Fig. 2. Switches in parallel.
Fig. 3. Switch open—current flowing.
Suppose that we have two switches 1, 2 in parallel ([see Fig. 2]). When do we get current flowing from the source to the sink? Answer: when either one or both of the switches are closed. Therefore, this circuit is an exact representation of the statement “Switch 1 is closed or switch 2 is closed.”
Suppose that we have a switch that has two positions, and at any time must be at one and only one of these two positions ([see Fig. 3]). Suppose that current flows only when the switch is open. There are two possible cases and results ([see Table 10]).
Table 10
| Switch 1 is closed | Current flows |
|---|---|
| Yes | No |
| No | Yes |
This is like the truth table for “not”; and this circuit is an exact representation of the statement “Switch 1 is not closed.” (Note: These examples are in substantial agreement with Shannon’s paper, although Shannon uses different conventions.)
We see, therefore, that there is a very neat correspondence between the algebra of logic and automatic switching circuits. Thus it happens that: