Harry Porter's Relay Computer

Relay's existed in Babbage's time, this raises the interesting question of whether Babbage could have succeeded by choosing the right technology.

Babbage was defeated by the cumulative demands of engineering tolerances. Any inaccuracy in the position of art A would have a knock-on effect on part B and so on. Although a replica has been bought to tolerances that were individually achievable in Babbage's day it was not possible to build and debug the ensemble in Babbage's day.

Digital technology is more practical because it is much more tolerant of errors. TTL logic generates 0V to represent a 0 and 5V to represent a 1 but it will accept anywhere in the range 0-0.7V or so for 0 and 4.3-5V for a 1. That makes it much more resilient in the face of noise or slight manufacturing errors.

The other major difference with electronics over mechanics is the lack of friction of course.