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The Combo Ringing Machine

Switchboard operators offered something automatic exchanges could not -- the human touch. A reportedly true story,  


Caller: Can you help me? I'm trying to call my doctor, and I'm having trouble.

Operator: What kind of trouble are you having?

Caller: I have a pain in my side and it's going up my back.


Ha Ha… When operators were replaced by automation, some people complained about the lack of personalization. Operators were a community antenna of sorts, sometimes offering news and opinions. Callers were sometimes family or friends.  How can human interaction be replaced by some beeping tones?  


In the early days of operator assisted calling, common phrases a caller may hear were, “What number please?”  or “That person is engaged” or “ Mr. Strowger is not answering”, and so on.    

With no operator, the automatic exchange needed to offer something roughly corresponding. This ended up being a variety of call-progress tones and bell ringing sequences. Here are examples,


  • Caller goes off hook, then a dial tone is heard (dial the digits)

  • Ring the bell on called phone, send audible ringing tone to caller or send audible busy tone (often 60 IPS) to caller

  • Optional party line ringing sequences

Other tones were also used.  


  • Off hook, loud howler signal (audible alarm that a phone is off-hook)

  • All circuits busy; fast busy tone (120 IPS)

  • Toll operators heard tones indicating the collection or return of coins on pay telephones.  

  • Also needed were many types of recorded announcements, time of day, etc.   


First, let’s separate out the ringing problem from the tone generating problem. Today, generating tones or ringing a caller’s bell would be a snap. In 1915, not so. First the ringing problem.


Simple ringing plans


A mechanical ringer was introduced with the first Bell telephones (~1877). It consists of two closely spaced bells (sometimes of different resonance frequencies), a movable metal clapper, and an electromagnet (EM). Alternating current passed through the EM causing the clapper to alternately hit each bell. Turns out that Thomas Watson is credited (US patent 210,886) with inventing the two-chime ringer (yeah, a bell) as we know it today.

icon for information
Watson telephone ringer

                    Fig 1, Watson-like ringer, Smithsonian Institution, circa 1885

Due to the mechanical nature of the device, a 20 Hz (cycles per second) AC signal was ideal, usually at 90 volts. Many hobbyists (and from personal experience) have attempted to ring a phone bell using wall current at 60/50 Hz. The problem is the device just hums since there is no time for the clapper to move between positions. Hence, they settled on 20 Hz.

Sure, ringing one bell is relatively easy. In the early days it was done using a hand cranked AC magneto generator. But ringing say 50 phones or more at once in a large automatic exchange requires significant continuous ringing power. More on this later.

Tone plans

Worldwide there are MANY different tone plans and they each varied over the years. There is no standard. Many European plans use single frequency tones. The North American plan often used multiple tones, summed. Summed tones can sound more pleasing than single tones, but it’s an acquired taste. Here is one plan from AT&T in the 1960’s.

  • Audible dial tone is a continuous signal of the addition of 350 and 440 Hz tones.

  • Audible ringing tone sums 440 and 480 Hz with a cadence of 2 seconds On and 4 seconds Off.

  • Audible busy tone sums 480 and 620 Hz with a cadence of a half second On and a half second Off (60 interruptions per second, IPS).

To experience a variety of tones, have fun with Demo 2.

Party Line plans

For almost 100 years telephone companies offered “party lines.” Long telephone lines (2 copper wires per phone) are expensive to install and maintain. So, from the very start two or more subscribers often shared a long pair of wires leading to the central office (CO), maybe miles away. This arrangement was called a party line. Two subscribers were the min and ten the max (rural). My parents had a 2-party line in the 1960’s in San Francisco.  

There are many iterations here. With just 2 parties, each phone w
ould ring independently from the other. However, each could hear the conversation of the other if both were off-hook at the same time. Awkward at times. With a 4-party line, subscribers were assigned a distinctive ring sequence and only answered if they heard their assigned ring sequence. Two distinct ring sequences were needed for a 4-party line and five for a 10-party line (yikes!). 

For example, using Fig 2, ring Code 1 is a 2 seconds on and 4 seconds off ringing cycle and ring Code 2 is a “1 on, 1 off, 1 on, 3 off”
sequence. All ringing codes repeat after 6 seconds. Sure, it’s inconvenient for users but it saved them money and for telephone companies too on outside wiring.

telephone party line ring codes

Fig 2, five party line ring codes [Goddard]

telephone party line types

                                          Fig 3, Bell telephone magazine (1922)


With this preamble, let’s see what equipment Western Electric and others provided to meet the requirements outlined above.  

The All-in-One Machine


Western Electric and others devised and built special purpose combo-machines to provide tones, signal interrupters, ringing voltage and more. These machines were heavy and located on the first floor or basements of exchanges.  Fig 4 is a photo from 1932. This is a P-Type ringing and tone generating set. There are two units side-by-side. This is quite a contrivance and it’s not easy to grasp what it does just by looking.  

Type-P ringing machine for telephone exchange with tone sources

Fig 4, Two P-Type ringing machines with tone sources [Stone]

So, let’s break this behemoth down piece by piece. Fig 5 is a simplified diagram of the four main sections of a sample combo-machine.  The order of the components on the shaft may differ on production machines like the one in Fig 4. Also, the motor drive could also be a rotation regulated DC motor, not AC. 

diagram of an all-in-one ringing and tone generating machine for a telephone exchange with interrupters

Fig 5, All-in-One ringing and signal generator

From the left;

  • Main AC line powered drive motor.  Some designs use a DC drive motor. This rotating shaft extends from end to end. 1200 RPM is a common speed.

  • Ringing generator. Its output supplies ringing power for all the called phones at about 90 VAC, 20 Hz. For an AC generator with 2 internal poles, a 1200 RPM (20 RPS) shaft will generate a 20 Hz output.  

