour, Jul 22, 1884, 370/304 - uses the above tuning fork and motor, but it's not clear how the wheels at each telegraph station are synchronized.  Synchronization was a big part of stock ticker patents.

The Poul La Cour Museum in Denmark -

1828660
Method of and apparatus for driving tuning forks, Buckingham William D, Homer Edward C, Western Union Telegraph Co, Oct 20, 1931, 84/409, 984/260 - force along axis of base.    
1906985 Vibratory frequency standard, Marrison Warren A, Western Electric Co, May 2, 1933, 331/156, 310/25, 333/200, 84/409 - frequency standard where fork is mounted resiliently mounted & with Sine wave output.
1912343
Tuning fork temperature compensation and frequency adjustment, Buckingham William D, Western Union Telegraph Co, May 30, 1933, 84/457, 984/260 - by a special mounting
1913331 Tuning fork drive, Buckingham William D, Western Union Telegraph Co, Jun 6, 1933, 84/733, 84/743, 84/409, 331/156 - drive up/down at stem
1937583
Oscillation generator, Norrman Ernst B, Rca Corp, Dec 5, 1933, 331/156, 84/409, 331/69, 331/157, 331/185 - 3 aluminum boxes with insulation and controlled temperature + damping factor 4 to 15X lower than standard tuning forks (see fork dimensions in Fig 1 & Fig 2).   An optional construction is to use a magnetostrictive bar (Fig 4) instead of the tuning fork.
1956350 Electrical musical instrument, Hammond Laurens (Hammond Organ), Apr 24, 1934, 84/601, 84/DIG.400, 200/17.00R, 84/715, 307/27, 84/DIG.250, 307/1, 84/433, 310/170, 984/355 - uses tuning-fork w/stem drive (Fig 15) to allow operation from a DC power source and can be used to tune the organ to match other instruments.
1958071 Electrically driven vibrator, Scofield Philip F, Heintz & Kaufman Ltd, May 8, 1934,
1963719 Mechanical vibrating element, Schelkunoff Sergei A, Bell Telephone Labor Inc, Jun 19, 1934, 84/402, 984/260, 367/191, 310/26 - loop = two tuning-forks connected at the tips of the tines.   

1958071
                      Electrically driven vibrator
2015410 Vibrating system, Harold R Prescott, Continental Oil Co, Sep 24, 1935, 84/409, 368/202, 331/156, 367/186, 310/25, 367/178, 368/167 - 100 Hz time marks for seismic oil exploration.
By choosing the base (1) material and the tuning fork (13) material and the spacing (dimension d) between supports (3) temperature compensation is achieved.  Similar to the Harrison pendulum clock temperature compensation (Wiki).

2034787 System for generating alternating current, Williams Jr Albert J, Leeds & Northrup Co, Mar 24, 1936, 331/156, 310/25, 331/183, 84/409 - magnetic field away from tuning-fork.
2049179 Frequency generator system, Burrell Stallard, Bell Telephone Labor Inc, Jul 28, 1936, 331/156, 331/183, 330/145, 330/164 - coil axis parallel to tines?
Controls the drive power at constant level independ of power supply variations.

2147492 Oscillation generator, Mead Jr Milton S, Gen Electric, Feb 14, 1939, 331/156, 327/530, 310/25, 310/19, 358/416, 84/409, 327/331, 327/322, 968/481 - constant drive current leads to better frequency stability.
2247960
Tuning fork, Eugene Michaels Simon, Bell Telephone Labor Inc, Jul 1, 1941, 84/409, 310/25, 984/260, 310/328 -
2309853
Rate and attitude indicating instrument, Norden Elwood, Lyman Joseph, Sperry Gyroscope Co Inc, Filed: Apr 10, 1941, Pub: Feb 2, 1943, 73/504.15, 33/360, 33/351, 73/178.00R, 33/300 - Gyroscope
2556342 Mounting for tuning forks, Dickran Sebouh, Filed: May 5, 1944, Pub: Jun 12, 1951, 84/409, 84/457, 984/260 - attach at nodal points
2687338
Synchronous time system for oscillographs, Davis William LHathaway Instr Company, Aug 24, 1954, 346/107.4, 250/208.4, 324/88, 318/85 -  
GB1967955A means for controlling the amplitude of vibration of an electromagnetically maintained tuning fork vibrator or the like, Donald Crisp Gall, Jack Sutcliffe, H TINSLEY AND CO Ltd, 1955-07-07 ,
2707234 Tuning fork oscillators, Frank Dostal, American Time Products Inc, Apr 26, 1955, 331/69, 331/156, 331/176, 324/76.49, 368/167 -   Gyroscope
2817779 Drive and mounting means for a tuning fork structure, Barnaby Roland E, Morrow Charles T, Sperry Rand Corp, Dec 24, 1957,  310/25 Gyroscope
Calls:
2513340 Angular velocity responsive apparatus, Lyman Joseph, Sperry Corp, Jul 4, 1950, 73/504.16 - Gyroscope
2683596 Rate of turn instrument, Roland E Barnaby, Charles T Morrow, Lloyd A Nevala, Sperry CorpJul 13, 1954, 73/504.16, 55/359, 310/15, 336/30 - Gyroscope  
2753173 Turn rate measuring instrument, Roland E Barnaby, Charles T MorrowSperry Rand Corp, Jul 3, 1956, 73/504.16, 84/409 - Gyroscope   
2843742 Device for maintaining mechanical oscillations, Meyer Cluwen Johannes, Philips Corp, Jul 15, 1958, 318/127, 318/160, 368/158, 331/181, 318/132, 331/116.00M, 318/130, 310/39, 310/36, 968/475 - a very simple circuit with a battery, one transistor but two coils, one for sensing and one for driving a clock motor.  The drive pulse is at exactly the zero position of the magnetic balance wheel - spring.  When driven at the zero position variations in the amplitude of the drive pulse have a minimal effect on the frequency of oscillation.   
2888582
Tuning fork oscillator, Max Hetzel, Bulova Watch Co Inc, Filed: Jun 19, 1953, Pub: May 26, 1959, 310/25, 968/481, 984/260, 968/482, 968/483, 84/409, 968/486 -
2956242 Tuning fork oscillator, Grib Boris F, Philamon Lab Inc, Oct 11, 1960, 331/156331/177.00R333/138331/182331/135 - 1E-7 adjustments made by changing phase of tube circuit feedback.
 
