Summary: This is another on/off switch, and the plain physigram is just like the light switch push down/pull up. The interest here is that when the kettle boils it also can turn the switch off – system–initiated physical transition. Furthermore it does so the same way as you do yourself – compliant interaction.
In some ways an electric kettle on/off switch is just like a light switch: you press it down and the power goes on: you lift it up and the power goes off.
Indeed this is precisely the case for very old kettles (a long, long time back).
This diagram shows the physigram and associated power state for one of those early kettles. On the left of the diagram is the kettle switch as a simple toggle switch (with a little give), and on the right is the logical system state – power on or off. We can also see the one-to-one mapping – an exposed state device.
Of course old kettles would boil dry if left unattended … and maybe even set the house on fire.
Happily a modern kettle does not do that. If you leave it on eventually the water in the kettle boils and after a while the kettle will switch itself off.
The revised physigram below shows this. The kettle toggle switch has an extra dashed (system-initiated) transition. This is labelled ‘system down’; however, while previous events have been generated by the user’s actions, in this case the ‘system down’ event is triggered by the kettle itself.
On the right-hand side is the logical system state. There is still a one-to-one mapping between the visible (and tangible) switch state and the power on/off. However, the system state when the power is on has been revised to distinguish boiling from non-boiling water. Of course there are many possible water temperatures, not just boiling and non-boiling, and the temperature varies when the power is off as well as on; however, we have only distinguished the parts of the system state that are pertinent for the description.
The boiling state (within the power on state) is shown as triggering the ‘system down’ event denoted by the red lightning bolt.
Note that the kettle is not powered down by a separate relay (although many kettles also have a safety cut out which does operate invisibly like this); instead the switch physically springs back up into the ‘off’ position. In other words, the way the system controls the state is identical to the way the user does so – compliant interaction.
Also note that for some kettles the switch is actually maintained in the ‘on’ position using some form of electromechanical relay. When the user pushes the switch down is does no really stick in place physically, but instead the switch being down triggers the relay, and then the switch is held down actively by the electronics. The boiling of the water breaks the relay connection and the switch pops up. This is evident if the kettle is really disconnected from the power as it is then a bounce back switch. For this kind of kettle, the physigram shown above would still be valid, but as a depiction of feedback loop B.