Table 9
| Switch 1 is closed | Switch 2 is closed | Current flows |
|---|---|---|
| Yes | Yes | Yes |
| No | Yes | No |
| Yes | No | No |
| No | No | No |
Now what does this table remind us of? It is precisely the truth table for “and.” It is just what we would have if we wrote down the truth table of the statement “Switch 1 is closed and switch 2 is closed.”
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).