What is the most useful tool you ever made?

Probably the hoe or whatever preceded it.  The day we started breaking up the earth to plant seeds was the first step on the path to civilization.

Whoops I better learn to read more carefully. LOL.  Somehow missed the word 'you'

I was going to say - you are way older than I thought.....

Hi dougw, that's several good questions! You'd think the manufacturers would put some sort of transparent tape or coating over the grid to protect it from small impacts, but now you mention it I remember having had 3 or 4 element failures on different cars that could have been caused by objects on the back parcel shelf making physical contact due to potholes and level crossings. Sometimes they could be repaired using silver-bearing varnish but my experience was that such repairs didn't last long.

My next train of thought was to wonder what the resistivity of a layer of condensation is? And would it matter if a current flowed somewhere as a result? Surely it would be tiny, insignificant even. So my view is that the thickness of the condensation would be very thin, and that it would get even thinner very quickly when the demister was switched on. It also 'shrinks away' from the elements as they heat up so once everything has settled down there will be no current from element to condensation, which will disappear within a few minutes at most. So I'm a bit baffled by that one - too many unknowns, no 'standard' set-up or way of getting results that I know of, etc.

(This paragraph contains inuendo, poor attempts at humour, unlikely scenarios, and exaggeration. You have been warned!)    One thing is sure though - I won't be standing behind my car in the pouring rain with the tailgate up manipulating my probe and squeezing the water out of it while my multimeter floats off the back parcel shelf down into the rear passenger foot-well ending up in a shower of transparent plastic shards and a bent pointer while I try to scribble things down with a fountain pen on a sodden piece of paper...

I am surprised the grid is not coated with insulation. I would have thought any condensation on the window would short out the grid if the surface was conductive.

It measures the voltage at any point on the 'element' which, in most cases, is a horizontal stripe across the rear window. My car has seven of them. All the left ends are joined by one slightly larger vertical stripe which is connected to the +12v supply from the battery via the dashboard switch. Similarly, all the right ends are connected by a second vertical stripe to 0v, a.k.a. the battery negative terminal, the chassis, an unpainted and non-rusty bolt head, etc. The meter negative terminal is connected to all the right ends of the demister elements via the car metalwork, the meter positive terminal 'connects' to each demister element in turn by the user holding the probe and pressing it very gently into contact with the element so that the conductive foam deforms very slightly and becomes a low-value resistor in series between the element and the copper tube. The foam is for protection of the element rather than for any electrical reason.

From now on, 'track' refers to all the elements in parallel connected between the +12v and the 0v vertical stripes, and 'probe' and 'slider' are synonymous.

Basically, what I've done is to construct a potentiometer (usually pronounced 'pot') such that one end of the track is connected indirectly (via dashboard switches etc.) to the +12v battery terminal, and the other end of the track to the negative battery terminal (via car bodywork etc.) The slider can be positioned anywhere along any one of the elements by the user, and a good element will cause the meter to display anything between +12v at the left-hand end and 0v at the right-hand end. A dodgy element will cause a sudden transition from 0v to 12v as the probe is moved across a break from right to left and vice versa.

N.B. Other vehicles may have different demister configurations and different ways of switching them on and off. Check the wiring diagram first if it is available, or use the DVM with both its' norrmal probes and 15VDC selected to verify voltages before testing. To prevent damage do not touch any of the elements or either of the vertical stripes with the pointy end of a meter probe.

Cool. So this probe is measuring the electric field from the demister elements? (Or is it actually contacting resistive/conductive elements?)

Very clever.  Specialized knowledge in a number of different areas came together in this example.  Of course the sales people will probably shudder the next time you drop by the car lot.   

Very clever.  Specialized knowledge in a number of different areas came together in this example.  Of course the sales people will probably shudder the next time you drop by the car lot.