History

The first working programmable computer (!) was the Z3, designed by Konrad Zuse and completed in 1941. This computer was implemented with telephone relays. Here are some of its characteristics:

• Data format: 22 bit binary floating point (1 bit sign, 7-bit exponent, 14-bit mantissa).
• Provided operations: add, subtract, multiply, divide, square root
• Data memory: 64 words of 22 bits implemented with relays
• Program memory: punched film
• Frequency 5.3 Hz
• Relays: 2600 (1400 of them for the data memory)
• Power consumption: 4000 Watts
• Calculating speed: add .8 seconds, multiply 3 seconds

A good description is here: http://ed-thelen.org/comp-hist/Reckoners-ch-2.html

Although the Z3 was the first, it was not an influential computer since its details were not published beyond a rejected patent application.

Recent History

A number of people have made one-off relay computers in recent years, including:

Harry Porter's Relay computer: http://web.cecs.pdx.edu/~harry/Relay/

• 415 relays

Jon Stanley's Relay computer "RC-2": http://www.electronixandmore.com/projects/relaycomputertwo/index.html

• 281 relays

Phil Ryal's "RC-3": http://www.computerculture.org/projects/rc3

• 418 relays

Fredrik Andersson's "Zusie": http://www.nablaman.com/relay/

• 268 relays

Rory Mangles' TIM-8 computer: http://www.northdownfarm.co.uk/rory/tim/

• 152 relays

Kilian Leonhardt's relay computer

• 1500 relays