Description and Notes:
This is model TP007100B of the Gerber variable scale. It looks like a slide rule, but the similarity is only superficial, as the basic principle behind the two is almost completely different. Although this variable scale does have computational capability, but it operates more like a divider, therefore in my opinion, it would be more appropriate to be categorized as a measuring device.
As can be seen from the above picture, it does have a slider and there is a logarithmic scale printed on the base plate. However, the "CPU" of this ingenious device is a triangular shaped spring that has one hundred calibrated coils, of which every fifth is colored blue-green, every tenth red and the others white. It also has an auxiliary indexed round coil spring to facilitate the reading of the coils of the triangular spring. One end of the two springs are attached to the slider and the other end to the left block of the base plate, therefore they will simultaneously extend as the slider moves from left to right, or contract vise versa. A hex wrench is provided for adjusting the friction between the slider and the sliding rail to prevent which been pulled back from the desired stop position by the spring force. Since these are linear springs which obay the Hooke's law, the coils will be equally spaced with respect to any stop position of the slider. Consequently, this scale can be used for linear interpolation or extrapolation of any plotted data points without any computation. The logarithmic scale on the base plate allows it to be used on graphs or plots with logarithmic coordinates. This device certainly can also be used as a precision spacing divider up to one hundred points.
This scale is out of production now, but the Gerber Scientific Instrument Company still exists, and the history of the company can be found in their official website. A copy of the instruction manual of this scale in PDF format can be downloaded from this page.