Page:Researches on Irritability of Plants.djvu/39

16 variation of the length of the coercer will either bring about full resonance or make it entirely ineffective. When the tuning is nearly but not quite perfect, then we have the phenomenon of beats. In this case, in the successive dots of the record, there will be periodic blanks. For the purpose of exact adjustment of length of the coercer I employ a micrometer-screw, by means of which the most delicate adjustment of length may be carried out.

For periodic interruption the coercing and vibrating coils may be put in series, but I find it is much easier to obtain a persistent vibration when the coercer coil is placed in a multiple arc with the vibrator coil. An electro-motive force of 4 volts should be sufficient for the purpose of maintaining a steady vibration of both the coercer and the vibrator.

Having thus secured the requisite perfection of the resonating-writer, it is necessary to describe the complete apparatus by which to obtain records of responses in Mimosa and other sensitive plants. For this purpose we require a slide-carrier to hold the recording-plate, and this is to be dropped at a definite speed, without jar; also the clockwork by which it is to be actuated. Besides these is needed some special means by which the recording-point may be brought to the proper distance from the recording-surface. It is necessary, again, that the response-movement of the writer should be absolutely parallel to the writing-surface, and that its tip or contact-point should be capable of delicate adjustment as regards distance. It should be possible, moreover, to bring this writing-point to any position on the recording-surface that may be required. I will now proceed to relate the devices by means of which all these conditions have been met. Some of these will be seen in fig. 3, which illustrates only the upper part of the Resonant Recorder.

A gunmetal upright, the upper part of which is of triangular section, stands on a large disc of the same metal, which is screwed to a larger wooden base-board. The slide-carrier,