- #36
jim hardy
Science Advisor
Gold Member
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@CRT Will you pardon an old guy a bit of nostalgia ?
This digression has a purpose, so please bear with me.
This is why kids should be shown how to work on old cars. It teaches them a lot.
I knew i'd encountered a simple, earthy "Joules per second " meter someplace in my distant past.
Just now came to me.
My 1953 Ford Stationwagon (wish i still had it)
had gages operated by "Heated Wire" technology of the 1930's. Rugged and reliable, if less sophisticated than today's computer dashboards.
I can't find a Ford shop manual explanation but this old Chrysler one will suffice to make the point:
http://www.allpar.com/history/mopar/electrical2.html
Basically a wire is heated by a current proportional to what it is you wish to measure.
The wire expands with temperature and that mechanical movement is transmitted to the indicating needle.
Now think about this --- the temperature of that wire is in proportion to the rate at which heat is put into it.
That'd be watts, or joules per second.
Deflection then is in proportion to Joules per second.
That gets me one step closer to your direct indicating "Joules per coulomb" voltmeter.
Old brain hasn't clicked onto it just yet..
BUT
This beats watching TV commercials !
Those old gages were well suited to their task
and provoked thought - as one drove down the road, all gages would shift slightly when you hit a shady part of road and the dashboard cooled a degree or two.
As i said - an old car is good for a kid. But he needs the shop manual and some tools.
thanks mentors for indulging me...
old jim
This digression has a purpose, so please bear with me.
This is why kids should be shown how to work on old cars. It teaches them a lot.
I knew i'd encountered a simple, earthy "Joules per second " meter someplace in my distant past.
Just now came to me.
My 1953 Ford Stationwagon (wish i still had it)
I can't find a Ford shop manual explanation but this old Chrysler one will suffice to make the point:
http://www.allpar.com/history/mopar/electrical2.html
Basically a wire is heated by a current proportional to what it is you wish to measure.
The wire expands with temperature and that mechanical movement is transmitted to the indicating needle.
Now think about this --- the temperature of that wire is in proportion to the rate at which heat is put into it.
That'd be watts, or joules per second.
Deflection then is in proportion to Joules per second.
GIBRALTAR: the original Chrysler gauge design
Virtually all Mopar RWD passcars used thermal-type instrumentation (shown at right). Simply put, this means that the pointer of each gauge (excluding the ammeter, of course) is mechanically linked to a bimetallic strip. The strip is wrapped with resistance (heating) wire, just like what’s in old toasters, etc. As the current passed through this wire is increased, the wire gets hotter, transferring this heat to the bimetal strip, which bends more and more as the temperature is increased, deflecting the pointer. As you probably guessed, this gauge design is inherently very well damped and very slow to respond - which is probably a good thing. Nobody wants to see the gas gauge, for instance, swing wildly as the fuel in then tank sloshes around. And the designers clearly were just as happy that Vern can’t see the temp gauge fluctuate as the thermostat opens and closes, or the oil pressure drops down quickly to 20 PSI in traffic on a hot day.
One end of the Nichrome (resistance) wire is connected to the sending unit for that particular gauge: a thermistor (temperature-variable-resistance solid-state device, see above) in the case of the water-temperature gauge; a simple variable resistor linked to a float in the example of the gas gauge (fig. 2 in the photo section), and a variable resistor linked to a diaphragm in the case of the oil gauge (“A” in fig. 4, photo section.) (Cars without an oil gauge have a warning light, which is activated by means of a simple switch - “B” in fig 4).
The other end of all these gauges need a reference voltage supply. Something rock-steady and unwavering. If this voltage varies, so will the gauge readings. Take a look a the crude regulator in fig. 6, at right. This relic of the 1930s is all that’s there for your gauges to work off of. It’s junk!
That gets me one step closer to your direct indicating "Joules per coulomb" voltmeter.
Old brain hasn't clicked onto it just yet..
BUT
This beats watching TV commercials !
Those old gages were well suited to their task
and provoked thought - as one drove down the road, all gages would shift slightly when you hit a shady part of road and the dashboard cooled a degree or two.
As i said - an old car is good for a kid. But he needs the shop manual and some tools.
thanks mentors for indulging me...
old jim
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