Comptometer

Introduction
Calculation
"Controlled-Key" mechanism
Videos
Comptometer Simulator
Models
My Model E Comptometer
My Model J Comptometer
Super Totalizer
My Super Totalizers
Books and Manuals
Tables
Comptometer Magazines
Advertisements, Booklets, Leaflets, and Letters
Newspaper Articles
Paraphernalia
Patents
Links

The Comptometer is a mechanical calculation machine. It was invented Dorr E. Felt, and made by the Felt & Tarrant company in Chicago, USA. Felt gave a lecture about the history of mechanical calculators, which was published in 1916, and in it he briefly mentions how he came to make the first prototype of the machine in 1885. Its various models were in production from 1888 to 1961, peaking during the 1930s.

The "shoebox" case of the most common models is made of copper plated steel that is painted brown. Any Comptometers that seem to be made of shiny copper have had their original paint stripped or polished away.

The keyboard usually consists of 8, 10 or 12 columns. Each column has 9 buttons numbered 1 to 9 from nearest to furthest. To help with touch-typing, the tops of the even-numbered keys are almost flat, while the odd-numbered keys are concave.

In front of the keyboard there is a register. It is a row of small round windows through which you can see the digits of the calculation result. There is one more window/digit than there are columns of keys.

A top plate of the case extends some distance beyond the keyboard, and onto this is attached a chromed plate that bears the "Felt & Tarrant" name.

Calculation

The Comptometer is a direct adding machine. This means that when you press and release any of the numbered keys, its value is immediately added to the total shown in the register. Adding a set of numbers is therefore as simple as typing them all in. It has "duplex" functionality, meaning that it is possible to press keys from different columns at the same time to speed thing up.

The large lever on the right will clear the register, setting it to zero.

Subtraction is done by adding the tens-complement. The small digits on the keys are helpful for achieving this. To subtract a number, you must first mentally subtract 1, and then type it using the small digits. Precede the number by as many leading zeros (actually nines) as is needed to reach past the left of the current value in the register. The small metal tabs are used to block a carry, and using one here avoids the necessity of pressing any further leading zeroes in the number you are subtracting.

Multiplication is done by repeated addition. If you can press the keys for entering one of your factors all at the same time, it is easy to press that set of keys several times to get multiples of that number. Shift your fingers one column to the left to add multiples of ten times your number, and so on for higher columns.
For example to multiply 6798 by 24, press 6798 down four times in the last 4 columns, shift a column to the left and press the same pattern of keys twice.

Many tricks were devised for other kinds of calculations. To increase speed and touch-typing accuracy, operators often only used the bottom half of the keyboard. Instead of entering a high digit such as 8 directly, they would press 4 twice. This reduces the need for hand movement, and is easier to do without looking.

"Controlled-Key" mechanism

If you press down on a key but release it before it is pressed all the way in, the mechanism of the Comptometer would not be able to register the digit you wanted. To avoid this leading to calculation mistakes, a safety mechanism was used. When a partial key press occurs, all the keys in the other columns become locked. The partial key press will not have affected the number in the register, so you can then press the intended key completely to rectify the problem. You will have to press the red button at the back right to release the locked columns to continue.

Videos

Here is a video where I demonstrate the various features of my Model J Super Totalizer Comptometer.

In this video I demonstrate how to do the four basic arithmetic operations on the Comptometer.

This video shows the Model E Comptometer.

I gave a talk at the Gathering for Gardner in 2016 about the history of the Comptometer. The first video was a practice run, and the second video is the actual talk.

Comptometer Simulator

I have created a simulation of the Comptometer using javascript. If you click the link below, the simulation will open in a new window. If you are using a touch screen, then on most browsers you will be able to press multiple keys at the same time.

Start the Comptometer Simulator

Models

The first model (with a wooden case) and the subsequent models A to E are very rare, and won't be discussed here in detail. The most common shoebox models are F, H and J. The serial number limits are based on observed examples, so machines with numbers outside these ranges may exist.

