This collection began as a rather minimal attempt to acquire a few rules that I would like to have had back when I used slide rules, had I been able to afford them at the time. Unfortunately, or fortunately, depending on your viewpoint, the collection now seems to have grown somewhat past that. In theory, it is still based on two predominant types of rules:
general purpose 'standard' (~10 inch) linear duplex log log and hyperbolic rules, and
specialty rules that I might have used in one of the fields in which I worked, or in one of the hobbies I pursued.
But now the collection also includes a lot of other kinds of rules I would never have run across or used. They're here for a variety of really good 'reasons'. Sometimes I was just curious about a certain rule (and I thought that spending money to satisfy that curiosity was a good idea?). Sometimes a particular acquisition just seemed like a good idea at the time (but maybe not so much later!). Sometimes I got a 'good deal' (yeah, right!). Sometimes I bought one because I thought it helped the collection towards a certain hard-to-define 'completeness' (and no, I don't really know what that means). Sometimes a particular rule just 'spoke to me' (but unfortunately, usually in Russian, Czechoslovakian, Chinese, or some other language I don't read or understand). And of course there are a few cases where I just don't have a clue about why I got the rule or where it came from.
Why Collect?
Anyway, when people learn that you collect slide rules, their first reaction (after possibly, but not necessarily, polite amusement) will usually be to ask, 'Why?'. After I'd been asked that question the first twenty times, I found my reactions ranging from impatience with the questioner (why couldn't they just see how cool slide rules were anyhow?) to frustration with myself (why couldn't I quickly come up with a 25-words-or-less answer to such an apparently simple question?).
Well, whether the question is a good one or not, it has been useful for me because it has caused me to really think about my motivations. Now, I wouldn't maintain that all such 'Why?' questions demand carefully thought-out answers. Sometimes the appropriate answer is just, 'because I like it'. But personally, I'd rather have a more detailed answer than that. I suppose it's curiosity on my part. Anyway, here's what I have come up with so far.
I think the physical appearance and utility of slide rules appeal to my sense of order and 'rightness'. Math and its physical manifestations have in some ways always been comforting to me. Unlike the basically chaotic nature of a lot of our life experiences, there is a predictability and dependability to mathematical relationships that can be counted upon. Some slide rules just make me feel 'calm'.
Aside from any sense of order they might convey, many slide rules appeal to my sense of the aesthetic. Simply put, some of them are just beautiful. And some of them feel good, too.
Taken as an evolutionary category, slide rules tell a story of how the practice of math has evolved to meet man's technical demands. From the basic pre-Mannheim C-D scale devices, to the development of log-log scales, to the addition of scales to display hyperbolic trig functions, to the complexity of a Faber-Castell 2/84 Mathema, to the enormous diversity of special-purpose rules, the evolution of the slide rule has at least in some sense, mirrored the ever-increasing complexity in man's calculational needs.
Slide rules also provide interesting examples of the differences in priorities between different technical/industrial/marketing cultures. For instance, even though the mathematical demands should have been pretty much the same worldwide, why did the Pythagorean scale apparently never 'catch on' in the U.S., despite its use in Europe since the 1930's? And why did it take European manufacturers 20 years or longer to adopt the hyperbolic trig function scales that had been in use in America since 1930 or so?