Star Ford

Essays on lots of things since 1989.

Complete and incomplete covers in engineering

I confess I have been irritated my whole life about car dashboard controls for heating and cooling because they are an incomplete cover for the complexity that is going on inside. It has been a rough few decades for user interface enthusiasts!

What is a complete cover? It is a layer or shell over some machine complexity that completely hides it and does not let any of the complexity out. A cover is incomplete if it forces you to understand what is going on underneath, or if it is confusing when you do not understand, or if the cover is insufficient to operate all aspects of the machine. A cover can be thick or thin – the thicker the cover, the more it changes the paradigm of the machine interaction. A cover is optimal when it is complete, regardless of whether it is thick, thin, or absent. Sometimes it is optimal to have no cover.

I will explain this with some of examples, starting with a mechanical mercury thermostat. There are three kinds of people in relation to these devices: (1) Those with a gut fear reaction when they look at dials and numbers; (2) those who understand the two exposed dials – measured temperature and set point – but do not know or care how it works inside; and (3) those who understand that the rotation of the temperature-sensitive coil which is superimposed on the rotation of the set point tips a mercury switch, that the bi-stable 2-lobed shape of the mercury chamber affects the temperature swing, and why mercury is used in the first place. It is a lovely thing, but not really the scope of this paper. I am mainly concerned with the middle category of people who are functional operators of the cover and what kind of cover it is.


The thermostat is a complete cover because you can operate every aspect of the heater with it, without needing to know how it works. It is also a fairly thick cover in the sense that it translates one paradigm to another. The actual heater requires an on/off switch to work, thus the only language it understands is on/off. But the thermostat exposes a set point to the user. It translates the language of on/off to the language of set points. Someone could replace the whole heater and wiring with a different inside paradigm but leave the exposed paradigm there, and the user would not need to know that anything changed, because the operation stays the same. In many systems – especially software systems, the replaceability of layers is an important design point, and complete coverage is one of the factors that makes it possible. Read the rest of this entry »

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Appliance model of computing

This longish article about what is now called cloud computing was before its time in a way, with none of the new terminology. It’s still ahead of the implementation, and I keep thinking I’ll implement some of it.  In short, it describes an internet operating system (i-glue) based on blackbox standards, rich protocols, and replaceable appliances. Read the rest of this entry »

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