In the previous post, you wired two knife switches in series — one after the other in a single line — and discovered that the lamp only lights when both switches are closed. The current has exactly one path through the circuit, and that path has two gates. Both must open for anything to get through. You had, without quite naming it yet, built an AND gate from copper and spring steel.
Now take those same two switches and rewire them side by side, sharing the same two wires. Put switch A on the left path, switch B on the right path, and hang the lamp at the bottom where both paths rejoin. Something quietly remarkable happens: the lamp lights when A is closed, or when B is closed, or when both are. The only time it stays dark is when neither switch gives the current anywhere to go.
Two arrangements, two rules. Series said both. Parallel says either. And buried in that word — either — is the whole logic of OR.
A tale of two paths
Before we name this OR, let’s feel it physically. In a series circuit there is one path for current, and two gates guarding it. In a parallel circuit there are two paths, and blocking the lamp requires closing both at once. Leave one path open and current simply takes the other.
Think of it like a road that splits around a hill. If both routes are clear, traffic flows either way. If a fallen tree blocks one side, drivers take the other. Only when both routes are blocked does everything stop.
That dual-path picture carries more structure than it first appears. The physicist sees current following the path of least resistance — actually, all available paths at once. The engineer sees a redundant design: no single point of failure. The logician sees a precise four-row rule, which we’ll meet in a moment as the OR truth table. But the historian, looking back at the 1840s, sees something even more striking: this same design philosophy was already keeping words flowing across continents, years before anyone dreamed of a logic gate.
The parallel-path lesson from the telegraph
When long-distance telegraph networks spread across the world in the late 1840s and 1850s, the engineers running them learned a hard lesson fast: wires break. Ice storms snap them, floods wash out poles, railroad crews accidentally sever them. A single line between two cities was brittle. The message business was too important for that.
The solution was almost immediate: run a second line in parallel. If the primary path went down, operators could reroute through the backup. A message bound for New York from Boston no longer depended on a single strand of copper stretched across a hundred miles of unpredictable countryside. It had alternatives. The system became resilient.
The same thinking spread to the telephone networks that followed, and from there to every layer of communications infrastructure since — undersea cables doubled and tripled across ocean floors, fiber-optic backbones crisscrossing continents, and ultimately the internet itself. The designers of the early internet were explicit about it: the network should find routes around damage. That’s packet switching, but the underlying logic is the same we’re looking at right now. Give the signal more than one way through, and losing one way doesn’t stop the message.
There’s a name for what those engineers were doing. They were applying OR logic to physical infrastructure. An outage anywhere shuts down the network only if every path fails — which is AND logic applied to failure. As long as any path survives, the message gets through — OR logic applied to success. These aren’t just gate behaviors. They’re patterns of reasoning that engineers had internalized long before Shannon gave them a mathematical name.
Two switches, one lamp, four possibilities
Let’s be precise. Call the left switch A and the right switch B. Each is either closed (1) or open (0). The lamp is either on (1) or off (0).
Working through every combination:
| A | B | Lamp (A OR B) |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
Three out of four combinations light the lamp. The only dark row is the first — both switches open, both paths blocked, no route for current anywhere.
Compare that to the AND table from the previous post, where the lamp lit in just one out of four cases — the single row where both switches were closed. Parallel OR and series AND are, in a very precise sense, mirror images of each other. AND is strict: it demands everything. OR is generous: it needs only something.
Notice the last row, where both switches are closed. The lamp lights, as expected — because at least one path is complete. Current isn’t confused by having two routes; it flows through both. The logical result — lit or dark — depends only on whether at least one path is clear, regardless of what the other path is doing.
This four-row table is the truth table for OR. You can derive it entirely from the physical setup, without ever knowing the word “OR” or the word “gate.” The logic is already in the wire, waiting.
Try it yourself
Reading a truth table is useful. Throwing the switches yourself makes it unforgettable. Below is a live, simulated parallel circuit — two knife switches sharing a single lamp — running a real DC circuit simulation inside this page. Turn your sound on.
Three experiments worth running while the circuit is in front of you:
- Close either switch alone — does the lamp care which one? Try A first, then open it and close B. The lamp responds identically. Current doesn’t prefer either path — it takes whichever one is open. That indifference is exactly what OR means: the source of the 1 doesn’t matter, only that at least one exists.
- Close both switches, then open just one. The lamp stays on. Trace the circuit in your head: current is still flowing through the remaining path. This is redundancy made visible. Knock out either link and the signal survives. Only losing both at once causes failure. That’s the telegraph engineers’ insight, live and interactive.
- Contrast it with AND. If you have the previous post’s series circuit open in another tab, flip back and forth. In series, closing only one switch leaves the lamp dark — the open switch is a veto. In parallel, closing only one switch is enough — the closed switch is a vote. AND requires unanimity. OR requires only a majority of one.
OR is the logic of resilience
There’s a deeper observation worth sitting with before we move on.
AND logic is the logic of requirements: every condition must be met before anything happens. You use it when failure anywhere is fatal — a lock that opens only when the right key is inserted and the correct code is entered. It’s strict, safe, and unforgiving.
OR logic is the logic of alternatives: any one condition is enough. You use it when you can afford to lose some paths but not all — an alarm that sounds when the front door or the back door opens; a redundant server that handles a request when the primary or the backup is healthy. It’s inclusive, flexible, and tolerant of individual failures.
These aren’t just gate behaviors. They’re patterns of reasoning woven into the world. The telegraph engineers who laid parallel cables were applying OR logic to their infrastructure without calling it that. A person who sets two alarm clocks is using OR logic to make sure they wake up. An immune system that recognizes a pathogen if any of several molecular signatures are present is running OR logic at a biological scale.
Shannon would go on to show, in his landmark 1937 master’s thesis, that AND and OR — together with the inversion (NOT) we met in the very first post on how relays work — are sufficient to compute anything that can be computed. Arithmetic, memory, comparison, decisions: all of it falls out of just three primitive patterns. We’ll reach that complete picture soon. But first, those patterns need names and a mathematics. That’s the story of the next post.
What you just learned — and where we go next
The ground covered in this post:
- Two switches in parallel give current two independent paths — closing either one is enough to light the lamp.
- The rule that falls out is OR: the lamp is on when A is on, or B is on, or both are — and off only when neither is.
- The truth table has three lit rows and one dark row — the mirror image of AND’s one lit row and three dark rows.
- Parallel paths are the physical expression of alternatives and resilience, the same design principle that prompted engineers to duplicate telegraph lines nearly two centuries ago.
- AND (series), OR (parallel), and NOT (the normally-closed relay contact) together form a complete toolkit for logical computation.
In Post 4, we climb one rung higher: we’ll meet George Boole, who in 1854 published an algebra of true and false — decades before relays existed — and discover why AND, OR, and NOT are not arbitrary engineering choices but the deep skeleton of logical thought itself. Shannon didn’t invent the logic; he recognized that switches had been doing it all along. Once we have Boole’s notation, you’ll be able to design any logic circuit on paper before touching a single switch.
Until then, every circuit in this series — including this OR circuit and the AND circuit from the previous post — is open in the DigiSim Relay Lab. Wire them together, add more switches, and start combining the patterns. The click is waiting.