Collected Physical Papers/The Magnetic Crescograph and the Magnetic Radiometer
XXVII
THE MAGNETIC CRESCOGRAPH AND THE MAGNETIC RADIOMETER
In winter the growth appears to be in a state of arrest; in reality growth may persist but its rate is too slow to be detected even by the High Magnification Crescograph in which two levers produce a compound magnification of 10,000 times. It may be thought that further magnification could be obtained by a compound system of three levers; there is, however, a limit to the number of levers that may be employed with any advantage, for the slight overweight of the last lever becomes multiplied, exerting a tension so great on the plant as to interfere with the normal rate of its growth. The friction at the bearings also becomes added up by an increase in the number of levers which obstruct the free movement of the last recording lever. Magnification requiring additional material contact between different levers have, therefore, to be abandoned and it became necessary to devise an ideal method of magnification without contact. In the device perfected for this purpose a single magnifying lever attached to a growing plant upsets a very delicately balanced magnetic system (cf. fig. 114). The indicator is the reflected spot of light from a mirror carried by the deflected magnet. I have thus been able to produce a magnification as high as 50 million times. This order of magnification would lengthen a single wave of sodium light to 2,500 cm. It is obvious that this method of super-magnification would be of great help in many physical investigations.
The Demonstration Crescograph
Such an enormous magnification can not be employed in ordinary investigations on growth, for the indicating spot of light rushes on like a flash. I had, therefore, to reduce the magnification to a million times by which demonstrations of a striking character on various phenomena of growth can be given before a large audience. The following may be taken as a typical example:—
The normal rate of growth of the experimental flower-bud of Crinum Lily was 0.0006 mm. per second. A scale 3 metres long divided into centimetres was placed against the screen. A metronome beating half seconds is started at the moment when the spot of light transits across the zero division; the number of beats is counted till the index traverses 300 cm. At the normal temperature of the room (30° C.) the index traversed 300 cm. in 5 seconds. As it is easy to measure 1 mm. on the scale, the Magnetic Crescograph enables measurement of growth in a period shorter than 1/500th of a second.
After observing the normal rate at the temperature of the room, the plant chamber was cooled to 26° C. by blowing in cooled water vapour; the time now taken by the spot of light to traverse the scale was 20 seconds, i.e., the growth-rate was depressed to one-fourth. Under continuous lowering of temperature there was a continuous retardation, till at 21° C. growth was arrested. Warm vapour was next introduced raising the temperature of the plant-chamber to 35° C. The spot of light now rushed across the whole length of the scale in 112 seconds, growth being enhanced to more than three times the normal rate. The entire series of experiments on the effect of variation of temperature on growth was completed in the course of 15 minutes.
Experimental demonstrations of the action of light, and of various chemical agents can also be given in a similar manner.
The Magnetic Radiometer
As an example of the application of the method of high magnification in physical investigations, I describe the Magnetic Radiometer in determination of the energy of different rays of the solar spectrum by measuring the elongation of a metallic wire coated with lamp-black. The particular spectral ray falling on the wire is absorbed, and thus raises the temperature proportionately to the energy of radiation. The resulting increase of length is so minute as to be undetectable by any method of magnification hitherto available.
A diagrammatic representation of the apparatus is given in fig. 114. W is a length of zinc wire which becomes lengthened by rise of temperature produced by absorbed radiation. It is attached by a hook to the short arm of a long magnetic lever, the N end of which is lowered by any elongation of the sensitive wire. In front of the N end of the magnetic lever is suspended a small magnetic needle with an attached mirror. As the N pole of the magnetic lever is lowered, it produces an increasing deflection of the suspended needle which is magnified by the spot of light reflected from the attached mirror. The sensitiveness of the apparatus is very greatly enhanced by the employment of a perfect system of astatic needles.
Fig. 114. The Magnetic Radiometer.
This method for the measurement of radiation is extraordinarily sensitive. For instance, there is an apparatus permanently adjusted in the Institute in which the sensitive element is a thin strip of ebonite which is enclosed in a tube with a narrow slit in front. When any person walks at some distance past the instrument, the indicating spot of light remains perfectly quiescent until the individual is in line of sight of the sensitive strip; a sudden deflection is then produced by the radiation which is emitted from the human body, through thick warm clothing and a heavy overcoat; when the person walks past the line of sight the indicating spot of light returns to its original zero.
The zinc wire was surrounded by a wooden tube with a narrow slit for the passage of the spectral rays. The slit was open through a length of 10 cm., the brass piece to which the strip was soldered being protected from radiation. A very thin piece of mica covered the slit, to prevent air-currents getting access to the narrow chamber in which the sensitive strip was enclosed. The exceedingly thin piece of mica was found practically to have no effect in obstructing radiation. The whole apparatus was placed inside a larger wooden box covered with non-conducting felt. A round hole in front of the box (covered with a piece of thin glass) allowed the passage of the spot of light reflected from the mirror attached to the suspended magnetic needle.
It is necessary to give a detailed account of precautions to be observed, because the instrument is so sensitive that it detects the slightest difference in the temperature of the different portions of the same mass of air. Gases are highly non-conducting, and their temperature hardly attains a perfect uniformity. In these circumstances it is best to take precautions against contact of the strip with the outside air, which, again, should not be disturbed in any way. The observer never moves from his place in the dark room, which is closed on all sides.
The spectrum produced by the carbon disulphide prism was found to extend beyond the limit of the visible red, this extension into the infra-red region being almost 6 cm., or about one-third the breadth of the visible spectrum.
The extreme sensitiveness of the Radiometer was also exhibited by its discrimination of the radiation components of the morning and midday light. In the morning the intensity of the blue rays in the spectrum was found to be slightly less than at midday. This is due to the greater scattering of the short waves by the thicker stratum of the atmosphere through which the light has to pass in the morning.
The energy of radiation in the different regions of the spectrum was compared on January 16 and 19, 1923. It should be remembered that the determination of the energy of the different rays was made upon the spectrum given by the carbon disulphide prism. The following results for the 16th and 19th January are seen to be practically the same.
Table showing the Distribution of Energy in the Spectrum produced by the CS2 Prism
| Wave-length. | Energy of Radiation. | Mean. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 16th January. | 19th January. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Radiation was detected at some considerable distance beyond the extreme red. At about 850 μ μ the mean deflection of the Radiometer was found to be 85 divisions; at 790 μ μ the energy was found to be 147, which was the maximum. At A (760 μ μ) the deflection declined to 145; at a (720 μ μ) it was reduced to 132; C (656 μ μ) gave a deflection of 97. At the sodium line D (590 μ μ) the reading was 71; the energy of the spectrum underwent a further and continuous decline; at E, F, and G the deflections were 42, 27, and 10 respectively.