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Collected Physical Papers/On A New Electro-Polariscope

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II

ON A NEW ELECTRO-POLARISCOPE

In a paper read before the Asiatic Society of Bengal "On Polarisation of Electric Rays" (May, 1895), I gave an account of some experiments which showed that crystals which do not belong to the Regular System, produce double-refraction of the electric ray, and that the refracted beams are plane-polarised.

Among the numerous crystals examined, I found some exhibiting the so-called depolarising action in a very marked degree, even when the thickness of the crystal was less than the wave-length of the electric radiation. I found, for example, Nemalite, a fibrous variety of Brucite, exhibiting this action with pieces which were comparatively thin. Different varieties of Satin Spar, Serpentine, Tourmaline and a few others were found to be very effective depolarisers.

From the various experiments to be presently described, I was led to suppose that these crystals transmit the ordinary and the extraordinary rays with unequal intensities. It would thus seem possible to quench one of the two rays by absorption, ordinary radiation after transmission through these crystals thus becoming plane-polarised. It should, however, be mentioned here, that crystals as a rule are far more transparent to electric radiation than to ordinary light, and as a consequence greater thickness of crystals would be required for the complete absorption of one of the two rays.

The apparatus with which the following experiments were carried out, consists of a Radiator emitting short electric waves, a cylindrical lens of sulphur for rendering the electric beam parallel, a pair of Analyser and Polariser, and a Receiver for detecting the electric radiation.

Electric oscillation is produced by sparking between two small platinum beads attached to hollow hemispheres and an interposed small platinum sphere (cƒ. fig. 1). The two electrodes are connected with the secondary ends of a modified Ruhmkorff coil. The usual vibrating interrupter was found to be a source of trouble, and was therefore rejected. A flash of radiation consisting of many electric oscillations is sufficient for a single experiment, and this was easily obtained by a sudden break of the primary current. To economise space, the coil was taken out of the supporting condenser box. Long strips of paraffined paper with tin-foil coatings were wound round the coil, and appropriate connections made with the interrupting key. The coil, about three inches in diameter and five inches in height, was placed vertically at one end of a small tinned-iron box, 6×4×7 inches. The metallic box screens the Receiver from electric and magnetic disturbances produced by the making or breaking of the primary current, the electric radiation being transmitted through a tube along the desired direction.

The box also contains a small storage cell and a tapping key, the press button projecting out of the box through a small hole. In front of the box there is a tube one inch in diameter and three inches long. To the inner end of this tube is fixed an ebonite square on which the radiator is mounted.

Polariser. Analyser and the Crystal-holder

At the further end of the tube is placed the Polariser,which may be a grating or any other form of Polariser, to be presently described. By means of a variable diaphragm at the end of the tube, the amount of electric radiation for an individual experiment may be varied.

Next to the Polariser is the crystal-holder, which allows the principal plane of the crystal to be inclined at any azimuth.

The Analyser is similar in construction to the Polariser, and is mounted at the open end of the cell containing the sensitive Receiver, which is a spiral spring coherer, with numerous points of contact.

The resistance of the coherer varies within wide limits. The one I use gives the best result when the resistance is reduced by compression to about 4,000 ohms; the corresponding current circulating round the circuit is then about 10-4 ampere. The incident radiation reduces this resistance to 10 ohms or less. The sudden increase of current, due to the diminution of resistance, produces a deflection of the spot of light reflected from the galvanometer.

The coherer is enclosed in a small metallic cell open in front for the reception of the Analyser. The metallic cell has also a tubular projection behind, through which the wires from the ends of the coherer pass out.

Adjustable E. M. F. and the Galvanometer

The wires from the ends of the coherer lead to an adjustable E. M. F. and a dead-beat galvanometer of D'Arsonval type. The wires are placed within a double coating of tin-foil, and the galvanometer and the voltaic cell enclosed in a metallic case with a slit for the passage of the reflected spot of light. These precautions are taken for shielding the receiving circuit from the disturbing action of stray radiations.

The sensitiveness of the Receiver depends greatly on the proper adjustment of the E. M. F. For all-round work, an E. M. F. of about ·45 volt, is best suited with the particular coherer used in my experiments. This was obtained from an Iron-Zinc cell, the Iron being immersed in a solution of ferric chloride, and Zinc in dilute sulphuric acid. After the Receiver has been subjected to radiation for a length of time, its sensibility is diminished; this may be restored by slightly increasing the E. M. F. The Receiver is placed in a derived circuit, the main current, and therefore the derived E. M. F., being varied by means of a rheostat.


Method of experiment

When the spark-gap is vertical, the electric radiation is to a great extent polarised, the vibration being accompanied by electric force in a vertical plane, and magnetic force in a horizontal plane. I shall, for simplicity, consider only the electric vibration. When the partially-polarised radiation is transmitted through a horizontal grating, the emergent beam is completely polarised, the vibration taking place in a vertical plane passing through the axis.