Model	Years	S/N range	Number	Characteristics
Wood	1887-1903	10-6244	6300	Wooden case Front of case is right-angled, with a flat glass window over the register. Complicated clearing mechanism with a wheel and a small lever.
A	1904-1906	15000-20603	5700	First model with copper-plated steel case. Front of case is right-angled, with a flat glass window over the register. First model to have Duplex (multiple key press) functionality. First model to have a clearing handle.
B	1906-1909	25000-33461	8500	First with S-curve shaped front of case. Carry-blocking tabs are shaped and closer to keys.
C	1909-1912	35000-38602	3700	Keys made of composite
C-light	1911-1912	40000-48648	8700	Keys made of celluloid Lighter key action
D	1913-1913	49000-49154	200
E	1913-1915	54000-61612	7700	First to have the Controlled Key system. Only model to use guards on the side of the keys to detect inaccurate keystrokes. White Controlled Key release button
F	1915-1920	100000-142337	42400	Front side panel of case has no decoration. Clearing lever pivot is located just beyond the 9 keys. Clearing lever angled towards the user. Rear side panel of case lists series of patents. Embossed decoration on the side panels is the same colour as rest of the case. First with a red controlled key release button Keys are black and white.
H	1920-1927	200000-248508	48600	Front side panel of case has Comptometer logo. Clearing lever pivot is located between the 4 and 5 keys. Clearing lever angled vertically. Rear side panel of case has Comptometer logo. The name plate lists the patents. Decoration on the side panels of the case are painted black to contrast with the case. Keys are black and white. Case is a centimetre longer at the front, with the S curve less steep, to make room for the new clearing mechanism.
J	1928-1938	J245489-J346384	100900	The serial number is preceded by the letter J. Keys are green and white.
K	1934-1950	350000-??????		Electrically driven. Metal casing, painted very dark brown. Keys are all the same length. Casing has a rear leg to tilt the keyboard. Clearing lever ends shaped like a tab rather than a round handle.
M	1939-1950	400000-??????		Rounded metal casing, painted dark green. Clearing lever inside the casing instead of on the side. New Comptometer logo on the front. Carry-blocking tabs have small white/green key tops. First with shutters that hide leading zeroes automatically.
3D11	1950-1957?			Controlled-key release button in front of keyboard, and green instead of red. Automatic Controlled-key release button on the left of keyboard, useful in multiplication/division. No Comptometer logo on the front. Decimal pointers are below the register.
992/99C	1950-1961?			Electrically driven version of the model 3D11. No automatic Controlled-key release button on the left of keyboard. No Comptometer logo on the front. Decimal pointers are below the register.
616CE	1957-1961?			Better known as the "Customatic". Electrically driven, similar to the 992. Electric clearing mechanism. A slider to the left of the keyboard for setting the "Customatic Key Control". Light grey casing. Decimal pointers are below the register.

There is little or no external difference between the H and J models. They use exactly the same casing, and even their serial numbers overlap during the time when the stock of H model mechanisms were used up and the production of the J model mechanisms was built up. There were apparently several internal mechanical improvements, so the ones with the improved mechanisms had their serial number preceded by the letter J. They started using green keys on the model J instead of black ones, but as keys were often replaced this is not a completely reliable characteristic.

The electrically driven model K was like an electric typewriter in that the electricity was used to generate the force needed to activate the mechanism so that the key presses could be kept very light. It had a redesigned case that raised the rear end so that the keyboard sloped at the same angle as the previous models to compensate for the fact that all keys now had the same amount of travel.

The model M also has a variant built during the war, dubbed the model WM. Due to a metal shortage its parts were redesigned to use less metal (e.g. internal plates and levers were thinner and had holes punched in them). It looks the same as the normal model M but is much lighter in weight.

My Model E Comptometer

This is a 12-column Model E Comptometer. This model was the first to include the "Controlled-Key" error detection, but also has guards on the side of the keys. If the metal guard is pressed down without first pressing the button itself, then the key will not travel further. This guards against the user accidentally catching an adjacent button during a button press. This system was dropped in subsequent models, probably because it was too complicated, the keys were prone to breaking, and because the problem it is guarding against is not common in trained operators.

