Researches on Irritability of Plants/Chapter 2
CHAPTER II
THE RESONANT RECORDER
It was stated in the last chapter that however light the contact may be, and however smooth the glass recording-surface, the record was still apt to be either arrested, or seriously distorted, on account of friction. As long as I employed the ordinary method of continuous contact of the writing-point with the glass surface, it was impossible to overcome this particular difficulty. It occurred to me at last that the problem might find a solution if I could succeed in making an intermittent instead of a continuous writing-contact. I have solved this problem by devising two different types of apparatus, which I have called respectively the Oscillating Recorder and the Resonant Recorder. In the former, the recording-surface itself is made by an electro-magnetic device, to vibrate to and fro, thus bringing it into periodic contact with the writing-point.[1] This apparatus is extremely convenient for the general purpose of recording responses in which the measurement of excessively short intervals of time is not essential.
But there are many important problems, such as the determination of the latent period, and accurate determination of the velocity of transmission of excitation, in which 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 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 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 itself is maintained in a state of persistent vibration by the usual electro-magnetic arrangement. I shall for the sake of convenience refer to this reed-interrupter as the Coercer; and the writing-index simply as the vibrating-recorder or Vibrator. The reed was at first purposely selected of too great a length, so that the natural frequency of the coercer should be slower than that of the vibrator. The free end of the coercing reed carries a platinum wire which, dipping into a cup of mercury, completes the electric circuit. The other end is clamped, and by shifting the clamping-point the vibrating length of the reed is gradually and continuously shortened. This has the effect of gradually raising the vibration-frequency of the interrupting reed. A time soon comes when the frequency of the coercer is exactly the same as that of the vibrator. The latter, which has been hitherto more or less inert, now suddenly breaks out, as foreseen, into very regular and sustained vibrations of large amplitude. For some purposes it is important that the vibration-frequency of the recording-index should have a definite value. The various frequencies most suitable for these researches were 10, 20, 50, 100, and 200 vibrations per second. The exciting reed was previously calibrated by means of frequency-meters, or standard tuning-forks, to give these values. Then, with great expenditure of time and patience, different vibrating-recorders having various standardised frequencies were constructed. For this we have to select fine-steel wires of differing lengths and thicknesses. The final tuning is accomplished by careful filing or hammering. Filing the tip of the vibrator raises the frequency; and hammering of the wire, near the point of suspension, lowers it. A general flattening, along the whole length, tends to maintain the vibration in a definite plane, otherwise the free tip is apt to execute an elliptical vibration. The tuning of the vibrator with the coercer is not very difficult when the vibration-frequency is low, say 10 per second. But when the frequency is high, say 100 or 200 times in a second, an exact tuning is essential. The slightest 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.
The Slide and Clockwork
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, holding the glass recording-plate, moves up and down the top part of the upright. It is essential to have this slide so accurately fitted that the plate-carrier may be able to drop
Fig. 3.—Upper part of Resonant Recorder. (From a Photograph.)
Thread from clock, not shown, passes over pulley P, letting down recording-plate; S′, screw for adjusting distance of writing-point from plate; S, screw for vertical adjustment; T, tangent-screw for exact adjustment of plane of movement of recorder, parallel to writing-surface; axis of writer supported perpendicularly at centre of circular end of magnet; C, coercer; M, micrometer-screw for adjustment of length of coercer; V, vibrating recorder; G, smoked-glass plate.
