• digital
• binary
• electronic

Electronic "Switches"
As we have already seen, begining with the ENIAC era computers have been electronic because the device that represents a digit is not "mechanical"—that is, it does consist of a moving part, such as a gear—but, rather, the devices at the heart of electronic computers switch states purely by means of a change in the flow of electricity through them.

Vacuum Tube
The first such electronic mechanism to represent a digit was the vacuum tube.

Interestingly, the ENIAC was not binary, but decimal! It used 10 vacuum tubes to represent the digits 0–9.

Let's consider the number 128 . Here is how ENIAC stored this number (= the vacuum tube is on):

The ENIAC required 30 vacuum tubes to store the decimal 128. Ten for the 1, ten for the 2 and ten for the 8. In addition, it had to turn on 11 tubes.

The Atanasoff-Berry (ABC) computer used vacuum tubes, too, but, unlike the ENIAC, the ABC was binary. The number one hundred twenty-eight is written like this in binary: 10000000. Thus, the ABC stored the number this way:

Because each binary digit can only be 0 or 1, only one on/off switch (here, a vacuum tube) is required for each digit.

Thus, although 10000000 is longer than 128, it can be stored with only 8 switches (instead of 30) and only requires turning on one switch (instead of 11)! This disparity grows even worse with larger numbers.