Vacuum Valves and Mercury Memory: Remembering Sir Maurice Wilkes

Maurice Wilkes inspecting a mercury delay line memory. Copyright Computer Laboratory, University of Cambridge. Reproduced by permission.

The semiconductor industry is constantly producing new innovations that enrich our lives. But, we stand on the shoulders of giants. Sir Maurice Wilkes, who passed away on November 29th this year, was one of the tallest.

Sir Maurice is credited with several pioneering developments in computing, but is best known for leading the team at Cambridge University in England that developed EDSAC, the first truly modern computer, which ran its first program in 1949.

The UK’s Daily Telegraph newspaper wrote an excellent obituary to Sir Maurice which tells the stories of both Sir Maurice and EDSAC. It’s well worth a read.

EDSAC was not the first computer; that is generally credited to Charles Babbage and his never-finished Difference Engine. It wasn’t even the first electronic computer. That was ENIAC, developed at the University of Pennsylvania and designed to calculate artillery firing tables. EDSAC was, though, the first computer that resembles what we use today – one that stores both program and data in temporary storage, allowing any program to run without reconfiguring the hardware. Before EDSAC, the computer that Sir Maurice’s team built, reprogramming a computer took days and meant flipping thousands of switches and moving cables around. EDSAC could instead load a program from paper tape, making the machine the first modern computer.

It’s interesting to compare EDSAC with, say, a modern laptop computer:

Source: Daily Telegraph and Wikipedia.

It would be a mistake to dismiss EDSAC as a fossil. The level of ingenuity is staggering. Today, we solve problems with brute computing force. Throw enough processors and memory at a problem and you can solve anything. It’s an impressive approach, but not an elegant one. Back then, every bit counted. With less memory capacity than you’d find in a cheap digital watch today, EDSAC found a 79-digit prime number and solved differential equations in gene theory.

The memory technology is simply astonishing. A mercury delay line was a long tube filled with liquid mercury. Data was stored as acoustic waves rushing back and forth in the liquid. Hundreds or even thousands of bits could be stored at any one time. Today, we store information in the nanoscale capacitors of a DRAM chip, one bit a time.

An unexpected (and unfortunate) side effect of this breakthrough was the discovery of debugging. In his memoirs, Sir Maurice said: “I can remember the exact instant when I realised that a large part of my life from then on was going to be spent in finding mistakes in my own programs.”

Today’s nanoengineering is amazing, of course, but it’s far too small to see. Mercury sloshing back and forth in a tube and the orange glow of racks of vacuum tubes? Now that’s impressive.