  • Timing cams: Each cam has an associated switch that follows the cam’s outline to create on/off interruptions. While three switches are depicted here, some interrupters feature up to 15 cams/switches.  For party lines, up to 5 ringing sequences are needed (Fig 2).

  • Tone wheels. There are several ways to generate the call-progress tones needed for an office and using tone wheels is one. Using a segmented rotating ring is another means, covered below. 

Tone generation with wheels

Using a rotating toothed iron wheel to generate a tone was common place in the days before vacuum tubes.  Fig 5A (Western Electric 3A tone generator) shows such a tone generator.  If there are 25 teeth on the wheel and the motor turns at 2,400 RPM (40 RPS) a 1000 Hz tone is generated.


A horseshoe-like shape made of iron is wrapped with a wire coil.  A static electromagnetic (EM) field is created in the iron with 24 VDC applied to this coil. As the rotating iron teeth pass the horseshoe end points the EM field is modulated to create a near sinewave (AC) signal. A separate pickup coil detects the modulated field and its output is filtered to create a clean sinewave. [Van Duyne]


By adjusting the motor RPM and tooth count other commonly used tones (400, 480, 620 Hz) may be generated.   

telephone exchange tone generator Motor driven alternator with 1000 Hz output

Fig 5A. Motor driven alternator with 1000 Hz output [Fagen]

Check out an amazing instrument that was the world’s first streaming electronic music synthesizer.  It used tone wheels extensively [Telharmonium]. 

Early small versions

For a small office exchange, there was a need for a small ringing generator. Fig 6 shows a smallish combo machine typical in circa 1903. It has an AC motor (or DC) at one end and 20 Hz AC ringing generator at the other.  There is a single segmented wheel for generating the busy tone and the busy interrupter (.5 sec on, .5 sec off).

Why wasn't there a dial tone generator? It became necessary only around 1906 with the invention of the Line Switch, akin to a Line Finder.  Before that date
, callers just went off-hook and dialed. 

ringing machine with tone generators for a small telephone exchange 1905

  Fig 6, Motor (left) and ringing generator (right) with busy tone generator (right attachment) for small office [Miller]

The Dynamotor

Engineer Charles Bradley is generally credited with the invention of the rotary converter circa 1888 as seen in Fig 7.  This device is a combination of a DC motor and AC generator inside the same housing (e.g., Fig 6 inside one case minus the tone wheels). It was also called a dynamotor or a hybrid motor-generator. The idea was to input a DC current (may be at 48 VDC) to energize the armature's rotation. As it rotated, it also generated an AC output suitable for ringing subscriber's telephones. 

Restored Western Electric dynamotor circa 1895

Fig 7, Dynamotor 

The DC current was input on the left side using a commutator to pass the current to the moving armature.  This is typical for most DC motors of this time period. The AC (ringing voltage) was output on the right side using two slip rings. It is very clever to build a device that was both a motor and generator.

However, setting and maintaining the required 20 Hz output frequency under varying loads was not easy. Some systems used a mechanical governor to set the shaft RPM.  Another method
controls the current in the two stationary field coils to set the RPM. 

The combo unit also had limited flexibility.  Separate motor-generator sets (Fig 6) can be easily adjusted or replaced to accommodate different voltage, frequency and speed requirements. Plus separate systems can scale to support large AC current demands. 

The video below explains how a dynamotor works by integrating a DC motor and AC generator.  It includes a neat kitchen tabletop demo too.  

Fig 8 shows a modern version of a dynamotor. This was used primarily in smaller public and private exchanges in the 1960's. Notice the many switch/cam interrupters on the right and some tone generators on the left.


The audio frequency of a generated single tone is related to the number of segments in its associated ring and the shaft RPM. You can see the ring’s segments on the far left and in Fig 9. This method does not use a tooth tone wheels as discussed above. Both methods were used in combo-machines. The tone wheel method is more durable because the pickup uses proximity not an eroding contact brush.  

Western Electric model 2000 ringing machine - tone generators and timing interupters

Fig 8, Model 2000, Western Electric Ringing Machine
Source: JKL Museum of Telephony

telephone exchange ringing machine tone generators slotted rings

Fig 9, segmented rings for generating tones
Source: JKL Museum of Telephony

Fig 10 depicts a device equipped solely with interrupters, devoid of tone or AC generators. This unit (two are shown) was developed by the Stromberg-Carlson company. The picture clearly shows the cams with individual on/off patterns and the switches that follow the cam’s patterns to create the on/off actions. Many of the cams generated interrupt sequences for party line ringing codes.  

telephone exchange Stromberg-Carlson multi cam/switch interrupter

Fig 10, Stromberg-Carlson multi cam/switch interrupter

Source: JKL Museum of Telephony

Here is a short, repurposed video showing Figs 8, 9 and 10 in live operation. Thanks to the JKL Museum of Telephony and Phil McCarter (sxsphil).

Ringing machine and interrupter demos

Ringing machine and interrupter demos

Play Video

All told, every exchange from the 1900’s until the 1950’s needed mechanical devices like the ones discussed in this section. Starting with the machines described above, we arrive today where all similar functions are produced using solid state methods.  





Fagen, M.D., A History of Engineering and Science in the Bell System. The Early Years (1875-1925)

Goddard, M.C., Ringing selection in No. 5 crossbar, Bell Laboratories Record, April 1950


Stone, J.R., Commercial Construction Adopted for Ringing-and-Coin Control Generators, Bell Laboratories Record, Nov. 1932

Miller, Kempster, American Telephone Practice, 1905

Van Duyne, A Tone Alternator, Bell Laboratories Record, Sept.1932



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