A Self-Recording Poroweter and Potometer. 363 
relay R in connection with the recorder are operated by a cell Z', 
in the event of the circuit becoming closed by the mercury index 
in the tube BD' reaching the terminals t 1 1'. 
If the apparatus is started thus arranged the following is the 
course of events. The mercury index passing up the tube BB', 
reaches the terminals t't' and completes the circuit in which the cell 
Z is included. The relay R is therefore brought into action and 
a record registered : the relay R' is also affected and the valve at C 
opened as described above. The mercury index then passes back 
towards B, when contact is broken at t't 1 and the valve at C closes. 
The mercury index is thus left somewhere between tt and t't'. 
In order that the valve may remain open until the index has 
returned to the lower end ( B ) of the tube BB' the following device 
is used. 
The armature P of the magnet M carries, on an arm, an 
adjustable screw IV. A small electro-magnet N with a moveable 
armature S—such as is used in electric bells—is fixed so that the 
spring V attached to S makes contact with W when the armature 
P moves upwards. V is connected with the battery Z and W with 
the terminal in' by wires as shown. 
The magnet N is included in a circuit consisting of a cell Z", 
the relay R and the terminals tt. 
Thus, after the valve at C 1 has been opened by contact of 
the index with the terminals at t't 1 , the mercury commences to fall 
back in the tube BB 1 as described above : but in spite of the contact 
at t't 1 being broken the valve remains open owing to the establish¬ 
ment of the circuit through VW. 
When the index reaches tt on its downward journey, the circuit 
containing the magnet N is completed, the armature S moves 
upwards and the contact at VW is broken. 
The valve at C' thereupon closes and the index is in position 
to start a fresh record. 
Despite this somewhat involved description the actual working 
is quite straightforward when the electrical connections are 
correctly made. 
Briefly—(a), the electro-magnet M serves to open the valve 
at C‘ when the mercury index reaches t t', (b) the contact VW serves 
to maintain the valve open until the mercury index reaches tt , (c) 
when the magnet N comes into operation and by breaking the 
connection VW allows the valve to close. 
In addition, whenever the index passes either tt or t't' a record 
of the fact is made on the recording drum. 
