Tuesday, July 17, 2007
This evening I sat down to start restoring a fine old Dring & Fage slide rule, and I finally remembered to do something I've been meaning to do for a long time: show a “before” and “after” photo.
This particular slide rule dates back to the mid-1800s. It's an unusual style, with two slides on each side of the body. The condition of this example is superb, with almost no damage or even signs of wear. The only notable problems are (1) it is filthy, as you might expect from something 150 years old, and (2) the parts fit too tightly to move smoothly.
In the photo above, the top piece is a segment of an uncleaned slide and the bottom piece is a segment of a cleaned slide. This rule has four slides plus a rather large body – it took me 45 minutes to clean the first slide, so the whole rule will probably take about 5 hours to clean.
The cleaning process is simple, but tedious: I soak an old washrag, fold it into quarters on my worktable, sprinkle some Bon Ami onto a corner of the washrag, and then carefully scrub one little piece of my work at a time. The trick is to scrub hard enough to get off all the soiling, but not hard enough to do any damage to the finish – the only way I know how to do this is trial-and-error, starting with very little pressure and working my way up.
Once I've thoroughly scrubbed the entire workpiece, I inspect it carefully for any firmly adhered stuff. These chunks I carefully work off with dental picks. This particular piece had a few specks of something that looked like wax, plus a few paint droplets. Most of the pieces this old that I restore have far more adhered junk.
The last cleaning step is to rinse the workpiece, which is more of a challenge than it sounds. Slide rules like this have very finely engraved lines on them, and if they're made from certain woods (such as mahogany) open pores. These engravings and pores accumulate soap, feldspar powder from the Bon Ami, and fine particles of whatever the slide rule was exposed to in its life. All of this stuff must be rinsed out, and usually the force of the tap water is not enough – I also have to scrub out all the pores with a very fine, stiff-bristled brush. That last bit can be quite tedious!
Once I've cleaned the entire slide rule, and all the parts are thoroughly dried, I'll apply several coats of Johnson's Paste Wax. That will protect any bare wood that's been exposed, and it will fill in small pores and scratches. I let the wax dry for a few hours and then polish it up with a shoe-polishing rag. In the photo above, the bottom slide has been cleaned but not waxed; it will be a little darker and much shinier once it's been waxed.
One interesting thing I've learned about these old wooden slide rules is that they are impervious to water. When I first started restoring these instruments, I was afraid to get them even slightly wet – I figured that would raise the grain and ruin the finish. After a few accidental soakings caused no harm, I started getting bolder about the use of water – and now I don't hesitate to put a 200 year old instrument directly in the tap water. The only slide rule I've ever damaged with water was an unfinished wooden rule made by Lawrence (an American manufacturer of crude, low-end slide rules), and even that one was easily fixed with a little fine sandpaper. The combination of water and Bon Ami cleans the old wooden instruments better than anything else I've ever found…
This is a fine example of a watch-style circular slide rule, albeit a very large one – it is 87mm (about 3½ inches) in diameter. Click on the photo at right for a large version, or visit my slide rule collection for all the details.Using one of these slide rules is a bit more work than an ordinary linear slide rule (turning the knobs is slightly tedious), but they pack a lot of accuracy into a very small package. With its three-segment D scale, this one can multiply and divide with the same accuracy as a linear slide rule that is 49 cm (over 19 inches) long – not back for something that fits conveniently into your pocket!
The Fowler watch-style slide rules (they made many models) seem somehow quintessentially English to me: heavy, sturdy, practical, and yet quite sophisticated. I know of no American-made slide rule even remotely like this one, though several American manufacturers made circular slide rules. There were several English manufacturers of fine watch-style circular slide rules, at least one French manufacturer, and even the Soviets made a cheap imitation. But the English models have a unique style that, for whatever reason, was never duplicated outside of England…
Via NBC San Diego, this news (more photos here) of a hit-and-run wreck that seriously injured a mother of four, but spared her kids:
Police said that the driver of a pickup truck took off running after he hit a truck with a mother and her four children inside.
The wreck took place on Monday afternoon on Melody Road in Jamul.
According to investigators, the truck with the family inside was following behind the suspect's pickup, which had pulled over to the shoulder. When the family drove by the truck, its driver pulled out to make a U-turn and slammed into her truck. Authorities said the driver fled the scene. A passenger riding inside the vehicle stayed with the pickup, investigators said. The children were apparently unharmed in the wreck, but their mother was seriously injured.
The driver fled the scene on foot. Given that the police have his truck and his passenger, one would think the police would be able to identify, locate and apprehend him quickly. I sure hope so...
