Editing Heat transfer (section)

==== Conductivity experiments ====

===== "New Experiments upon Heat" =====
In 1785 Thompson performed a series of thermal conductivity experiments, which he describes in great detail in the ''[[Philosophical Transactions of the Royal Society|Philosophical Transactions]]'' article "New Experiments upon Heat" from 1786.{{Sfn|Martin|1951|p=147}}{{Sfn|Thompson|1786|p=273-304}} The fact that good [[electrical conductor]]s are often also good [[Thermal conduction|heat conductors]] and vice versa must have been well known at the time, for Thompson mentions it in passing.{{Sfn|Thompson|1786|p=274}} He intended to measure the relative conductivities of mercury, water, moist air, "common air" (dry air at normal atmospheric pressure), dry air of various rarefication, and a "[[Torricellian vacuum]]".
{{Blockquote|text=From the striking analogy between the electric fluid and heat respecting their conductors and non-conductors (having found that bodies, in general, which are conductors of the electric fluid, are likewise good conductors of heat, and, on the contrary, that electric bodies, or such as are bad conductors of the electric fluid, are likewise bad conductors of heat), I was led to imagine that the Torricellian vacuum, which is known to afford so ready a passage to the electric fluid, would also have afforded a ready passage to heat.}}
{| class="wikitable floatright"
|+
!Medium
!Relative conductivity
|-
|Mercury
|1000
|-
|Moist air
|330
|-
|Water
|313
|-
|Dry air (1 atm)
|80.41
|-
|Dry air (1/4 atm)
|80.23
|-
|Dry air (1/24 atm)
|78
|-
|Torricellian vacuum
|55
|}
For these experiments, Thompson employed a thermometer inside a large, closed glass tube. Under the circumstances described, heat may—unbeknownst to Thompson—have been transferred more by [[Thermal radiation|radiation]] than by [[Thermal conduction|conduction]].{{Sfn|Martin|1951|p=147-148}} These were his results.

After the experiments, Thompson was surprised to observe that a vacuum was a significantly poorer heat conductor than air "which of itself is reckoned among the worst",{{Sfn|Thompson|1786|p=277}} but only a very small difference between common air and rarefied air.{{Sfn|Thompson|1786|p=300}} He also noted the great difference between dry air and moist air,{{Sfn|Thompson|1786|p=296}} and the great benefit this affords.{{Sfn|Thompson|1786|p=297-298}}
{{Blockquote|text=I cannot help observing, with what infinite wisdom and goodness Divine Providence appears to have guarded us against the evil effects of excessive heat and cold in the atmosphere; for if it were possible for the air to be equally damp during the severe cold of the winter ... as it sometimes is in summer, its conducing power, and consequently its apparent coldness ... would become quite intolerable; but, happily for us, its power to hold water in solution is diminished, and with it its power to rob us of our animal heat.}}{{Blockquote|text=Every body knows how very disagreeable a very moderate degree of cold is when the air is very damp; and from hence it appears, why the thermometer is not always a just measure of the apparent or sensible heat of the atmosphere. If colds ... are occasioned by our bodies being robbed of our animal heat, the reason is plain why those disorders prevail most during the cold autumnal rains, and upon the breaking up of the frost in the spring. It is likewise plain [why] ... inhabiting damp houses, is so very dangerous; and why the evening air is so pernicious in summer ... and why it is not so during the hard frosts of winter.}}

====== Temperature vs. sensible heat ======
Thompson concluded with some comments on the important difference between temperature and [[sensible heat]].
{{Blockquote|text=The ... sensation of hot or cold depends not intirely upon the temperature of the body exciting in us those sensations ... but upon the quantity of heat it is capable of communicating to us, or receiving from us ... and this depends in a great measure upon the conducing powers of the bodies in question. The sensation of hot is the entrance of heat into our bodies; that of cold is its exit ... This is another proof that the thermometer cannot be a just measure of sensible heat ... or rather, that the touch does not afford us a just indication of ... real temperatures.}}