766 



10 Ohms, is short-c'ifciiiled, and so on at every following contact. 



If we place the whole of the platinnm wire in a Wheatslone 

 bridge, it is clear that when the gas is compressed, so when the 

 mercury in the tube rises, the resistance will be subjected to abrupt 

 changes. This is observed by replacing the galvanometer needle of 

 the bridge at zero, whenever a part of the platinum resistance has 

 been vshunted out by the mercury. At the outset of the experiment, 

 when the measuring tube is still entirely filled with gas, the resistance 

 of the whole platinum wire, which we shall call the volume wire, 

 is in the bridge. Whenever a reservoir has been filled with mercury, 

 the part of the volume wire wi-apped round that reservoir, is short 

 circuited. 



The resistances of the different parts of which the volume wire 

 consists, are known by the gauging of the measuring tubes. We 

 shall now proceed to a discussion of this gauging. 



An exceedingly narrow capillary CD is sealed to the measuring 

 tube. This capillary has the same length as the measuring tube and 

 is bent somewhat further round. The measuring tube is still open 

 at the top, and has a prolongation E, to which a rubber tube can 

 be fastened. An accurate scale-division G is attached to the narrow 

 capillary. The capillary is drawn out thin at the top and bent. 



If the end D is put in a vessel with mercury, the tubes will be 

 filled with mercury, when the air is sucked off at E. When the 

 mercury in the measuring tube is close to a point of contact, the 

 mercury can be made to move to and fro past it by suction or pres- 

 sure with two pumps ^) connected with E by means of a three-way 

 cock. The volume wire is then again inserted into a Wheatstone 

 bridge. The galvanometer needle deviates whenever the place ot 

 contact is passed. At the same time the mercury in the narrow 

 capillary tube passes up and down along the scale. After some 

 practice it is not difficult to read the position of the mercury in 

 the latter tube at the moment that the galvanometer needle deviates. 

 The best waj^ to do this is of course when the mercury in the 

 measuring tul)e rises, because the meniscus has then the same position 

 as during the measurements. 



If this has been done at a place of contact, the mercury may be 

 pressed from the reservoir under it by increase of pressure of E, 

 and the quantity that flows out at D may be received in a weighing 

 bottle; then the level of the mercur}' in the capillary tube is again 

 observed at the moment that the galvanometer deviates. It is clear 



^) Two cycle pumps, in one of wliich the leather valve has been put reversed. 