My machine has serial number 56337, and was made in 1913 or 1914. This Comptometer is in fairly good condition, except that the left-most column of keys is locked. Presumably the guard system for that column is stuck. The zeroing mechanism often fails to fully reset when the handle is moved slowly. The Controlled-Key reset button is normally white in the Model E, but has been replaced here with a later red one. Some of the keys have broken during its working lifetime causing the key rod to come away from the flat stem of the guard. These keys have been patched up either by binding them together with a bit of metal wire, or by riveting them together with the key in the depressed state, effectively disabling the guard for that key. It is interesting to see that the base plate of the case for the 12-column model was built by joining together two smaller ones.

Comptometer Model E, register cover removed

Comptometer Model E, removed fixed key, front

Comptometer Model E, removed fixed key, back

Comptometer Model E, removed fixed key, side

See the Video section on this page if you want to see the key guard system in action.

My Model J Comptometer

This 10-column Model J Comptometer looks rather different from its original state. In the late 1940s or the 1950s it has been reconditioned, during which the casing has been wrinkle painted green, and the key-tops replaced with those from a model M or later. It has unfortunately been stored in poor conditions - the paint has discoloured and has begun to flake off.

Its internal mechanism is in fine condition, and needed only a little bit of lubrication to make it work perfectly.

This machine has serial number J314420. It has been said that the first two digits of the J number roughly match the year of production, but that is not quite accurate. The numbers range from about J245489 to J346384, which were produced from 1926 to 1938, so the machines were made about 2 to 4 years later than the first serial number digits indicate. My machine was therefore probably made in 1934.

Comptometer Model J, Register internals close up

Comptometer Model J, Mechanism bottom side

Super Totalizer

A special version of the Model J was made, called the Super Totalizer. The mechanism and case of the model J is extended at the front to add a second register. This register has a lever on the right that causes the number shown on the normal register to be added to it, and which also then clears the normal register. The lever on the left clears the extra register.

The extra register also has a row of buttons that when pressed and locked down move a cover over the corresponding digit to hide it. Any carry operation from a covered digit to a still visible one is suppressed.

This version has the normal J serial number, but also a second serial number especially for the Super Totalizer.

Below is a list of serial numbers I compiled mostly found in online collections and auctions:

J/M number	S number	Keyboard	Source
J314265	S602	8 columns	Online auction
J315705	S691	12 columns	Zsolt Árpád Papp
J315804	S739	10 columns	Paul Breedveld
J316912	S843	10 columns	Fred Haeghens
J316821	S1131	8 columns	Rechenmaschinen illustrated, H.van Noort
J316833	S1151	8 columns	eBay, Bob Otnes
J317799	S1308	12 columns	Jay M. Goldman
J317808	S1318	12 columns	Computarium, François Koeller
J317834	S1335	12 columns	Worthpoint / eBay
J317903	S1415	12 columns	My own.
J321777	S1625	10 columns, sterling (no farthings)	John Wolff
J324639	S1854	10 columns	Computer History Museum
J324703	S1919	10 columns	Robert Gotlieb
J324720	S1928	10 columns	Rechner lexikon, Valéry Monnier
J324716	S1939	?? columns	Arithmeum
J324725	S1957	10 columns	Cris Vande Velde
J327695	S2321	12 columns, sterling (no farthings)	John Wolff, not shown on his website
J339128	S2594	8 columns	Cris Vande Velde
J339163	S2620	8 columns	National Museum of American History
J342586	S2708	12 columns	Online auction
J342835	S2973	10 columns	Arithmeum
J342848	S2992	10 columns	Bart Depuydt. In poor condidtion.
J344930	S3080	8 columns	Mathematisches Maschinenmuseum
J345247	S3150	10 columns	My own. In very poor condition.
M514283	S3729	10 columns	Zsolt Árpád Papp

It is not clear whether the S numbering started at 0 or 500, and it probably did not go further than 3300 on the model J, so it is likely that between 2800 and 3300 J Super Totalizers were produced.
In some intervals, such as between S1308 and S1415, the S numbers rise a little faster than the J numbers. Note also that the J316912 entry on the list has a contradictary S number. This might mean that some previously produced J Comptometers were being converted to STs, or that the numbers in a production batch were not assigned in a strictly matching order.

