Collected Physical Papers/The Self-Recording Radiograph
XXV
THE SELF-RECORDING RADIOGRAPH
A diurnal periodicity is generally exhibited in the various activities of the plant and in the movements of its different organs. This periodicity must be related to external variations, notably of temperature and light. It would be impossible to analyse the resulting phenomenon unless a continuous record of changes in the intensity of light is secured with the same exactitude as that of variation of temperature. As regards these two principal factors, the effect induced by the rise of temperature is often antagonistic to that of increasing intensity of light. A rise of temperature, for example, enhances the rate of growth up to an optimum; light, on the other hand, acts as a stimulus in retardation of growth.
For full analysis of diurnal periodicity in plants, it thus becomes necessary to devise means for continuous record of variation of temperature and the changing intensity of light. In regard to the variation of temperature, a simple and reliable type of thermograph has been devised, by which it is easy to obtain a continuous record of the variation of temperature throughout the day and night. No apparatus is, however, available for the continuous registration of variation of the intensity of light.
The Selenium Cell
Selenium is well known for the characteristic diminution of its electric resistance under illumination. When an electric current is maintained in the circuit, exposure of the selenium cell to light causes an increase of the galvanometer deflection. Several difficulties are, however, encountered in employing it for a continuous record of fluctuation of light for the whole day. The resistance of selenium undergoes a change under the polarising action of an electric current, the variation increasing with the strength and the duration of the current. The effect of polarisation is however negligible, if the current be feeble and of short duration. There is a possibility of another difficulty arising from the effect of daily variation of temperature on the normal resistance of the selenium cell. Allowance for this could be made by taking a continuous record of the effect of hourly variation of temperature on the resistance of the cell kept in darkness. Finally means have to be devised for automatic record of galvanometer deflections under changing intensities of light.
The Radiograph
The difficulties encountered in obtaining automatic record have been removed by the following devices:—
(a) The Wheatstone Bridge for balancing electric resistance of the selenium cell in dark and its upset on exposure to light.
(b) The arrangement of three electric keys which are automatically put on and off in regular sequence and at pre-determined intervals.
(c) The Self-recording Galvanograph.
The Wheatstone Bridge
This is diagrammatically represented in B (fig. 106). The resistance of the particular selenium cell S is 76,000 ohms in the dark. An approximately equal resistance is placed in the second arm of the bridge. A rheostat having a large number of turns of fine wire with a sliding contact is used for the two variable arms of the bridge, diagrammatically represented by a straight line. Under exact balance the galvanometer deflection is reduced to zero. The balance is upset when the selenium cell is exposed to light and the resulting deflection gives a measure of the intensity of light.
The Automatic Keys
After previous adjustment of the balance in the dark the electric circuit is completed by the closure of key K₁;
Fig. 106. The Self-recording Radiograph.
the selenium cell is next exposed to light by an automatic electro-magnetic shutter T. The deflection of the galvanometer is recorded by means of electric sparks on a piece of moving paper. The different operations are carried out in proper sequence by the automatic devices described below.
K₁ completes the battery circuit for about 10 seconds, by which time the record is completed. The successive records for variation of light are taken at intervals of 15 minutes; the periodic closures of the circuit are thus for 10 seconds at intervals of 15 minutes. This short passage of the current is found in practice to cause no polarisation.
The second key K₂ actuates an electromagnetic device by which the trap-door, T, is opened for the definite period of one second; the selenium cell S inside the dark box is thus exposed to light for this length of time. The trap-door is seen in the diagram immediately above the dark box. In reality it is at the upper end of a vertical tube the inside of which is coated with lamp-black to prevent side reflection. The light that falls on the selenium cell is thus from a definite area of the sky. The intensity of light from the sky at different periods of the day causes deflection of the galvanometer which is proportional to that intensity. The maximum deflection of the galvanometer employed is attained in the course of 3 seconds after the exposure.
The third key K₃ is for completion of spark circuit R for record of the maximum galvanometric deflection three seconds after the exposure of the selenium cell. This key actuates a sparking coil R, the vibrating interrupter of which is not shown in the figure. The spark, thus produced, punctures the maximum deflected position of the galvanometer index on a moving piece of paper attached to the plate M.
