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:
Although the Z3 was the first, it was not an influential computer since its
details were not published beyond a rejected patent application.
A number of people have made one-off relay computers in recent years,
Harry Porter's Relay computer: http://web.cecs.pdx.edu/~harry/Relay/
Jon Stanley's Relay computer "RC-2": http://www.electronixandmore.com/projects/relaycomputertwo/index.html
Phil Ryal's "RC-3": http://www.computerculture.org/projects/rc3
Fredrik Andersson's "Zusie": http://www.nablaman.com/relay/
Rory Mangles' TIM-8 computer: http://www.northdownfarm.co.uk/rory/tim/
Kilian Leonhardt's relay computer