356 
DR. J. T. BOTTOM LEY AND MR. F. A. KING ON THERMAL 
Method of Conducting an Experiment. 
16. Before commencing an experiment, the apparatus, and the tubes connecting it 
to the Sprengel pumps, are thoroughly dried out. The sooted or silvered globe, as the 
case may be, is then attached to the copper plug o, into which the thermojunction is 
soldered. When an experiment is being made with the sooted globe, the globe 
receives a fine dead-black coating of soot from a small gas jet when it is hanging in 
position on the thermojunction wires. The enclosure aa is also given a coating of soot 
on the inside, and is then brought up into place and clamped to the upper flange by 
the set screws ff, which pass through holes drilled in lugs cast on the flanges. The 
joint h between the flanges is next made tight by pouring molten fusible metal into 
the cup x till the joint is well covered. A small blowpipe jet is run over the surface 
of the fusible metal just after it has set, to make sure that no pin-holes are left in the 
joint. 
The preliminary exhaustion of the apparatus is made with a two-cylinder Fleuss 
mechanical pump, which very rapidly produces almost a barometric vacuum. Then 
the Sprengel pumps are' set working, and they continue to run until a high vacuum 
is obtained. When, after repeated heating, the last traces of moisture and the 
occluded gases have been driven from the enclosure and charcoal bulb, the pumps are 
shut olf temporarily, and the M‘Leod gauge and charcoal bulb, /, are left in 
communication with the enclosure. The charcoal bulb is then immersed in a vessel 
of liquid air, which causes the collapsible gases to be absorbed by the charcoal and 
condensed, or frozen. This greatly improves the vacuum, and it clears the enclosure 
of traces of mercury vapour which may have diffused through from the pumps, a 
matter of the highest importance. 
17. The next operation is to check the galvanometer with the standard Clark 
cells, in order to ascertain that its constant has not altered since the calibration of the 
thermojunctions. When this has been done, liquid air is poured into the Dewar vessel 
surrounding the enclosure, and as soon as the violent ebullition has subsided, denoting 
that the enclosure has cooled down to the temperature of the liquid air, observations 
commence. 
18. Readings are taken every 2 minutes at the beginning of the experiment, and 
later on at intervals of 5 minutes, the frequency depending on the state of the 
vacuum, and the rate of cooling of the globe. 
A chronometer, beating half-seconds, is placed close to the observer at the reading 
telescope. At 15 seconds before the exact minute the mercury switch is closed 
and the deflection to the right of zero observed. The switch is instantly reversed, 
and at 15 seconds after the minute the galvanometer has come to rest and the 
deflection to the left of zero can be read. The switch is then opened and the zero oi 
the galvanometer is taken. The vacuum pressure, and the temperature of the liquid 
