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          Washington Sabatini’s impressive calculator
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
    This impressive device, the Ferrero H 39 circular slide rule, was developed in Italy by Washington Sabatini in the 1930’s. Its function is explained clearly in the Italian manual:
The purpose of the “WASHINGTON” calculators is to give engineers a quick and certain means of calculating the dimensions of reinforced concrete structures used in construction. It is also a means of relieving them from arithmetic and allowing them to choose the cheapest sections. In this way the efficiency of their work increases as well as the possibility of realizing considerable savings in the material.

Our DUCE, powerful driver of every activity, has concisely expressed his judgment with these words: “All engineers must have this device”.

-- Building Exhibition, Rome 1933      

I wouldn’t take Mussolini’s word for it (however powerful a driver he may have thought himself), but there is no question that the Washington is one impressive productivity booster.

Construction:

    At 27 cm diameter, this is one of the largest items in my collection. It comprises ten concentric aluminum rings covered with complicated scales and pointers. The rings are all movable except for the second largest; that one is fixed to the body of the device, as is the celluloid cursor overhanging the largest ring. The movable rings are rotated around the center of the device by use of the prominent crank handle that spans its radius.
    The key feature of the mechanism is the way the crank connects to the rings: each movable ring can at any time be either connected to the crank and rotate with it when it moves, or be disconnected and remain stationary. The exception is the wide outermost ring, which is always connected. What determines the state of a ring is the button on the crank that lies above it. Each button is a spring-loaded latch, and has a pin that can engage a slotted dome on the ring when it passes over it. However, should the operator pull the button up and twist it by 90 degrees, it will lock in the raised position and the pin will miss the dome, leaving the ring alone.
    This can be seen in the image below. In this photo the first two and the last four buttons (counting from the right) are lowered and engaged with their rings. The third button, marked M, is also lowered, but has not yet engaged its dome; once the crank is moved clockwise the pin will climb the shoulder of the dome and click into the slot, engaging its ring. Meanwhile the fourth button (marked Sigma) is locked in the raised position and will miss the dome as it passes over it; that ring will remain stationary.
Sabatini/Ferrero H 39 slide rule - crank latch mechanism
Click photo to enlarge
    Each ring carries a number of scales, and between one and three indices (the pink circles with a pointer) that point to the scale on the next larger ring. The multiplicity of scales and indices is due to the many types of concrete structures that can be calculated: rectangular beams, T beams, pillars, etc. Tiny icons reflecting the respective shapes tell you which calculation each index or scale is to be used for.
    The identities of the various scales are given in a detailed foldout page in the manual. They represent loads, dimensions, moments, stresses, and other parameters that I would no doubt appreciate were I a civil engineer.
    The bottom of the instrument is closed by a round plate that also carries four rubber feet; when you remove the plate you can see the complex baseplate that holds the rings snugly in their orbits.		 Sabatini/Ferrero H 39 reinforced concrete slide rule diagram
Click photo to enlarge
This baseplate has grooves where spring-loaded retainer pins both guide and constrain each ring’s motion. In the close-up photo you see some of these retainers, as well as the screws that hold the slotted domes in place. The screw at the left edge of the photo, beyond the grooves, fastens the fixed ring to the baseplate.
Sabatini/Ferrero H 39 reinforced concrete slide rule - inside view  Sabatini/Ferrero H 39 reinforced concrete slide rule - inside view
Click a photo to enlarge
    The calculator comes in an octagonal padded box, and in its lid are fitted two aluminium arcs that are plug-in additions to the scales in the fifth ring, where they can be secured with two pins that fit small holes on that ring.

Operation:

    To effect a calculation, the rings are all engaged to the crank, which is then turned to set each of them in turn -- going from the outside towards the center -- to point to the correct value on the next larger ring. After each ring is set its button is raised to disengage it from further motion. When all rings have been set, the wide outermost ring that has been carried with the crank throughout this exercise will be found -- mirabile dictu! -- to show the correct result under the cursor’s hairline.
    To better illustrate this, here is a step by step example taken from the manual. The photos represent the instrument’s state at the end of each step.

Sabatini/Ferrero H 39 reinforced concrete slide rule in case
Click photo to enlarge
 Problem: Calculate the height of a rectangular beam of 6 meter span which must withstand a load (including its own weight) of 12,000 kg. Assume a bending moment coefficient of 1/8, permissible stress of cement and iron 50 and 1400 kg/cm2 respectively, a modular ratio of 10 and a beam width of 30 cm.

Preparation: Lower all buttons and turn the crank full circle to engage all the rings.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 1: Rotate the crank to bring the P index [Total load] to 12,000 kg and raise the P button.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 2: Bring the L index [Beam span] to 6 m and raise the L button.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 3: Bring the M index [Bending moment coefficient] to PL/8 and raise the M button.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 4: Bring the Sigma index for rectangular beams to the intersection point of the arc 1400 [permissible stress of iron] with the curve 50 [permissible stress of cement] on the scale with a white background (the blue background is for iron calculations) and raise the Sigma button.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 5: Raise the m button [Modular ratio] -- the index is already on the value 10 so no rotation is required for this ring.

Step 6: Raise the s button. This parameter is not used in the calculation of rectangular sections; for such sections the two black arrows must always coincide, and since they do, no rotation is required for this ring.

Step 7: Bring the b index [Beam width] to the value 30.
Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
Step 8: Since the ::: index [Percentage of reinforcement in compression relative to that in tension] is already at zero, as it should be (there is no need for reinforcement in compression), this ring is left alone.
Step 9: The result can now be read under the cursor’s hairline on the outer ring. This ring has multiple scales, whose purpose is indicated by marks placed on the cursor itself. Here we use the outermost scale, marked "h" (beam height) on a rectangular icon (rectangular beam). The celluloid has yellowed, but this is clear if you enlarge the photo. The result, under the hairline, is 70 cm.

    That’s it! And if you think that was difficult, remember that we’ve just solved an equation with seven variables -- try that with pencil, paper, and a standard slide rule! The Duce, it seems, got it right that time: every construction engineer would benefit greatly from Sabatini's invention.

 Sabatini/Ferrero H 39 reinforced concrete slide rule
Click photo to enlarge
The inventor:

    Washington Sabatini (1890-1966) was a civil engineer from Oneglia in northern Italy. He served in the first world war as an officer in the Italian Engineering Corps, and subsequently applied himself to the development of calculating aids for reinforced concrete problems. He produced three calculators in this domain: the first model (seen here) came out in the mid 1920s, the second (here) around 1929, and the H 39 -- the device shown in this article -- arrived in the late 1930s (my unit’s manual was printed in September of 1939).
    Sabatini produced the first two models himself, but the H 39 was manufactured by the brothers Cesare and Angelo Ferrero in Savona. Cesare and Washington had, in fact, been classmates in engineering school, and the Ferreros had a workshop that produced various engineering instruments. Washington did well to involve them: the exquisite quality of their work is very evident in the H 39!
Exhibit provenance:
     eBay, from a seller in Romania.

More info:
    Here is a scan of the manual.

    The French version of Sabatini’s patent for this device, issued in 1938.
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