A photocopied coil winding slide chart

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The Xeroxed calculator

Back when I was a teenage radio amateur homebrewing my own gear, I'd go through a lot of vexing trial and error to figure the correct design for winding radio frequency coils of a desired inductance. One day I discovered that a friend working in an electronics lab had a special slide rule for coil calculations. It was made of cardboard, with an inner movable part visible through various cutouts in the outer shell.
Unfortunately, he couldn't spare it, and it wasn't available in any store I could access. Necessity being the mother of invention, I resorted to a Xerox copier and reproduced the faces

Click a photo to enlarge

of all the layers. I then produced a fairly accurate replica of the coil calculator and used it happily for many years.
One side of this slide chart handles the simple computation of resonant frequency and wavelength from the capacitance and inductance in an LC circuit. But of more interest is the much higher sophistication of the other side, with its sliding set of graphs in the top window and its numerous fixed scales (top photo). This embodies more complexity than one usually sees in slide charts. Furthermore, it illustrates the big advantage of analog calculating aids over digital ones: you can get a grasp of the “big picture”. An electronic calculator may tell you that a coil of such and such dimensions will have an inductance of nine micro-henrys; but you’ll have to redo the entire calculation to answer a question such as “Could I still get this inductance with a narrower diameter while keeping the length reasonable?” or “What wire size will give me the required turns-per-inch?” or “How fast does the required wire size change if I increase the desired inductance? How about if I replace enameled wire with D.C.C. wire?” The slide chart gives you all these “what ifs” and more, along a continuum, at a glance.

As with most of my hobby construction projects, I made do with available materials. The inside of this calculator, visible through the cutouts in the photo at right, identifies its humble origins in material recycled from some chocolate box. The xeroxed scales were glued on after meticulous alignment. The result looks rather less appealing than the glossy, colorful original (in fact, those being early years for photocopier technology, the thing had a faded, smudged gray look, and in places I had to use ink to overcome the fading). But it did the job, and provided the coil design for quite a few radio receivers and transmitters. In fact, it was so useful, and so much faster to use than the alternative, that I recall the joy of using it to this day. No wonder that I had proudly inscribed it with my ham station call sign, 4Z4GE.

A peek at the inside of the coil winding calculator

Click photo to enlarge

Incidentally, the original I worked from seems to have been a promotional item advertising the Allied Radio Corporation, and produced for them in 1960 by the Perrygraf company. This last specialized in producing cardboard "slide chart" and "wheel chart" calculators that were sold to companies as advertising give-aways; and it still does that today. A look at their web site will show an amazing range of simple, effective devices that can help you with anything from watching those calories to figuring mortgage payments. In fact, these simple computing devices are every bit as useful and elegant today as they were in 1934, when Lester Perry started his company.

		Exhibit provenance: I found this item unexpectedly while rummaging in my basement a few years ago. It must've been sitting there for decades, outliving the radio parts company it was created to promote.

The Xeroxed calculator