The successive closure and opening of the keys are made automatically and in proper sequence by means of a clock work, the whole process being repeated at intervals of 15 minutes.
The Galvanograph
The most difficult problem is the automatic record of galvanometric deflections. This can be secured without any difficulty by means of photography. A spot of light reflected from the galvanometer mirror is in this case allowed to fall on a photographic plate which descends at an uniform rate by clockwork. This, however, entails the use of a dark room and subsequent development of the plate. The trouble was avoided by the device of direct record of the galvanometer deflection by means of electric sparks.
The sparking method has been previously employed in which the deflected index of the galvanometer in connection with one electrode of an induction coil leaves a spark record on a moving piece of paper. Several difficulties are, however, encountered in employing this method for record with a highly sensitive galvanometer. There is a liability of leakage of the high tension current into the galvanometer circuit. The discharge of the spark gives moreover a backward kick to the index by which the normal deflection undergoes an unknown variation.
The above difficulties have been removed in the following manner. The moving coil of the sensitive D'Arsonval galvanometer, has a long glass index I, at right angles to the plane of the coil. The glass index is coated with shellac varnish to render it highly insulating. The index is projected to a short distance on the opposite side, for attachment of a counterpoise; this takes the form of a vertical vane of mica which acts as a damper. The galvanometer itself is of an aperiodic type, and the addition of the damper makes it perfectly dead-beat. The sensitiveness of the galvanometer is such that a micro-ampere of current produces a deflection of 10 mm. of the index. The recording index has attached to it a short vertical piece of thin platinum wire pointed at its two ends; this end of the index moves between a sheet of metal M, on which is spread the recording paper, and a semi-circular piece of narrow metal sheet C. The metal sheet M is mounted on wheels and moves at an uniform rate by clockwork. Record is made by sparks. One electrode of the sparking coil is in connection with C, and the other with M. The sparking thus takes place simultaneously, above and below the vertical and double-pointed platinum wire carried at the end of the index. There is thus no resultant kick, and the index remains undisturbed. The sparking, as previously stated, takes place three seconds after exposure of the selenium cell to light, by which time the deflection reaches its maximum. The record thus consists of successive dots at intervals of 15 minutes, the dots representing the maximum deflections of the galvanometer corresponding to the intensities of light.
The record given in figure 107 was taken about the end of January; the sun rose at about 6-45 a.m. and set at 5-30 p.m. The twilight is very short in the tropics; the sky is feebly lighted about 6 a.m.; it becomes dark about 6 p.m. The record shows the intensity of light to be exceedingly feeble at 6 a.m. The rise in the intensity was rapid, attaining the maximum at 12 noon.
This will be designated as the light-noon. The intensity of light then declined at a rate slower than the rise; after 5 p.m. the fall of intensity was extremely rapid.
It was stated that there was a possibility of change of resistance induced by diurnal variation of temperature. In order to determine the extent of this variation, a spark record was also obtained before exposure to light. The dotted record near the base (fig. 107) shows that the resistance remained practically constant, inspite of the variation of the temperature.
An important point arises as regards the diurnal variation of light and of temperature, and determination of their periods of maximum and minimum. For this purpose records of diurnal variations of temperature and of light were taken on the same day in summer with the
Fig. 108. Record of diurnal variations of light and of temperature in summer.
thermograph and the radiograph. The two curves are given in figure 108; it will be seen that while the maximum intensity of light is at 12 noon, the thermal maximum is at about 2 p.m. The thermal noon is thus two hours later than the light-noon. Light disappears at night from 6 p.m. to 6 a.m., that is to say, the period of minimum or zero light is prolonged for 12 hours. But the fall of temperature is gradual, and the minimum is attained at about 5 a.m. which is the thermal dawn. The characteristic variations of these two important factors should be borne in mind, since the diurnal movements of plants are modified by the algebraical summation of the effects of light and of temperature.
It is sometimes desirable to carry out researches during a period. when the intensity of light remains approximately constant. This period is found to be between 11 a.m. and 1 p.m. for the variation is then only ±5 per cent. of the mean.
The record given of the diurnal variation of light is true of days when the sky is clear. But the passage of clouds causes change in the intensity which is accurately recorded by the Radiograph. A record of such irregular variation in a stormy day is given in the lower record of figure 107.