smoothly and uniformly, without any jerking whatsoever. I have sometimes attained the same end by mounting the plate-carrier on wheels and letting it slide down vertical rails. The plate-carrier is allowed to drop by the running down of a clock or a phonograph motor, according to what may be the requirement of the speed. The quarter-plate size (11 X 8 cm.) is convenient for record, as it is not too large for book illustration. The suspending thread passing over pulleys is wound round the winding-wheel of the clock. This wheel is provided with click and ratchet, which allow it to be wound without interfering with the axis of the clock. Thus winding of the wheel in the left-handed direction pulls up the recording-slide. The running-down of the clock then allows the slide to fall at a uniform rate. The various speeds found necessary for different records were such that the entire length of the plate, 11 cm., travelled past the recording-point in ·5 second, 6 seconds, 15 minutes, 1 hour, or 3 hours. The first two of these rates were obtained from a phonograph motor employing two different-sized wheels. The last three were obtained by attaching three different-sized wheels to the clock-axis which carries the minute-hand. In these slow rates the movement of the plate is quite uniform from the beginning, but when, as in the first two cases, this has to be dropped at a relatively high speed, a short time will elapse, equivalent at most to the first fourth of the plate, before it becomes quite uniform. Should uniformity of such movement be specially desired for the record, it must be commenced after passing this first fourth. But on account of the chronographic signals which accompany the record, this uniformity is not absolutely essential, for they give us the data from which the time-relations of the curve may be derived.
I may here refer to a few practical points with regard to the preparation of the glass for record and its subsequent fixing. In order to produce an even layer of smoke on the recording-plate, it is moved over the gas-flame from a bat's-wing burner; and this deposit of smoke will be improved if the gas has been previously passed through a jar containing a small quantity of benzine. After the record is taken, it is fixed by pouring carefully over it a dilute solution of canada balsam in xylol. It is afterwards easy to reproduce this record by contact-print on a photographic paper.
Adjustment of the Writer
It is sometimes necessary to have the recording-point brought two or more times to the same place, in order that the successive records may be rendered the more strictly comparable. This is accomplished by a rack and pinion to adjust the height of the platform carrying the plant. When the platform is lowered, the petiole, which is attached to one arm of the recording-lever, pulls it down, and the recording-point is moved to the left. When the platform is raised, then by the action of the counterpoise attached to the other arm of the lever the index is moved in the opposite direction. In this way the recording-point can be brought to any position that is desired. The same end is secured through adjustments of a micrometer by which the carrier of the writing-index is raised or lowered.
Another adjustment that is necessary is the bringing of the recording-point near to the writing-surface without actual contact; so that, when the index is set in a state of resonance, it may trace a dotted line. The necessary adjustment is brought about by means of a micrometer-screw at the top of the instrument, by which the lever can be made to approach or recede from the writing-surface. When the speed of the plate is slow, the successive dots may be so close together as to appear like a continuous line.
More troublesome is the adjustment necessary to render the plane of movement of the index exactly parallel to the writing-surface. If this be omitted, the writing-point in one part of the record, say to the right, will be too far away to strike the surface, whereas in another part, say to the left, it will press against the plate and lose its freedom. This difficulty I have been able to overcome by mounting the vertical rod, carrying the writer, inside another tube. An attached tangent-screw, T, then causes a very slow rotation of the vertical rod, either in the right-hand or the left-hand direction. By this means it is possible to bring the
Fig. 4.—General view of the whole apparatus, and the electrical connections by means of which excitatory shock of a definite duration may be given to the plant; duration of shock determined by metronome which completes electrical circuit. (From a Photograph.)
plane of movement of the writing-index exactly parallel to that of the writing-surface. The complete apparatus for obtaining response of Mimosa is shown in the accompanying illustration (fig. 4).
We have next to consider the practicability of devising, for the excitation of the plant tissue, perfect methods of stimulation, the intensity of which can either be maintained constant or varied in a perfectly known manner. We have moreover to render the successive stimulations, and consequent scripts of the plant, a perfectly automatic process; so that the experimenter may be comparatively relieved of personal participation in the securing of the records. This will have the incomparable advantage of having no element of personal error in the results so obtained. The question of the effects of the various forms of stimulus will be dealt with in the next chapter.
Summary
In the response-records of plants, errors are introduced on account of friction of the writing-point against the recording-surface.
These errors are eliminated by the method of intermittent instead of continuous contact for the record. By employing the principle of resonance, the writer is made to vibrate to and fro at a known and definite rate. The record consists of series of dots giving definite time-intervals. The record is thus its own chronogram.
- ↑ Bose: British Association Report, Dublin, 1908, p. 903.