MEASUREMENT OF TEMPERATURE. 
173 
equal to that of the bulb of the air thermometer : a little below the bulb the gauge 
tube widens out into a cylindrical reservoir C, from which a side tube communicates 
with the three-way tap T, which also serves to close the entrance to the large cylindrical 
bulb D. A side tube o, near the lower extremity of this limb of the V gauge, serves 
for the introduction of the requisite sulphuric acid, and can be sealed off when 
required. 
The bulb B serves to determine the volume of the bulb of the air thermometer at 
any time, on the principle of the volumenometer, and so to check any alteration in the 
volume of the bulb without the necessity of dismounting and filling it afresh ; it may 
also be used for several other purposes. 
When it is desired to fill the bulb of the air thermometer with pure dry air by 
repeated exhaustion and re-admission at a high temperature, the whole of the acid (if 
the gauge is already full) is first drawn back into the bulb D, which it completely 
fills, and the tap T is turned thus (_L) : the contained air is then exhausted, and the 
dry air re-admitted repeatedly by way of the tube CeT. When this has been satis¬ 
factorily effected the tap T is turned thus (T), and the acid allowed to flow down into 
B, completely cutting off the mass of dry air in the bulb. 
The mass of air enclosed may also be easily adjusted by means of the manometer, 
to give any desired pressure at any desired temperature. 
The volume of the bulb B gives the necessary range to the gauge in heating or 
cooling; if the tap T be turned thus (H), so as to put C and D in communication and 
cut off the manometer, the instrument may be left to itself to cool or heat through a 
large range. 
When the air thermometer has been thus left to itself to cool or heat, it often 
happens that the pressure is only roughly known, so that, if communication were 
opened direct between the gauge and the mercury manometer, a sudden and disas¬ 
trous movement of the acid in either direction might take place if the manometer did 
not happen to be exactly adjusted. It is convenient, therefore, to introduce the small 
auxiliary piece of apparatus ST' on the way between the H 3 S0 4 gauge and the 
mercury manometer. 
The U tube S is of small bore, about 1 mm., and is used as a kind of safety indicator 
to show when the pressure is the same on both sides of the three-way tap T v ; for this 
purpose it is half filled with mercury. 
The pressure of the air in D is always known to within 20 cm. or so of mercury, so 
that the manometer can be approximately adjusted; the tap T is then turned thus (T); 
the tube S indicates the residual difference of pressure on either side of T'; the mano¬ 
meter is adjusted till this difference vanishes, and is then put into direct communica¬ 
tion with the HoSO^ gauge. If these precautions were not taken, the acid might 
sometimes get sucked back into the bulb or the pressure tubes, either of which events 
would ruin the series of observations. 
