14 RESEARCHES ON IRRITABILITY OF PLANTS 



strictly symmetrical position, the movement of the writer 

 caused by the excitation of the leaf would draw it into an 

 asymmetrical position, resulting in a one-sided pull which 

 would seriously interfere with the reliability of the record. 



The Resonant Recorder 



It is therefore absolutely necessary so to arrange matters 

 that the electro-magnet shall be without laterality. This 

 condition I was able to fulfil by making the pole of the 

 electro-magnet in the form of either a cylinder or a ring. 

 The axis, from which is suspended the writing-index, is 

 accurately supported, perpendicular to the plane of the 

 circular section of the magnetic pole at its centre. Every- 

 thing was thus made symmetrical, and as there was no 

 laterality there could be no tendency whatsoever for the 

 index to execute its to-and-fro vibrations in any other 

 direction than that which was perpendicular to the plane 

 of the terminal pole of the magnet. As this plane may be 

 adjusted parallel to the glass recording-surface, the tapping 

 movement of the writing-index can be made to take place 

 perpendicularly to the recording-surface. 



Next, in order to overcome the difficulty of the irregular 

 timing of those electrical impulses which are to maintain 

 the recording-index or writer in a state of periodic vibra- 

 tion, I devised the Resonant Recorder. If we know the 

 natural frequency of vibration of the recording-index, and 

 if by means of some mechanism we can send periodic 

 currents of exactly the same frequency through the electro- 

 magnet, then the intermittent magnetic pulls will exactly 

 synchronise with the natural swings of the writing-index. 

 Owing to this perfect tuning the index will now resonate, 

 breaking out into a persistent and regular vibration of con- 

 siderable amplitude. In practice, all that is necessary in 

 order to secure this is to take a long steel reed, which in the 

 course of its regular vibration will periodically interrupt 

 the electro-magnet circuit of the vibrator coil. The reed 