The spiral-spiring coherer itself exhibits selective absorption. It absorbs more readily vibrations parallel to its length. Thus, when the spark-gap and the coherer are parallel, the response is very energetic, whereas the response is but feeble when the two are crossed. The analysing action of the coherer becomes more complete if it be further provided with an Analyser grating with wires perpendicular to the length of the coherer.

There are two principal positions of the Polariser and the Analyser:—

(1) Parallel position.—When both the gratings are horizontal.
(2) Crossed position.—When the polarising and analysing gratings are at right angles to each other.

In the first position, the radiation being transmitted through both the gratings, falls on the sensitive surface, and the galvanometer responds. The field is then said to be bright. In the second position the radiation is extinguished by the crossed gratings, the galvanometer remains unaffected, and the field is said to be dark. But on interposing certain crystals with their principal planes inclined at 45° to the horizon, the field is partially restored, and the galvanometer spot exhibits large deflection. This is the so-called depolarising action of double-refracting substances.


Experiment with Serpentine

I obtained a piece of fibrous Serpentine of greenish colour from the Geological Department of India; its thickneess was about 2 cm. I interposed it with its fibres inclined at 45°, between the crossed Polariser and Analyser. There was at once a restoration of the field.

The Polariser and the Analyser were now made parallel, and the piece of Serpentine placed with its fibres vertical or parallel to the vibration of the electric ray; the galvanometer now ceased to respond, proving the complete absorption of the ray by the Serpentine.

The piece of Serpentine was now held with the fibres horizontal, and the radiation was found to be copiously transmitted.

In other words, Serpentine transmits vibrations perpendicular to the fibres, but absorbs vibrations parallel to the fibres. Ordinary radiation would thus, after transmission through Serpentine, be plane-polarised, the vibration taking place perpendicular to the fibres. To ensure complete polarisation, the piece should be fairly thick.

An efficient Polariser or Analyser can thus be made of substances like Serpentine, provided that the thickness is sufficiently great.


Satin Spar, Tourmaline and Nemalite

I also found different crystals exhibiting unequal transparency to polarised radiation in different directions. Satin-spar exhibits it, the electric vibration being more easily transmitted across the fibres. I next tried some experiments with a piece of black Tourmaline about 2 cm. in thickness. With this thickness, it was not possible to obtain complete extinction, unless the intensity of the incident radiation was considerably diminished. I at first arranged the Polariser and the Analyser parallel, and the Tourmaline was successively held vertical and horizontal. The Receiver responded with unequal intensities, the response being more energetic when the length of Tourmaline was parallel to the electric vibration. With fibrous varieties of crystals, I found the vibration, as a rule, more easily transmitted perpendicular to the length of fibres.

I now held the Tourmaline horizontal, and by varying the aperture at the end of the radiating tube, diminished the amount of radiation, so that at a certain point there was no response in the Receiver. On now holding the Tourmaline vertical, the Receiver at once responded.

I next experimented with a piece of Nemalite. The crystal I used was a very small one, only one cm. in thickness, but this comparatively thin piece exhibited unusually strong depolarising action. It is therefore highly probable that a sufficiently thick piece of Nemalite would make an efficient Polariser.


Experiment with Jute Cell

The most efficient polarising substances I have come across are the vegetable fibres. Among these may be mentioned the fibres of Aloes (Agave), Rhea (Boehmeria nivea), Pine Apple (Ananas sativus), Plantain (Musa paradisiaca).

Common jute (Corchorus capsularis) exhibits the property of polarisation in a very marked degree. I cut fibres of this material about 3 cm. in length, and built with it a cell with all the fibres parallel. I subjected this cell to a strong pressure under a press. I thus obtained a compact cell 3×3 cm. in area, and 5 cm. in thickness. This was mounted in a metallic case, with two openings 2×2 cm. on opposite sides, for the passage of the radiation.

The polarising grating was removed, and the Analyser arranged with its vibration plane vertical. The jute cell was now interposed with its fibres horizontal, and the Receiver was found to respond energetically.

The cell was now placed with the fibres vertical, and there was now not the slightest action on the Receiver.

The fibres were now inclined at 45° to the horizon. There was an immediate response in the Receiver but the radiation was completely extinguished by rotating the Analyser till the grating crossed the fibres. On continuing the rotation, the Receiver responded most when the wires of the grating and the fibres became parallel. Keeping the jute cell fixed at 45°, it was found that during one complete rotation of the Analyser, there were four positions of extinction (when the wires and fibres crossed), and four positions of maximum transmission (when the wires and the fibres were parallel).

From the above experiments, it is quite clear that a jute cell produces complete polarisation of the transmitted ray, the plane of vibration being perpendicular to the fibres.

Two jute cells were then made, and the gratings discarded. One of these acts as a very efficient Polariser, the other as an Analyser. When the two are crossed, the electric radiation is completely extinguished.

In the polarisation apparatus described above, three different types of Polariser (or Analyser) can be used:—

(1) The wire grating Polariser.
(2) Polariser made of crystals like Tourmaline or Nemalite.
(3) Jute or vegetable-fibre Polariser.

The apparatus may also be used as a Polarimeter, the rotation of the Analyser being easily measured by means of the graduated disc.

(The Electrician, Dec. 1895.)