360 
W. Neilson Jones. 
is small in quantity; none of the forms of potometer in ordinary 
laboratory use, however, is self-recording. The self-recording type 
of apparatus now to be described has been found to work satis¬ 
factorily and can be used in conjunction with the recording 
porometer. 
For convenience of description it may be considered to consist 
of the following parts—(1) the recorder, (2) the potometer-tube, (3) 
the valve, (4) the electrical control. 
1. The Recorder. The recorder described above for the 
recording porometer is used without modification. If the two 
pieces of apparatus are required for use simultaneously, a duplicate 
recorder must be set up. 
Some means of lowering gradually the base of the recorder 
must be adopted—such as allowing its supporting cord to be slowly 
played out from a drum attached to a klinostat—so as to prevent 
overlapping of the record. 
2. The Potometer Tube. This consists of a straight length of 
tubing BB 1 of about 1 mm. bore the ends of which are bent up and 
fused to short lengths of tubing of wider bore (3-4 mm.) AB and 
A' B'. The whole tube has the form shown in the figure. Platinum 
terminals are fused into the tube BB' at tt and t't'. 
3. The Valve. A T-piece CC D (CC' being of wide bore tubing) 
has a rubber cork with a small central hole fitted firmly into the 
lower end C. A long piece of glass tubing EE’ of a diameter too 
large to pass through the hole in the rubber cork at C , has the lower 
end at E' drawn out to a much narrower diameter: this narrowed 
portion fits loosely into the hole in the rubber cork at C'. At F, on 
the tube EE', is fixed a rubber cork connected with the T-piece CC 
at C by a piece of thin black rubber tubing. This tubing must be 
thin walled and readily stretched : the attachment to the cork F may 
be made secure by binding with wire. The upper end of the tube 
EE' leads to a reservoir K. 
The lower end C of the tube CC' is joined to the end A of the 
potometer tube by means of a piece of rubber tubing : while the side 
arm D is joined to the end of the potometer tube by means of the 
T-piece G and rubber connections shown in the diagram. 
To the open end of G the experimental branch (or plant) is 
attached by an arrangement such as that shown at H. A mercury 
index about 1 cm. long is introduced into the tube ABB' A' and the 
whole apparatus filled with water. The method of working is as 
follows. 
