Collected Physical Papers

Front matter

COLLECTED PHYSICAL PAPERS

BY THE SAME AUTHOR

BY PROF. PATRICK GEDDES

The Resonant Recorder

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 refract 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.

Radiator.—Electric oscillation is produced by sparking between hollow hemispheres, and a small interposed sphere. The two beads attached to the hemispheres and the interposed sphere were at first thickly coated with gold, and the surface highly polished. This worked satisfactorily for a time, but after long-continued action, the surface of the ball became roughened and the discharge ceased to be oscillatory. After some difficulty in obtaining the requisite high temperature, I succeeded in casting a solid ball and two beads of platinum. There is now no difficulty in obtaining an oscillatory discharge; the sparks are made very small, as these are more effective with the receiver used. After a little experience, it is possible to tell whether the discharge is oscillatory or not. The effective sparks have a peculiar smooth sound, whereas non-oscillatory discharges give rise to peculiar cracked sound.

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 side by side, the sensitive surface being 1 × 2 cm. The springs are prevented from falling by a glass slide in front. The spirals may be compressed by means of a brass piece which slides in and out by the action of a screw. The resistance offered by this portion of the circuit can, therefore, be gradually varied. An electrical current enters along the breadth of the top spiral and leaves by the lowest spiral, having to traverse the intermediate spirals along the numerous points of contact. The resistance of the receiving circuit is thus almost entirely concentrated at the sensitive contact-surface, there being little useless short-circuiting by the mass of the conducting layer. When electric radiation is absorbed by the sensitive contacts, there is a sudden diminution of resistance, and the galvanometer is violently deflected.

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 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.)

On A New Electro-Polariscope

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.)

On Double Refraction of the Electric Ray by a Strained Dielectric

III

ON DOUBLE REFRACTION OF THE ELECTRIC RAY BY A STRAINED DIELECTRIC

The following investigations were undertaken to find whether a solid dielectric becomes double-refracting to the electric ray when it is subjected to a molecular stress by unequal expansion in two directions, or by mechanical compression.

The Electric Polarisation Apparatus, previously described, was used for these experiments. The Analyser and the Polariser were crossed. The strained substance was then introduced between the crossed Analyser and Polariser.

Effect due to unequal expansion

I cast a rectangular piece of paraffin, and chilled its surface unequally by a freezing mixture. On suitably interposing this between the Polariser and the Analyser, the galvanometer spot was at once deflected, proving the double-retracting nature of the strained dielectric. The piece was cast six months ago. It retains its unannealed property even now.

I took two zinc tubes, one cylindrical and the other rectangular, and made castings of melted pitch, which set on cooling. In the cylindrical tube the strain was the same in all directions at right angles to the axis, and this cylindrical mass of pitch did not exhibit any double-refraction. But in the tube with the square section, unequal strain was evident in rectangular directions, perpendicular to the axis; two of the opposite sides bulged out, and the other two bulged in. On interposing this piece between the crossed Polariser and Analyser, it exhibited double~refraction.

Effect due to compression

It appeared that stratified rocks, which, from the nature of their formation, were subjected to great pressure, would serve well for the purpose of this experiment. My anticipations were verified, as the following experiments will show:—

I took a cube of Sandstone, about 5 cm. on each side, and held it between the Polariser and the Analyser, with the plane of stratification inclined at 45°. The galvanometer at once responded.

I then rotated the piece of rock, and adjusted it with its plane of stratification parallel to the vibration plane of the Polariser. There was now no depolarisation effect, the galvanometer remained unaffected. On further rotation in the same direction, the depolarisation effect re-appeared, to disappear again when the plane of stratification was made parallel to the vibration plane of the Analyser.

Experiments with many other rocks, gave similar result.

(The Electrician, Dec. 1895.)

On the Determination of the Index of Refraction of Sulphur for the Electric Ray

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IV

ON THE DETERMINATION OF THE INDEX OF REFRACTION OF SULPHUR FOR THE ELECTRIC RAY

The indices of refraction of transparent substances have been determined by the usual optical methods. There are, however, numerous substances like the various rocks, wood, brick, coal-tar, and others which are not transparent to light, so that their indices could not be determined. These substances are, nevertheless, transparent to electric radiation; the present investigation was therefore undertaken to find a direct method of determining their indices with a high degree of accuracy.

Even in the case of optically transparent substances, the indices are only known for the narrow range of light waves. For greater wave lengths, the index is inferred from Cauchy's formula. Professor Langley has, however, shown that this formula fails to give trustworthy results when applied to the dark radiations in the infrared region of the spectrum. It does not, therefore, seem at all likely that it will give any reliable results when applied to the electric radiation.