Collected Physical Papers/On Polarisation of Electric Rays by Double-Refracting Crystals
COLLECTED PHYSICAL PAPERS
I
ON POLARISATION OF ELECTRIC RAYS BY
DOUBLE-REFRACTING CRYSTALS
A beam of ordinary light incident on a crystal of Iceland spar is generally bifurcated after transmission, and the two emergent rays are found polarised in planes at right angles to each other. The object of the present enquiry is to find natural substances which would polarise the transmitted electrical ray. It was thought that the analogy between electric radiation and light would be rendered more complete if the classes of substance which polarise light were also found to pdlarise the electric ray. The two phenomena may be regarded identical if the same specimen is found to polarise both the luminous and the electric rays.
As the wave length of electrical radiation is very large compared with that of visible light, it may be thought that very large crystals, much larger than what occur in nature, would be required to produce polarisation of the electric ray. I have, however, succeeded in obtaining polarisation effects with crystals of moderate size. This I was able to do by reducing the length of electric waves to about 5 mm. or so.
These experiments show that certain crystals produce double refraction, and that the transmitted beams are polarised. With the help of a rudely constructed apparatus, I was able, last year, to detect traces of these effects. The apparatus has since been improved in detail; it is now possible to detect the polarisation effect with certainty.
The usual optical method of detecting the bi-refringent action of crystals, is to interpose the double refracting structure between two crossed Nicols. The interposition of the crystal generally brightens the dark field. This is known as the depolarising effect, and is regarded as a delicate test for double refracting substances. There is, however, no depolarising action when the principal plane of the crystal coincides with the polarisation planes of either the Polariser or Analyser. The field also remains dark when the optical axis of the crystal is parallel to the incident ray.
A similar method was adopted for experimenting with polarised electric radiation. A parallel electric beam is first polarised by a wire grating. A similar grating acts as an Analyser. The two gratings are crossed, and the crystal to be examined is interposed. The Receiver is a modified form of "Coherer" with its associated voltaic cell and Galvanometer. Brightening of the field is indicated by a throw of the Galvanometer needle.
Apparatus used
The following are the different parts of a complete apparatus:—
Fig. 1. Polarisation Apparatus.
B, Metallic box enclosing the Ruhmkorff coil and Radiator.
K, The crystal to be examined. E, Voltaic Cell.
G, The Galvanometer. R, tube enclosing sensitive receiver.
As an electric generator, I at first used a small Ruhmkorff coil actuated by a battery. I, however, soon found that the usual vibrating arrangement is a source of trouble; the contact points soon get worn out, and the break becomes irregular. The oscillation produced by a single break is quite sufficient for a single experiment, and it is mere waste to have a series of useless oscillations. But the most serious objection to the production of secondary sparks, unless absolutely wanted, is their deteriorating action on the spark balls. Anyone who has tried to obtain an oscillatory discharge knows how easily the discharge becomes irregular, and the most fruitful source of trouble is often traced to the disintegration of the sparking surface. In my later apparatus, I have discarded the use of the vibrating interrupter. The coil has also been somewhat modified. A long strip of paraffined paper is taken, and tin-foil pasted on opposite sides; this long roll is wound round the secondary to act as a condenser. In this way there is a great saving of space. The two ends of the primary are in connection with a small storage cell through a tapping key. The coil, a small storage cell, and the key are enclosed in a tin-box; a small opening behind allows the stud of the press-key to pass through. In front of the box there is an opening, through which the radiating tube projects. The radiating box, thus constructed, is very portable. The one I have been using for some time past, is 7 inches in height, 6 inches in length, and 4 inches in breadth. There is another one which is still smaller.
The radiator tube is 2.5 cm. in diameter. As an instance of the efficiency of the radiating apparatus, I may here mention the fact that the storage cell was charged about a month ago; I have since been using the apparatus for electro-magnetic radiation almost every day. All the while it required no attention, and there was no further necessity of polishing the sparking surfaces. To obtain a flash of radiation I have only to press the stud and release it, and on an average, I require about fifty flashes for a day's work. For working an entire month it is therefore only necessary to have a little over a thousand breaks of the primary current. With the usual vibrating interrupter a larger number of breaks would have been necessary even for one hour's work.
Lens for rendering the beam parallel.—In my first apparatus, with the help of an ordinary glass lens and suitable diaphragms, the beam was made approximately parallel. This was more or less a guess-work, as the index of glass for the electric ray has not yet been determined. I have, however, succeeded in determining the electric index for Sulphur, which is very near 1.734. With the knowledge of this index, a cylindrical lens of Sulphur has been constructed, whose focal distance is known with accuracy. The source of radiation, the spark gap, is a line, and the lens is adjusted till its focal line and the spark gap coincide. In this way, a parallel beam of electric radiation is obtained.
Polariser and Analyser.—The success of the experiment depends greatly on the care with which the Polariser and the Analyser are constructed. Fine copper wire, 0.2 mm. in diameter, was carefully wound in parallel lines round two thin sheets of mica; there were about 25 lines in one centimetre. The mica pieces were then immersed in melted paraffin, and the wires thus fixed in situ. By cutting round, two circular pieces containing the gratings were obtained. One of these acted as a Polariser and the other as an Analyser.
Receiver.—The receiving circuit consists of a spiral-spring coherer in series with a modified Daniell cell and an aperiodic galvanometer of D'Arsonval type.