For some reason the Super Totalizer was hardly ever shown in advertisements. Instead, the electric model K was promoted much more heavily. There was no Super Totalizer version of the model K, but later there were ST versions of the mechanical model M. It seems that the ST models more popular in Europe and India than in the USA. It is probable that the model J Super Totalizer was assembled in the Netherlands, and is therefore more common there. Similarly, the model M ST is mostly found in Italy.

My Super Totalizers

Here are some pictures of my Super Totalizer (J317903/S1415). It is in very good working condition, but has had some repairs or restoration done. In particular, 9 of the key tops have been replaced with reproductions, and the celluloid windows have been replaced with more modern plastic.

I have recently acquired a second ST in rather poor condition (J345247/S3150), but which can be used for spare parts. It has 10 columns of keys, but their plastic tops have degraded, and some of them are missing. The black and white ink of the key numbers had expanded and spread, so I have cleaned the keys before taking these pictures. The clearing handle is badly bent, some screws are missing, as is one of the extra register's blinds and its plastic window. The mechanism had seized up, especially the mechanism of the extra register, but I have managed to get it mostly working again just by spraying in some silicone lubrication. Unfortunately the clearing mechanism tends to get stuck and I have not yet found the cause of that.
The most interesting thing about this ST is its late model number, which allows a fairly accurate estimation for the number of model ST Comptometers produced.

Books and Manuals

There are too many books to list here, so I have put them on a separate Comptometer Books and Manuals page.

Tables

Felt & Tarrant printed many small tables to help the operator with common calculations. Such tables were usually printed on thick cardboard for durability. In the 1920 edition of "Easy Instructions for operating the Controlled-Key Comptometer" a list is given of various tables that were available at that time. I reproduce that list here. The ones on this list that I own are marked by an asterisk.

Form Number	Title	Description
1	Tonnage Table	Gives the decimal part of a ton of any number of pounds.
3	Lumber Table	Gives result of dividing 12 into any dimension up to 11 7/8.
4*	Discount Table	Gives the net of chain discounts.
5	Lumber Table	Gives number of board feet in pieces of various sizes.
6	Gross Table	Gives the decimal for each fractional part of a gross and for each unit.
7*	Grain Table	Gives the decimal part of a bushel of any number of pounds of different grains.
8	Ten and 100 and 1000 X Inches and Fractions to Eighths Table	Used only in connection with the Engineering Model Comptometer for figuring beams, pipe, etc.
9	Reciprocal Table	Used for making division quickly and easily by multiplication.
10	British Currency Table	Gives the decimal part of a £ of any number of shillings and pence.
12	British Currency Table	Gives the decimal part of a £ for pence and 32nd fractions.
21	Quarters and Pounds of a Ton Table	Gives the decimal part of a ton of any number of Quarters and Pounds.
22	Quarters and Pounds of a Cwt. Table	Gives the decimal equivalents of a Cwt. from 1/2 lb. to 3 qrs. 27 lbs.
24	Interest Table	Gives the interest on $1.00 for different numbers of days at different rates.
25	Net Discount Table	Shows the net left from $1.00 after discounting for the required number of days.
26	Decimal Table	Used for figuring elapsed time. It gives the decimal part of a 360-day year up to any date.
29*	Decimal Equivalents of Fractions Table	Gives decimal equivalents of fractions from 1/4 to 63/64.
32	Cooperage Table	Used to determine how many staves of one size are equivalent to another size.
33*	Interest Table	Gives the interest on $1.00 for one day for rates from 1/8 % to 12 7/8 %.
35*	Payroll Table	Gives the decimal part of a month for any number of days and eighths of days.
36	Insurance Cancellation Table	Shows decimal part of the year for months and days.
37	Figuring Elapsed Time Table	Shows decimal part of a year for each day.
38	Pounds, Ounces, and Drams Table	Gives the decimals of a 16-oz. pound for any number of drams and ounces.
184	Interest Table	Gives the interest on $100. at 7% for from 1 to 364 days.
191	Grain Table	Gives the Dockage and the net bushels per 1000 lbs. for different rates of dockage.
205	Weights per Foot and Inch for Rounds, Squares, etc. of Different Thicknesses Table	Enables one to get the weight of a piece.
234	Iron and Steel Table	Gives the price per 100 lbs. at .01 to $100. per gross ton.
237	Interest Table	Used for figuring interest on a 365-day basis, where interest rates run from 1 to 12%.

