## Saturday, 17 March 2012

### Using Logarithms

I was watching a documentary on the history of modern computers and came across a few interesting items. I say modern computers because prior to the 1940s, a computer was a person, not a thing. He, primarily, was a person who computed numbers hours after hour. It wasn’t necessarily accounting figures, but anything, like artillery tables, navigational guides, or census statistics. Lots and lots of numbers. Numbers computed without aid of any mechanical devices.

If they wanted to add up a series of numbers, they had to add them up. If they wanted to multiply two numbers, they would consult tables to find the natural logarithmic of the numbers, add those numbers then find that sum in another table to give you the multiplied value. Every step of the process was an opportunity to create an error including the values in the tables. These books of tables were after all created by humans and contained errors. Imagine trying to copy out a telephone book by hand and not make a mistake.

Sometime in the 19th century Charles Babbage had the idea of creating a machine to eliminate the human error these computers created. It’s suggested his initial concept was the start of computers we know today. Perhaps. One of his difficulties was his choice to use the decimal numbering system instead of binary. Perhaps it wasn’t a choice, just standard operating procedure. Not thinking outside the box. If he had worked on a binary system, he may have built the prototype he had in mind.

And those tables? Here is an example. Say I want to know what 3.3 times 4.4 is because those are the dimensions of some object. I could do the multiplication by hand, double check my figures and report the result. What if I had to do that fifty times in a day? Or more? Look for a faster approach.

The natural logarithm of 3.3 is 1.19392247 and for 4.4, it’s 1.48160454. These log numbers come from a table. As with most people, I can add those numbers together faster than I can multiply them. The result is 2.67552701. Now I find what e to that number is and I come up with 14.52 which is 3.3 times 4.4.

So,

e(ln A + ln B) = A x B

The same works for division.

e(ln A - ln B) = A / B

I haven’t figured out how slide rulers work. No need, just curious.