we require time-measurements of the order of one-hundredth of a second. It will be shown that these determinations can be carried out with great precision, by means of the intermittent dots themselves, when the periodicity of their recurrence is rendered perfectly constant. For such purposes, then, we require a frequency of intermittent contacts amounting to something like a hundred times per second.
It was clearly impossible to make the heavy plate-carrier oscillate with such a high frequency. There remained only the theoretical alternative of causing the writing-point to vibrate to and fro, at the required frequency, so as to make the necessary intermittent contacts with the surface of the recording-plate.
The advantage of this intermittence may be understood from a concrete example. It will be remembered that the writing-point, under the action of the responsive fall, moves parallel to the surface of the recording-plate. If now, by means of some mechanism, the writing-point be made to vibrate to and fro, say, ten times each second, at right angles to the plate, this will in no way affect the record beyond the fact that instead of a continuous line a dotted line will be traced. The record will not now labour under the defects inseparable from the friction of continuous contact. Instead of this, we shall have the vibrating writer tapping a record which is practically free from friction. For it will be understood that, as in our concrete example, a recording-point which is vibrating ten times each second will execute one entire to-and-fro movement in one-tenth of a second. The duration of contact, at the extreme forward end of the swing, will represent only a small fraction, say one-fifth, of the entire period of one vibration. Hence after each contact, lasting only one-fiftieth of a second, the recording-point is absolutely free to take up the movement impressed upon it by the moving leaf. In a record lasting for one second the sum of the intermittent contacts will then amount to one-fifth, and the period of entire freedom to four-fifths of a second. We can thus see the theoretical
