advantages of an intermittent over a continuous contact. What has been said of the writer vibrating ten times in a second holds good equally in those cases where the vibration-frequency is much higher.
One great difficulty we encounter in carrying out this idea lies in giving the recording-point an impulse exactly perpendicular to the direction of its recording movement. If the recording-writer be made of fine steel-wire, and if we place behind it a small electro-magnet—the pole consisting of a rectangular piece of soft iron, at right angles to the wire—then, by sending a momentary strong current through this electro-magnet, a pull will be exerted on the wire which will make its recording-tip strike for an instant against the glass recording-surface. As the steel wire has to be made extremely fine, in order to reduce to a minimum the inertia of the recorder, the resulting pull exerted on it is very slight, unless an excessively strong current be sent round the electro-magnet. Again, unless the intermittent closures of the electric circuit be properly timed, the writing-index may be subjected to attraction in the course of its journey, now to the recording-surface and again away from it. In the latter of these cases its vibration, on which the intermittent contact depends, is totally destroyed.
But the most serious difficulty of all is that introduced by the edge of the attracting electro-magnet. It is known that the magnetic intensity of a pole is strongest at its edges. Should the writing-index by chance be placed exactly symmetrically, as regards the right and left edges of the pole, then the two lateral pulls, being equal, will neutralise each other, and the index will vibrate to and fro perpendicularly to the recording-surface. But should it be placed, however slightly, nearer to one edge than to the other, then one of the two pulls will be in excess, and the index will be drawn to one side, thus producing a disturbance in the record not due to the excitatory pull of the leaf. Even if, at the beginning, the index had been placed in a