Decimal Equivalents of Common Fractions To be used in Connection with the Comptometer.
Felt & Tarrant Mfg. Co., Chicago, U.S.A.
Series No. 2
108mm × 164mm

This card was included when I bought the 1911 edition of "Methods of Operating the Comptometer", so is likely from around that time, especially since the picture on it is of a model B or C Comptometer. The card is signed on the back by Adelina Artesani who according to the 1900 and 1910 census was born in 1894 and resident in Boston, MA.

Decimal Equivalents of Common Fractions To be used in Connection with the Comptometer.
Comptometer Division, Felt & Tarrant Mfg. Co., Chicago, Illinois
Form 29
102mm × 235mm

This card was included when I bought the "Table of Reciprocals" book, and it supplements that book by supplying some fractions that have numerators other than 1. Unlike most of the other cards, this one is printed partly in colour. It has no copyright year, but was probably printed in the 1940s as it has a picture of the model M.

Discount Table Showing net of $1.00 after discounts, shown at top and side, are taken off.
Comptometer Co., Chicago, U.S.A.
Form No. 4
165mm × 279mm

This card has no copyright year, but was probably printed in the 1940s as it has a picture of the model M. It is likely that earlier prints of this card exist.

Grain Table Pounds Expressed in Decimal Parts of a Bushel.
Felt & Tarrant Mfg. Co., Chicago, U.S.A.
Form 7
117mm × 267mm

This card has no copyright year, but was probably printed in the early 1920s as it has a picture of the model H.

Interest Table (360-day basis) To be used in Connection with the Comptometer
Felt & Tarrant Mfg. Co., Chicago, U.S.A.
Form No. 33
225mm × 159mm

This card has no copyright year, but was probably printed in the early 1920s as it has a picture of the model H.

Pay Roll Table Decimal Equivalents of Days and Eighth Fractions of Days, for 24-31 Day Months also Minutes expressed in Decimal Equivalents of an Hour.
Comptometer Co., Chicago, U.S.A.
Form No. 35
195mm × 322mm

This card carries a copyright year of 1925, but it must have been a reprint from the 1940s as it has a picture of the model M.

Decimal Equivalents of Fractions from 3rds to 26ths inclusive To be used in Connection with the Comptometer.
Comptometer Co., Chicago, U.S.A.
Form No. 368
185mm × 235mm

This card has no copyright year, but was probably printed in the 1940s as it has a picture of the model M.

Decimals of a Foot for each 1/8 of an Inch To be used in Connection with the Comptometer in Figuring Lumber, Steel Beams, Angles, Etc.
Comptometer Co., Chicago
Form No. 386
152mm × 253mm

This card carries a copyright year of 1925, but it must have been a reprint from the 1940s as it has a picture of the model M.

Decimals of a Foot card, hi-res scan front

Decimals of a Foot card, hi-res scan back

Ounces and Drams Reduced to Decimal of a Pound Table of Decimal Equivalents to be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 3
253mm × 192mm

This piece of paper has no copyright year, but it has a picture of the Model H or J, and I suspect it is from about 1928. It is printed by the UK company, and lists the various Felt & Tarrant branch offices around the country.

Ounces and Drams Reduced to Decimal of a Pound, scan

WAGES TABLE FOR 48-HOUR WEEK To be used in connection with the Comptometer
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 5
304mm × 178mm

For Calculating TIMBER SPECIFICATIONS Table to be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 6
354mm × 240mm

For Calculating Timber Specifications, scan

For Calculating the Weight of Deals, Battens, and Boards at 66 cubic feet to the Ton Table of Decimal Equivalents to be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 7
228mm × 240mm

For Calculating the Weight of Deals, Battens, and Boards, scan

GROSS, Each Fractional Part & each 144th To be used in connection with the COMPTOMETER. For Figuring Gross Invoices, Inventories, Plate Glass, etc.
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 8
209mm × 260mm (1925), 203mm × 256mm (1934)

Gross, Each Fractional Part and each 144th

Gross, Each Fractional Part and each 144th, scan

This is a later print of the same table. It has a picture of the Model K so probably dates from about 1934.