In a square piece of ebonite a shallow rectangular depression is cut out, and a single layer of steel spiral springs 2 mm. in diameter and 1 cm. in length is laid 
A pair of insulated wires from the ends of the receiver are led out to a galvanometer placed at a distance. The leading wires are shielded from radiation by enclosing them inside two coatings of tin-foil. As an additional precaution, the galvanometer and the voltaic cell are also enclosed in a metallic case with a slit in front of the galvanometer mirror. A spot of light reflected from the mirror is received on a scale. By adjusting the electromotive force of the circuit, the sensitiveness of the receiver may be increased to any extent desirable.
This is most simply effected by the following arrangement of a modified Daniell cell and a shunt. A small U tube is taken and the two limbs filled with copper sulphate and sulphuric acid, respectively; the bent portion of the tube is plugged with asbestos to prevent rapid mixing of the two liquids. A sliding ebonite piece carries a rod of zinc and a rod of copper, which are plunged in the two electrolytes. The cell is shunted with a resistance of about 10 ohms and the current flowing through the shunt, and therefore the derived E. M. F. is varied by varying the resistance of the cell by raising or lowering the electrodes. When no current is required, the rods are raised out of the liquids. A cell thus constructed is ready for use at a moment's notice, and will work for several days. The receiving circuit does not respond to the incident radiation unless a suitable E. M. F. acts on the circuit. The above simple method of adjusting the proper E. M. F. will be found very simple and effective.
When the Polariser and the Analyser are properly constructed, and the two exactly crossed, no radiation can reach the sensitive surface, and the galvanometer will remain unaffected. The field is then said to be dark. Any slight rotation of either the Polariser or the Analyser partially restores the field, and the galvanometer spot of light then sweeps across the scale.
Method of Experiment
The spark gap of the Radiator is adjusted in a vertical line. The wires of the Polariser are horizontal, and the transmitted electric ray is plane-polarised, its plane of vibration being vertical. The Analyser is now adjusted in a crossed position; on producing a flash of radiation by a single break of the primary, there is no effect on the galvanometer. The crystal to be examined is now interposed with its principal plane inclined at 45° to the horizon.
Rhombohedral System
(1) Beryl.—The first piece experimented on was a large crystal of Beryl. It is a hexagonal prism with basal planes. The specimen examined has each face 11 × 5 cm. The three axes lying in the same plane are inclined at 60° to each other, the fourth axis, which is also the optical axis, is at right angles to the plane containing the other three. The crystal was optically opaque.
On interposing this block with its principal plane inclined at 45°, the galvanometer spot flew off the scale. The crystal had thus produced the well-known depolarising action. The crystal was now turned round till its principal plane coincided with the vibration plane of the Polariser. There was now no action on the galvanometer. On continuing the rotation, the galvanometer at once responded. The spot became quiescent a second time, when the principal plane coincided with the plane of vibration of the Analyser.
The crystal was now placed with its optical axis parallel to the direction of the incident ray. There was now no action on the galvanometer. Rotation of the crystal round this axis did not produce any effect on the galvanometer. The field continued to be dark.
The piece of Beryl used in the above experiment was unusually large. But in the following experiments the crystals were quite small.
(2)Apatite.—In repeating the experiment with this crystal, strongly marked double refraction effect was exhibited.
(3) Nemalite.—This is a fibrous variety of Brucite, silky in appearance. In its chemical composition it is a hydrate of magnesia. This specimen exhibited a very strong depolarisation effect with a thickness of less than one cm.
Rhombic System
Barytes.—A piece of this crystal was found to be strongly double-refracting.
Triclinic System
Microcline.—This is a greenish blue crystal of the double oblique type. It exhibited polarisation effect in a remarkable degree.
Regular System
Rock Salt.—A large piece of this crystal did not produce any effect. This is what was expected.
Having satisfied myself of the fact that systems of crystals, other than regular, produce double refraction and consequent polarisation of the electrical ray, I tried the action of electric radiation on crystals ordinarily used in optical apparatus.
I got a fairly large piece of black Tourmaline. On interposing this with its plane inclined at 45°, there was prompt movement of the spot of light. There was no galvanometric indication when the principal plane of the Tourmaline coincided with the vibration planes of either the Polariser or the Analyser.
With ordinary light, a piece of Tourmaline of sufficient thickness absorbs the ordinary, but transmits the extraordinary ray. With the piece of Tourmaline used in the last experiment, I found both the rays transmitted, but, it seemed to me, with unequal intensities. In other words, one ray suffers greater absorption than the other. It seems probable that with greater thickness of crystal one ray would be completely absorbed. I found other crystals behaving more or less in the same way. I reserve for another communication particulars of experiments bearing on this subject.
Lastly, I tried an experiment with a crystal of Iceland Spar taken out of a polarising apparatus. With this I got distinct depolarising action.
Summary.—Crystals which do not belong to the Regular System, polarise the electric ray just in the same way as they do a ray of ordinary light. Theoretically, all crystals, with the exception of those belonging to the Regular System, ought to polarise light. But this could not hitherto be verified in the case of opaque crystals. There is now no such difficulty with the electric ray, for all crystals are transparent to it. As a matter of fact, all the above experiments, with one exception, were performed with specimens opaque to light, and it was an interesting phenomenon to observe the restoration of the extinguished electric radiation, itself invisible, by the interposition of what appears to the eye to be a perfectly opaque block of crystal, between the crossed gratings.
(Asiatic Soc. of Bengal, May 1895.)