Difficulties of Unscientific Thermometry
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TWICE ashot is an .expressive phrase, but 1-generally quite meaningless, as our correspondent "C.W.G.114." pointed out in his letter,published last week, relating to an article by our tyre expert. We are indebted ta him for putting us right, and We stand 'corretted.
However, the real culprits are .Messrs. Fahrenheit and Celsius (the inventor of the Centigrade scale), who perpetrated the• unscientific deed of crediting with no heat that which possessed "heat, which act has-led to nearly all heat rna:tters being based, ever since, on a wrong.
Nevertheless, " C.W.G.M." is not guiltless, for he has tripped up in another direction. Surely he is not juStified in saying that ".it would be more accurate to refer these temperatures (40 degrees.F. and 80 degrees F.) to the absolute zero of — 273 degrees C., when they would become 313 degrees and 353 degrees respectively."
To put it mildly, it is rather irregular to add degrees F. to degrees C. His"methOd is right,. but he applies it wrongly ; 40 and 80 should be added to 460, the number of degrees. F. of absolute zero below 0 on the Fahrenheit scale. Then. a 'substance having a temperature of SO degrees F. Would be 100 per cent, hotter than another at 270 degrees F. absolute. or — 190 de-grees F.
Absolute zero implies the existence of no heat. It is defined by one -authority, in homely but not strictly correct terms, as "the temperature to which the earth _would fall were the heat of the sun withdraivn entirely." There is little doubt that it exists naturally in the universe... A mechanical definition is "the temperature at. which particles, Whose. motion constitutes heat, would be at rest." • A third, which strikes us as the most satisfactory, becanse (for one reason) it suggests a way by which the figure was arrived at, is "the tern_ perature at which the pressure of a given mass of an ideal gas, 'maintained at a constant volutine, becomes zero." This• agrees well with the mechanical definition, and it ina:y beargued that, _ in the absence.of means,for maintaining a constant volume, the condition of no .pressure means that there would actually, be no N.-ohm-ie. Accordingly. a--fourth definition is the temperature at which a gas, if it remained a gas, would occupy no space.
In' endeavours to create absolute zero, we understand that recently — 272 degrees C. was reached by an ingenious heat-exchange system. This is a notable advance on the achievement of Fahrenheit, who, in 1714, according to Ganot, attained the lowest then known temperature by means of a , mixture of sal-ammoniac and snow, and, thinking it to represent absolute cold, used it as zero for his scale. —He did not know, as we do now, that on the shady side ofthe moon it is about — 244 F.
That he thought -his zero really was zero mitigates the offence referred to earlier, so far as he was concerned.
Whilst the Fahrenheit and .Centigrade scaleg are commonly employed, that named the Reaumur is less usual, and the Kelvin scale is known almost . only in specialized fields. Reaumur puts absolute zero at 218 degrees below' the freezing point of water, but the Kelvin scale is characterized by the fact that its own, zero is absolute zero.Kelvin was on the right tiack.
Thus, boiling points o5water on the four scales are 212 F.. 100 C.. 80 R. and 3•th K., and freezing points 32 F., 0 C., O. R. and 273 K. Another as it were, 4re-stage point in the temperature route is that at • which solid . carbon-dioxide changes directly to gas without an intermediate liquid state, namely, 1%5 K. What the ultimate destination May be one cannot say, Jeans gives 40 million F. as the temperathre inside the sun, . where, incidentally, he estimates the pressure to be 40,000 rhillion atmospheres.
Yet ahother landmark is the freezing point of mercury, which happens (unless, for some obscure reason, it was deliberate) to be where the F. and . scales coincide, that -is, — 30 degrees C.
and/or F. • • • 4 We deplore We fact that the Kelvin scale is not more -widely used.: If it had been employed 'by our tyre expert, -our cOrrespondent would have had no cause for criticism.