Some time ago I came across this online tool in a newsletter article – this is a very cool slide-rule-emulator that will not just let you move two slides, but actually slide it for you as you run an equivalent digital calculator calculation to the right!

If you haven’t ever used a slide rule before, it works on properties of logarithms, and the principle that log(a)+log(b) = log(a*b). Now it wouldn’t be very interesting to just have two normal rulers together, as sliding and adding would just let you do problems like 5+5 = 10 or 50+50 = 100 if you scale the numbers. With logarithmic scale, the spacings are off and it allows you to do multiplication in adding the numbers.

In his development blog, Mr Griffiths describes some interesting features of this slide rule brought in to this web app. For example, A and D lines let you square/square root a number, but interestingly enough the left side will let you square root 4 (2) or 400 (20), but an even number of digits such as 40 or 4000, should read from the right (finding the number to be about 6.3^2 or about 63^2) as described here. Magnitudes (that is, how big the number is, how many digits to push the final answer to) is a tricky part as most slide rules will not have hundreds of numbers.