A thermostat can display exactly 19°C on two occasions and make opposite decisions. It can be heating the room the first time and staying off the second. Both decisions can be correct.
This is a useful entry into hysteresis, a system property in which the present state depends partly on the path taken to reach it. The current input alone does not tell you everything.
1. The mechanism
In magnetic materials, previous exposure to a magnetic field can leave a measurable effect after that field is removed. This remaining magnetization is called remanence. When researchers vary the applied field up and down, the material may follow different paths, creating a hysteresis loop. Materials differ greatly in how strongly they retain magnetization. This is physical behavior, not a metaphor about human memory. IEEE's overview of remanence and NIST's research on hysteresis-loop measurements document the terms.
Engineers also design hysteresis into switches. A controller can use different thresholds for switching on and off, preventing repeated toggling near one temperature. That creates the same broad pattern of history-dependent state, through a different mechanism.
A time delay alone is not necessarily hysteresis. The key question is whether the same present input can be associated with different states because of the earlier path.
2. Outside CPG: a heating thermostat
Consider an illustrative heating control: it turns the heat on when a room falls to 18°C and turns it off when the room reaches 20°C. The gap between thresholds is 2°C.
At 19°C, the heater can be on if the room has just warmed up from 18°C. But it can be off if the room has just cooled down from 20°C. You need the previous switch state, not just the thermometer reading, to predict what happens.
The US Department of Energy's Federal Energy Decision System explanation describes thermostat dead bands. Actual equipment can use additional controls, minimum run times, or different logic, so the two-threshold model should not be assumed for every thermostat.
3. A careful application to beverage distribution
Imagine a hypothetical beverage brand suffers a six-week stockout. During the gap, a retailer gives its cold-box slot to another brand. Supply returns, but the shelf position does not automatically return with it.
The old input, product availability, has been restored. The old commercial state has not. There may now be a new planogram, changed retailer expectations, or a competitor whose sales justify keeping the space.
This is an analogy and a hypothesis to investigate, not a claim that stores follow a magnetic law. The practical decision is to budget for a re-entry conversation and shelf audit after a material outage, rather than assuming restored inventory automatically restores placement.
Compare stores with similar present availability but different recent stockout histories. Then investigate shelf position, store policies, consumer demand, and promotion. Differences may have several causes.
4. The assumption to challenge
“When you remove the problem, things go back to how they were.”
Sometimes they do. A system may reset quickly, or it may never have changed state in the first place. The absence of a recovery is not, by itself, proof of hysteresis. A competitor might simply be better, demand might have moved, or contracts might have changed.
The counterargument matters: the word hysteresis should sharpen an investigation, not become a fancy label for every persistent problem.
5. Your mental operator
Run a return-path test: What was the system doing before the disruption? What is the input now? If the input has returned to its previous level, has the outcome returned too? If not, what state variable, decision rule, or threshold changed?
A useful additional question: What specific action would reset the system, and what would that action cost?
6. Reasoning exercise: answer before reading on
Our illustrative heater starts off at 19°C. The room cools to 18°C, then warms to 19°C. Is the heater on or off? Now imagine instead it had reached 20°C and then cooled back to 19°C. What is the state?
Worked answer
In the first path, the heater switched on at 18°C and remains on at 19°C, waiting to reach 20°C before switching off. In the second path, it switched off at 20°C and remains off at 19°C, waiting to reach 18°C before switching on again.
Same present temperature. Different history. Different state. That is the insight worth keeping.
Research sources and notes
- IEEE Technology Navigator, “Remanence”. Technical overview of residual magnetization, coercivity, and hysteresis loops.
- National Institute of Standards and Technology, “Interlaboratory Comparison of Magnetic Thin Film Measurements” (2003). Research comparing measurements of hysteresis-loop properties including remanence and coercivity.
- Pacific Northwest National Laboratory / Federal Energy Decision System, “What is the thermostat dead band?”. US government technical example of different heating activation and cutoff temperatures.
Notes: 18°C and 20°C are illustrative thresholds, not a thermostat recommendation. The six-week stockout, retailer response, and prospective actions are hypothetical. The thermostat and magnetic examples show two distinct mechanisms producing a history-dependent state. The business application is an analogy, not a scientifically demonstrated hysteresis process.
Educational editorial content. Hypothetical examples are labeled in the original edition.