22 CHAP. IV. AUTOMATIC RECORD OF EVOLUTION OF OXYGEN 



gas pushes up the oil and the drop of mercury with it through 

 a short length at each bubbling. The movement of the 

 mercury, though slight, is sufficient to complete an electric 

 circuit through the pencil provided with double contact. 

 The pencil consists of a conical thin rod of ebonite, on 

 opposite sides of which are sunk two platinum wires which 

 project slightly beyond the pointed end. The pencil 

 is carried by a clamp, which is fixed to the neck of the 

 plant- vessel. Before exposure to light, the platinum points 

 are so adjusted by the micrometer screw, A, that the circuit 

 is just completed through the drop of mercury, and 

 then the electromagnetic writer strikes against the drum. 

 A slight turn of the micrometer screw now lifts the pencil, 

 breaking the circuit, and the writer is released. By this 

 means a very sensitive adjustment is made, such that the 

 movement of the mercury-drop at bubbling completes the 

 circuit with the utmost certainty and the record of suc- 

 cessive bubbles takes place without any possibility of 

 failure (fig. 3). 



The Electromagnetic Writer. — A small horseshoe electro- 

 magnet attracts the armature to which the writer is 

 attached, the stroke of which makes a mark on the moving 

 piece of smoked paper. The armature is polarised, so that 

 the feeble current given by a dry cell is sufficient to ensure 

 record. 



The Condenser. — -A serious difficulty arises in prolonged 

 records from the oxidation of the mercury caused by the 

 spark occurring at the break of the electromagnetic circuit. 

 The scum thus formed causes an uncertainty in the con- 

 tact. This difficulty is, however, obviated by the use of 

 a condenser, c, connected in parallel with the two platinum 

 wires by which the circuit is alternately made or broken. 

 Sparking is thus eliminated and no uncertainty in the 

 record can exist. 



The Recording Drum. — An experiment by this method 

 may, in a large number of cases, be completed in the course 

 of about ten minutes ; in special cases, such as in researches 