Decimal Part of a Year for Each Day on a 365-Day Basis To be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 9
184mm × 360mm

This piece of paper has a copyright year of 1914, though it was printed at a later date. From the dates used in the examples printed on the back I suspect it is from 1928. It is printed by the UK company, and lists the various Felt & Tarrant branch offices around the country.

Decimal Part of a Year for Each Day on a 365-Day Basis, scan

Annas & Pies as Decimal of One Rupee Table of Decimal Equivalents to be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 10
328mm × 211mm

Annas and Pies as Decimal of One Rupee, scan

QRS. & LBS. into CWT. and QRS. & LBS. into TON To be used in connection with the COMPTOMETER
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 21, Dec. Card No. 22
151mm × 239mm (1925), 164mm × 264mm (1934)

This is a later print of the above tables. It has a picture of the Model K so it is probably from about 1934.

Decimal Equivalents of Common Fractions To be used in Connection with the Comptometer.
FELT & TARRANT LTD., Aldwych House, Aldwych, London, W.C. 2.
Dec. Card No. 29
127mm × 203mm

This card has no copyright year, but I suspect it is from about 1928. It is printed by the UK company. It has a list of useful conversion factors on the back.

Decimal Equivalents of Common Fractions, front

Decimal Equivalents of Common Fractions, back

Decimal Equivalents of Common Fractions, scan, front

Decimal Equivalents of Common Fractions, scan, back

Pence & Sixteenths of One Shilling and Pence & Thirty-Seconds of One Shilling To be used in connection with the COMPTOMETER
Felt & Tarrant, Ltd., London.
267mm × 209mm

This piece of paper has no copyright year but probably dates from 1934. It is printed by the UK company, and lists the various Felt & Tarrant branch offices around the country.

Comptometer magazines

This section has been put on a separate Comptometer Magazines page.

Advertisements, Booklets, Leaflets, and Letters

There are too items to list here, so I have put them on a separate Comptometer Advertisements, Booklets, Leaflets, and Letters page.

Newspaper Articles

I don't own any physical copies of Comptometer newspaper articles. I have however found quite a few interesting articles in online newspaper and magazine archives. There are too many to list here, so I have put them on a separate Comptometer Articles page.

Paraphernalia

There are too many paraphernalia to list here, so I have put them on a separate Comptometer Paraphernalia page.

Patents

The Felt & Tarrant company, and Dorr Felt in particular, filed a lot of patents for all aspects of their inventions and designs. Below is a list of those that relate to the shoebox Comptometer. I have omitted from this list all patents for their other office machines, as well as patents for motorized or powered versions of the Comptometer. Some of the patents listed were never used in any production models, or used only for a short time on some models prior to the model F. The most important ones are displayed in bold type.

