The Gerber variable scale

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Joe Gerber's pajamas

This device acquainted me with a man I would no doubt have liked meeting: H. Joseph Gerber (1924-1996), a prolific inventor and able businessman who had been granted more than 650 patents over a lifetime of innovation. How could I not take a liking to someone who, we are told, was already building radios and motors at the age of eight?...

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Gerber had escaped Nazi persecution in his native Austria when he was 18, to come to the US, where he studied engineering at Rensselaer Polytechnic Institute. It was there, in his junior year, that he invented his first product, the Gerber Variable Scale we see here.
He was slaving at plotting data for his homework when he saw a way to save time by using an expandable ruler; this he promptly improvised from the elastic waistband of his pajamas! After graduating, he designed a more robust version and founded the Gerber Scientific Instrument Company to produce it. That company still exists (as Gerber Scientific, Inc.), producing An impressive variety of innovative technology tools.

The Variable Scale is based on a high precision triangular spring whose coils replace the pajamas elastic as the expandable ruler. The coils on the outward facing flat part of the spring are painted white, with every fifth coil alternating red or blue, forming a linear ruler -- but one that can be stretched so its 100 coils can span any length between one and ten inches. A second, parallel spring carries the numbers that mark this ruler’s major divisions. The two springs can be stretched to the desired length by a moving a metal slide that also has a hairline cursor that moves over

Gerber Variable Scale at different extensions

Click photo to enlarge

three scales: a linear scale that indicates the number of inches extended, a reciprocal scale that shows the number of coils per inch, and a logarithmic scale that indicates the antilogarithm of a tenth of the extension in inches. The design of this mechanism is truly ingenious: for example, the transparent plastic cover on the spring is designed to optically translate the ruler’s painted coils outward and downward, to the very edge of the device, where it can be used to measure with accuracy whatever drawing it lies on.

You can see the two springs, with the plastic cover removed, in the photo at the right.

Gerber’s scale can be used to solve a large variety of problems involving proportions, interpolations and scaling for data plotted on paper.
At the most basic, imagine you want to measure distances on a map. Instead of measuring centimeters with a ruler and then multiplying by the map’s scale factor, you can stretch the spring to where its full 100 coils correspond to (say) a kilometer on the map, and then measure directly any distances on this map with no need to multiply anything. More advanced calculations on graphs, oscillograms and drawings might use the spring in conjunction with the three calibrated scales. The manual, which you can download from the link below, has many examples, from the simple to the complex.

Click a photo to enlarge

One thing that bugs me in this story is that Joseph Gerber is practically unknown to the general public, despite having given us hundreds of inventions, and having pioneered entire domains of manufacturing automation that have had a major impact on industry worldwide. Let this article be a tiny step towards correcting this injustice...

Exhibit provenance:
eBay, from a seller in the USA.

More info:
This scale came without the instructions manual, so I wrote Gerber Scientific and they kindly offered to mail me a copy: Gerber Variable Scale Instructions Manual.

For more on H. Joseph Gerber, see http://www.gerberscientific.com/about_gerber/founder.htm .

Joe Gerber's pajamas