Vol. 7, 1921 
CHEMISTRY: A.W.C. MENZIES 
81 
A DIFFERENTIAL THERMOMETER 
By Alan W. C. Menzies 
DEPARTMENT OF CHEMISTRY, PRINCETON UNIVERSITY 
Communicated by Oswald Veblen, January 20, 1921 
One type of differential thermometer measures the difference in tem- 
perature existing at the same location at different times; a second type 
measures the difference in temperature existing simultaneously at different 
points in space. The thermometer here described is of the latter type. 
A well-known differential thermoscope of this type consists of two glass 
bulbs containing air, otherwise closed but communicating with each other 
through a U-tube partly filled with oil, whose change of level indicates 
change of temperature by responding to change of gas pressure within 
the bulbs. When this instrument is developed into a differential ther- 
mometer, certain disadvantages become apparent, of which three will 
here be mentioned. (1) If the manometric liquid is caused to run into 
one of the bulbs by accidental tilting, perhaps during transportation, then 
it is difficult to return the liquid into precisely the same position as it oc- 
cupied when the instrument was scaled. If stopcocks are introduced 
in the effort to avoid this inconvenience, the cure may become worse 
than the disease, because of zero-creep. (2) In the presence of permanent 
gas, the manometric liquid becomes, in practice, not infrequently broken 
into threads, separated by short columns of the gas. (3) Although oils 
furnish very sensitive manometric liquids, their use, or, indeed, the use of 
any liquid other than the insensitive mercury, allows the entrance of an 
error that has been too little appreciated. The incidence of this error in 
tensimetric work has been pointed out by the writer in another connection. 1 
The error in question is caused by the fact that a gas at higher pressure 
has a larger weight solubility than the same gas at a lower pressure. The 
permanent gas, always slightly soluble in manometric liquids other than 
mercury, therefore passes by a process of solution and diffusion from the 
side of higher to that of lower pressure. For this reason even stopcock- 
free instruments of the kind referred to suffer from slow zero-creep. 
In order to avoid these and other disadvantages, all that is necessary 
is to abandon entirely the use of permanent gas. One selects as manometric 
fluid not oil but some liquid whose change of vapor pressure per degree 
in the range of temperature where the differential measurements are to 
be made is such as to cause differences of vapor pressure in the two bulbs 
of the thermometer that will register themselves by adequate differences 
of level in the manometer. The diagram, figure 1, shows one simple form 
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