I don't know anything further about this incident. If any of my readers know more, please leave comments with the additional information.
Imagine you could travel back in time … say, a couple thousand years. Would you be able to teach people about technologies that are now common, but were unknown then? That's the question The Universe As asks:
If you were to travel 2000 years into the past, how useful would you be in jumpstarting technological advancements? This 10 question quiz will help you figure out your technological usefulness. If you do poorly on the quiz, as most people likely will, then just let that inspire you to study up more on how things work and where raw materials come from.
Take the quiz, and leave your score in the comments. I scored 8 out of 10.
This slide rule (click to enlarge the photo at right) was made around 1897 and sold by Keuffel & Esser. It may have been actually manufactured by Dennert & Pape (in Germany); they made the earlier versions of this model and I'm not sure when Keuffel & Esser actually took over the manufacturing themselves. I've posted this up on my collection web site, including high resolution scans and lots of details.
This one posed an interesting restoration challenge that I had not run into previously – sometime in its history, someone spilled a liquid onto one side of it, and then let the liquid evaporate without cleaning it up. This left a layer of gunk on the slide rule that looked a lot like rust, and was quite difficult to remove – no solvent I have would dent it; I had to soak and abrade it (using Bon Ami). Some staining still remains; I could see under a microscope that the colorant has migrated into the plastic.
In addition, this slide rule had 110 years worth of the grey “gunk” that seems to accumulate on anything that humans touch. I probably really don't want to know what's in it! Usually this gunk cleans off easily with soap or alcohol, but on this slide rule some of the gunk had hardened into something resembling granite. To clean the slide's tongues and grooves I had to resort to carefully picking off the rocky gunk with a dental pick – hard work for these old eyes...
I thought you loved me!
Each summer we depend on our air conditioner to keep our home's temperature at a livable level – otherwise it would be 100°+ in the house, and darned uncomfortable. We have a completely conventional air conditioner (for the U.S.): a standard electric unit that depends on the evaporation and condensation of a synthetic refrigerant. It works great, and keeps our house at a very comfortable temperature – but it costs a small fortune to run. Surely there must be a better way!
And there is.
In any dry part of the country (and we certainly qualify for that!), evaporative cooling is a technology that costs much less for any given cooling capacity. This technology (which comes in several forms) leverages a simple fact about water: it absorbs a lot of heat (2272 Joules) for each gram of water evaporated. Our air conditioner is a “five ton” unit (60,000 BTU/hour). To provide an equivalent amount of cooling, we'd need to evaporate water at the rate of about 7.4 gallons per hour.
Two forms of evaporative cooling are common in homes in certain areas of the U.S.: misting systems and “swamp coolers”. Misting systems force water at high pressure through tiny holes, creating extremely fine droplets that evaporate almost immediately, cooling down the air around them. Many people in Arizona, New Mexico, and other places use misting systems to cool their patios. There are a few of these in the San Diego area, but they are not common. Swamp coolers work by blowing air through a porous pad that is kept wet, usually by rolling the pad continuously through a tub of water. The air is then routed into the home, directly cooling the house. These systems are common in some areas (again, in Arizona and New Mexico), but they have one big drawback: they raise the humidity in the house to undesirably high levels.
The third form of evaporative cooling technology isn't found (to my knowledge) in homes at all, though it is quite common in industrial buildings. This is the “forced-draft cooling tower”, which works by forcing air over droplets of water to cool the water. A conventional heat exchanger then runs a separate loop (of refrigerant or chilled water) to heat exchangers in the building being cooled. These cooling tower system are slightly less efficient than swamp coolers (as there are some thermal losses in the heat exchanger loop), but they have the great advantage of not humidifying the air in the building being cooled. The only reason I can think of that these are not used for homes is that they are relatively complex pieces of machinery. A small capacity cooling tower is just as complex as a large one, and would probably be much more expensive to build than a small capacity refrigerant-based air conditioner. But much cheaper to run!
To cool our home on a typical summer day, I estimate that we'd have to evaporate about 40 gallons of water. I'm exploring the notion of building my own cooling tower to replace our conventional system. The basic engineering challenge is to create enough droplets, with air blowing over them, to evaporate water at the required rate of 7.4 gallons/hour; I've not yet found any reference that would help me design this. The rest of it is very straightforward, basically just plumbing.
Any “evaporation engineers” out there?
This evening I sat down to start restoring a fine old Dring & Fage slide rule, and I finally remembered to do something I've been meaning to do for a long time: show a “before” and “after” photo.
