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. Everything 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 vibration, 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 considerable 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