Patent	Filing date	Publish date	Name	Description
US 371,496	12 Mar 1887	11 Oct 1887	Dorr E. Felt	First version of the Comptometer, which had a wooden case.
US 568,021	14 Jun 1895	22 Sep 1896	Dorr E. Felt	Comptograph
US 661,121	2 May 1898	6 Nov 1900	Dorr E. Felt	Comptograph improvements.
US 671,109	24 Oct 1900	2 Apr 1901	Dorr E. Felt	Comptometer adapted for Sterling currency
US 733,379	3 Nov 1902	14 Jul 1903	Dorr E. Felt	Case lining to reduce sound. Presumably used in Model A.
US 762,520	29 Jun 1903	14 Jun 1904	Dorr E. Felt	Improved mechanism to reduce force needed. Presumably used in Model A.
US 762,521	29 Jun 1903	14 Jun 1904	Dorr E. Felt	Improved register. Presumably used in Model A.
US 767,107	3 Aug 1903	9 Aug 1904	Dorr E. Felt	New clearing mechanism. Presumably used in Model A.
US 960,528	16 Sep 1908	7 Jun 1910	Dorr E. Felt	Improved mechanism. Adds carry-blocking buttons. Used in Model B.
US 982,416	4 Apr 1910	24 Jan 1911	Dorr E. Felt	Mechanism to detect buttons pressed accidentally. Used later in Model E.
US 982,417	22 Aug 1910	24 Jan 1911	Dorr Eugene Felt	Mechanism to detect buttons pressed accidentally. Used later in Model E.
US 992,950	18 Dec 1909	23 May 1911	Dorr E. Felt	Mechanism to signal partial key stroke, via column indicator and bell.
US 996,009	4 Mar 1910	20 Jun 1911	Dorr E. Felt	Improved carry to reduce key force needed. Presumably introduced in Model C-Light.
US 1,003,723	6 Jan 1911	19 Sep 1911	Dorr E. Felt	Improved key-action (heavy to start, lighter through the down-stroke). Presumably used in the Model C-Light.
US 1,028,344	25 Nov 1911	4 Jun 1912	Dorr E. Felt	Controlled-key mechanism, locking other keys after partial key stroke.
US 1,066,096	12 Jul 1912	1 Jul 1913	Dorr E. Felt	Improved controlled-key mechanism, not releasing partially pressed key until fully pressed.
US 1,072,933	5 Apr 1913	9 Sep 1913	Dorr E. Felt	Complete controlled-key mechanism.
US 1,072,934	5 Apr 1913	9 Sep 1913	Dorr E. Felt	Improved clearing mechanism, reduced jamming with improper use.
US 1,074,689	30 Sep 1912	7 Oct 1913	George Steninger Bollensen	Keyboard with a fractions column
US 1,074,704	5 Apr 1913	7 Oct 1913	Dorr E. Felt	Keyboard with several columns for common fractions
US 1,074,705	5 Apr 1913	7 Oct 1913	Dorr E. Felt	More reliable clearing mechanism
US 1,110,734	3 Mar 1914	15 Sep 1914	Kurt F. Ziehm	New Controlled-Key mechanism. Presumably introduced in the Model F.
US 1,154,897	9 Oct 1911	28 Sep 1915	Joseph Abram Turck	Improved mechanism to reduce key force needed.
US 1,357,747	1 May 1918	2 Nov 1920	Joseph A. V. Turck	New clearing mechanism. Introduced in Model H.
US 1,357,748	1 Dec 1919	2 Nov 1920	Joseph A. V. Turck	Improvement on the new clearing mechanism, with bell. Introduced in Model H.
US 1,391,220	24 Apr 1920	20 Sep 1921	Joseph A. V. Turck	Early version of the Super Totalizer.
US 1,449,639	19 May 1921	27 May 1923	Joseph A. V. Turck	Controlled-key mechanism that also locks clearing lever.
US 1,714,964	12 Mar 1921	28 May 1929	Joseph A. V. Turck	Column indicator for controlled-key mechanism.
US 1,751,609	8 Oct 1923	25 Mar 1930	Joseph A. V. Turck	Block clearing handle when controlled-key mechanism active.
US 1,817,640	30 Apr 1929	04 Aug 1931	Neal Eberhart Newman; Frederick Adolph Niemann	Peg-Board system.
US 1,840,378	27 Oct 1926	12 Jan 1932	Carlos Wittenmyer	Carry blocking levers that are automatically released by carry.
US 1,902,597	29 Sep 1927	21 Mar 1933	Joseph A. V. Turck	More reliable clearing mechanism. Presumably introduced in Model J.
US 1,927,856	29 Sep 1927	26 Sep 1933	Joseph A. V. Turck	Operator-controlled locking of key columns and digits.
US 2,104,051	16 May 1935	4 Jan 1938	Frederick A. Niemann	Super Totalizer digit blinds.
US 2,130,364	5 Nov 1934	20 Sep 1938	Frederick A. Niemann	Super Totalizer.
US 2,149,817	5 Nov 1934	7 Mar 1939	Frederick A. Niemann	Super Totalizer.
USD 113,817	26 Aug 1938	14 Mar 1939	Frederick A. Niemann, Alfred L. Mell	Model M case design.
US 2,273,491	28 Apr 1939	17 Feb 1942	Haldon A. Leedy	Vibration proofing mounting of mechanism and case
US 2,346,601	24 Feb 1941	11 Apr 1944	Frederick A. Niemann	Hide leading zeroes
US 2,753,117	22 Sep 1954	3 Jun 1956	Emil C. Walker	Customatic mechanism

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