3085168 Tuning fork, Jones Albert C, Lingel Frederick J, Troup Peter B, Gen Electric, Apr 9, 1963, 310/25, 984/260, 331/156, 84/409 - for 400 Hz aircraft AC power supply
3122047
Tuning fork, Jones Albert C, Lingel Frederick J, Troup Peter B, Gen Electric, Feb 25, 1964, 84/409, 984/260, 84/457 - 400 Hz aircraft small size
3493292
Tuning fork structures, Dostal Frank, Bulova Watch Co Inc, Feb 3, 1970, 359/230, 331/155, 331/156  
6194817 Tuning-fork vibratory gyro, Masanori YachiYoshio SatohHiroshi IshikawaYoshitaka TakahashiKazutugu Kikuchi, Fujitsu Limited, Feb 27, 2001, 310/370310/351310/345

Related

Quartz Clock -
Chinese Spouting Bowl & Tibetan Singing Bowl -
Crystals
Crystal Equivalent Circuit
Crystal Impedance (CI) Meters
Crystal Temperature Compensation Patents
Joule Thief - Motor Circuits - simple one transistor control of a
Geek Stuff: Zendulum - a spherical magnet rolls in a curved track - very similar to a pendulum.
the TS-65D/FMQ-1 is related to the weather balloon tracking equipment, including PIBAL theodolites, radiosondes, CRT-1 sonobuoy, Roswell Incident
General Radio Sound Measuring Equipment (Also pages about: GR Strobotac & Harold "Doc" Edgerton)

Military Audio
Military Audio Accessories
U-229 Audio Accessories
U-229 Pinout by Function
phones
TS-585 Audio Level Meter
HP 4395A Network, Spectrum & Impedance Analyzer

Beltone 12D Audiometer -
Sound Powered Telephone - and the development of early speakers
Crystal Radios and early loudspeaker
Teledyne Avionics TA-3D Acoustic Impedance Meter -
Tuning Forks & Helmholtz Resonators -

Sonobuoys - CRT-1B Sonobuoy

References

Ref 1 On the Sensations of Tone as a Physiological Basis for the Theory of Music, Hermann von Helmholtz, 1912 -
Ref 2 The Theory, Design and Construction of Induction Coils, H. Armagnat (Translated by O.A. Kenyon), 1908
Ref 3 FUNDAMENTALS OF ACOUSTICS, 4TH ED,Lawrence E. Kinsler, Austin R. Frey, Alan B. Coppens, James V. Sanders, 2009
Ref 4
Journal of the Optical Society of America, Volume 6, Part 1 - 1922 An Electron Tube Tuning Fork Drive, E.A. Eckhardt, J.C. Karacher & M. Keiser pg 949.
Ref 5 Handbook of Piezoelectric Crystals for Radio Equipment Designers, PB-111586-R, Buchannon, Oct 1956

Links

120 Years of Electronic Music - Helmholtz Sound Synthesizer. Max Kohl. Germany, 1905 -
Naval Postgraduate School Physics - Ph 3451: Acoustics - Lecture 19 - Helmholtz Resonators - text: Ref 3.
Naval Postgraduate School Physics - PH3119 Tuning Fork Part 1 - note drive and sense coils at right angles to each other.
Naval Postgraduate School Physics - Helmholtz Resonator and other Maintained Oscillators -
NPS Physics - Ph 3451: Acoustics - Physics Experiment 2 - Helmholtz Resonator Part 1

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